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Effective masking levels for bone-conduction auditory steady-state response thresholds in infants Hansen, Erin Estelle 2010

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EFFECTIVE MASKING LEVELS FOR BONE-CONDUCTION AUDITORY STEADYSTATE RESPONSE THRESHOLDS IN INFANTS  by ERIN ESTELLE HANSEN B.Sc., The University of Calgary, 2007  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE  in  The Faculty of Graduate Studies (Audiology and Speech Sciences)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  April 2010  © Erin Estelle Hansen, 2010  Abstract To obtain ear-specific bone-conduction thresholds, masking of the non-test ear is often required. Masking is not currently utilized in the pediatric diagnostic test battery, partly because effective masking levels (EMLs) for bone-conducted stimuli in young infants are not known. The purpose of this study is to determine EMLs for auditory steady-state responses (ASSRs) elicited by bone-conducted stimuli in a group of normal-hearing infants under six-months of age and adults. Using a two-channel ASSR recording, single 1000- and 4000-Hz bone-conducted AM/FM stimuli were masked out with 1 and 4 kHz of narrowband noise presented binaurally. Taking into consideration maturational differences in real-ear-to-coupler differences (RECDs) and bone-conduction sensitivity (Small & Stapells, 2008a), it was predicted that infants would require more and less masking at 1000 and 4000 Hz, respectively. As expected, infants have higher and lower EMLs at 1000 and 4000 Hz, respectively, compared to adults. When RECDs are accounted for, infants have even higher EMLs at 1000 Hz and similar EMLs at 4000 Hz compared to adults. This is consistent with the frequency-dependent differences in boneconduction sensitivity for infants. When differences in bone-conduction sensitivity are accounted for, infants have lower EMLs at both frequencies. When RECDs and boneconduction sensitivity are taken into account, infants have lower EMLs at 1000 Hz and similar EMLs at 4000 Hz. Based on ipsilateral/contralateral asymmetries in masked amplitudes, adults were estimated to have inter-aural attenuations of at least 0-5 and 0-10 dB at 1000 and 4000 Hz, respectively. In contrast, infants were estimated to have inter-aural attenuations of at least 10 dB at 1000 Hz and minimum inter-aural attenuations of greater than 35 dB at 4000 Hz. Similar to behvaioural investigations, the amplitude findings of this study suggest processing efficiency may be immature at 1000 Hz, but not at 4000 Hz. Based on the findings of this study, the  following preliminary masking levels for bone-conduction stimuli are recommended: (i) 1000 Hz: 48 and 58 dB SPL at 15 and 25 dB HL, respectively, and (ii) 4000 Hz: 40 and 45 dB SPL at 25 and 35 dB HL, respectively.  III  Table of Contents Abstract  .  ii  Table of Contents  ivy  List of Tables  viii  List of Figures  ixx  List of Abbreviations  xiiii  Acknowledgements  xivv  CHAPTER 1: Literature Review: Effective Masking Levels  1  1.1  Introduction  2  1.2  Masking  6  1.2.1  Theory of Signal Detection and Minimum Masking Levels  8  1.2.2  Properties of Simultaneous Maskers  9  1.2.3  Effective Masking (EM) and Calibration of the Audiometer  13  1.2.4  Types of Masking  14  1.2.5  Adverse Effects of Tonal Maskers  18  1.2.6  Psychophysical Tuning Curves  19  1.2.7  Masking Level Difference (MLD)  21  1.2.8  Behavioural Masking Studies in Infants  22  Iv  1.2.9  Physiological Masking Studies in Infants  Maturation of the Auditory System  1.3  29 32  1.3.1  Maturation of the Cochlea  32  1.3.2  Maturation of the Outer and Middle Ear  33  1.3.3  Maturation of the Skull  35  Mechanisms of Bone Conduction  37  1.4 1.4.1  The Compression Mechanism  37  1.4.2  The Inertial Mechanism  38  1.4.3  The External Canal Mechanism  38  1.4.4  The Non-Osseous Mechanism  39  Auditory Steady State Responses (ASSRs)  1.5  40  1.5.1  Stimulus Rate and EEG Noise  41  1.5.2  ASSR Generators  42  1.5.3  Anaylsis of the Responses  43  1.5.4  Signal Averaging, Artifact Rejection, and Weighted Averaging  44  1.5.5  Objective Response Detection  45  1.5.6  Electrode Montage  46  1.5.7  Electrophysiological Thresholds  48  1.6  Rationale for Thesis Studies  52  V  CHAPTER 2: Effective Masking Levies for ASSR Thresholds in Infants  53  2.1  Introduction  54  2.2  Methods and Materials  60  2.2.1  Experiment 1A- Infant and Adult Effective Masking Levels (ASSRs)  60  2.2.1.1  Participants  60  2.2.1.2  Stimuli and Maskers  61  2.2.1.3  Calibration  64  2.2.1.4  Recording  65  2.2.1.5  Procedure  67  2.3.1  Data and Statistical Analyses  68  2.2.2  Experiment 1B- Adult Effective Masking Levels (Behavioural)  69  2.4  Results  70  2.4.1  Experiment 1A- Infant and Adult Effective Masking Levels (ASSRs)  70  2.4.2  Experiment 1B- Adult Effective Masking Levels (Behavioural)  86  CHAPTER 3: Discussion and Conclusion 3.1  Discussion  87 88  3.1.1  Experiment 1A- Infant and Adult Effective Masking Levels (ASSRs)  3.1.2  Experiment 1B- Adult Effective Masking Levels (Behavioural)  101  3.1.3  Clinical Implications  102  88  vi  3.2  Conclusion  .  104  References  106  APPENDIX A: Certificate of Ethics Approval  135  APPENDIX B: Individual Infant Data Experiment 1A  137  APPENDIX C: Individual Adult Data Experiment 1A  143  APPENDIX D: Individual Adult Data Experiment lB  151  VII  List of Tables Table 2.]. Conversion of bone-conducted stimulus levels in dB mSL to dB ilL for adults and  infant participants  63  Table 2.2. ASSR EMLs measured in the coupler and in the ear canal: Three-way mixed ANOVAs showing comparisons between bone-conduction intensity level (10 & 20 dB mSL), across age group (infants and adults) and carrier frequency (1000 & 4000 Hz)... 76 Table 2.3. Comparison of the mean real-ear-to-coupler differences (dB) between a group of 15  young infants (3-27 wk) and a group of 17 adults for 1000 and 4000 Hz. RECD values represent the average for the left and right ear  77  VIII  List of Figures Figure 1.1. Masking patterns produced by various pure tone maskers. The masker frequency in  Hz is depicted in a box in each frame. The intensity of the masker in dB SL is indicated by the numbers on the curves (Adapted with permission from Ehmer, 1959)  11  Figure 1.2. Acoustic spectra of bone-conducted stimuli (AM/FM) used in this study for carrier  frequencies of 1000 and 4000 Hz. Y axis ticks represent 10 dB intervals  14  Figure 1.3. Composite tuning curves for infants and adults, derived from averaged masked  thresholds from different subjects at different masker frequencies. Absolute (unmasked) thresholds are plotted with open symbols (Taken with permission from Olsho, 1985)  ...  20  Figure 1.4. Mean infant and adult thresholds for .5- and 4 kHz tones presented in four levels of  continuous masking noise. Tone durations were 10 and 100 msec. Values plotted at points labelled  “Q” are absolute thresholds. Error bars represent SD from mean (Taken  with permission from Berg & Boswell, 1999)  26  Figure 1.5. Mean bone-conduction ASSR thresholds (1 SD) at each carrier frequency for 35  young infants, 13 older infants, and 18 adults with normal hearing (Taken with permission from Small & Stapells, 2008a)  49  Figure 2.1. Acoustic spectra of stimuli (AM/FM) used in this study for carrier frequencies of  1000 and 4000 Hz. Y axis ticks represent 10 dB intervals  62  Figure 2.2. Spectral content for narrow-band noise maskers presented at 90 dB effective  masking level via an OB 802 clinical audiometer  64  ix  Figure 2.3. Representative ASSRs for 1000- and 4000-Hz bone-conducted stimuli presented at 25 dB HL with different levels of air-conducted narrow-band noise masker for an  individual infant and adult. Masker levels were calibrated in the coupler. Shown are amplitude spectra resulting from FFT analyses (75-105 Hz) of the ASSRs. Filled triangles indicate responses that differ significantly from the background noise (p. <05). Open triangles indicate no response (p.  05 and EEG noise 1 mV). Effective masking  level is defined as the lowest intensity of the masker that resulted in an absent response. 71 Figure 2.4. Effective masking levels calibrated in the coupler for 1000- and 4000-Hz bone-  conducted signals (dB HL) in adults (N=10-15) and infants (N=10-13)  72  Figure 2.5. Representative ASSRs for 1000- and 4000-Hz bone-conducted stimuli presented at  20 dB mSL with different levels of air-conducted narrow-band noise masker for an individual infant and adult. Masker levels were calibrated in the coupler. Shown are amplitude spectra resulting from FFT analyses (75-105 Hz) of the ASSRs. Filled triangles indicate responses that differ significantly from the background noise (p. <05). Open triangles indicate no response (p.  05 and EEG noise 1 mV). Effective masking  level is defined as the lowest intensity of the masker that resulted in an absent response. 74 Figure 2.6. Effective masking levels calibrated in the coupler and in the ear canal for 1000- and  4000-Hz bone-conducted signals (dB mSL) in adults (N=10-15) and infants (N=10-13). 75 Figure 2.7. Mean ASSR amplitudes for 1000 Hz (A.) 4000 Hz (B.) bone-conducted stimuli,  presented atlO and 20 dB mSL, at different masking levels, in the ipsi- and contralateral x  recording channel for infants (N=5 -14) and adults (N=5- 14). Results were only reported if at least 5 participants contributed to the mean  81  xi  List of Abbreviations Abbreviation  Definition  ABR  Auditory brainstem response  AM  Amplitude modulation  AM/FM  Mixed modulation  ANOVA  Analysis of variance  ASSR  Auditory steady state response  BMLD  Binaural masking level difference  CR  Circle radius  dB  Decibel  dB ilL  Decibels  dB mSL  Decibels  dB SPL  Decibels  df  Degrees of freedom  DPOAE  Distortion product otoacoustic emissions  EEG  Electroencephalogram  EM  Effective masking  EML  Effective masking level  F  Fisher’s F ratio  FFT  Fast Fourier transform  FM  Frequency modulation  Hz  Hertz  kilz  kilohertz (1000 Hz)  —  —  —  hearing level mean sensation level sound pressure level  XII  ms  millisecond  OAE  Otoacoustic emissions  p  Probability  RETSPL  Reference equivalent sound pressure level  RETFL  Reference equivalent threshold force level  SD  Standard deviation  SPL  Sound pressure level  TEOAE  Transient evoked otoacoustic emissions  VRA  Visual reinforcement audiometry  XIII  Acknowledgements I would like to thank the faculty, staff, and students at the UBC for their encouragement and support over the past two years. I am particularly grateful to Dr. Susan Small for her commitment to guiding me through my first research experience. I would also like to thank Dr. David Stapells and Dr. Navid Shahnaz for their input and participation on my thesis committee. I also owe special thanks to Lauren Hulecki and Ning Hu for their help throughout the summer in recruiting and testing young infants for my study. As well I would like to thank Elissa Rondeau for all of her help around the PAL lab. Finally I would also like to thank my family and friends for their support throughout my education.  xiv  CHAPTER 1: Literature Review: Effective Masking Levels  1.1 Introduction Although newborn hearing screening intervention programs, such as the British Columbia Early Hearing Program, have brought forth the goal of reducing the age of detection for congenital hearing losses to 3 months of age, with initiation of treatment (e.g.,amplification) by six months of age (Joint Committee on Infant Hearing, 2007), further research is needed to increase the accuracy of the diagnosis of congenital hearing loss and the effectiveness of treatment. Compared to adults, the diagnosis and treatment of hearing loss in young infants and children has inherent within it at least two additional challenges. First, unlike adults, infants are subject to on-going maturational changes early on in life; these changes affect the manner in which sound is both delivered to the ear and later processed by neural mechanisms. Second, unlike adults, behavioural methods of threshold assessment cannot be used for infants under six months of age (Joint Committee on Infant Hearing, 2007; Moore, Wilson, & Thompson, 1977) because infant behavioural responses are unreliable (Moore, Wilson, & Thompson, 1977; Muir, Clifton, & Clarkson, 1989). There are many maturational changes during infancy that affect the clinical diagnosis and subsequent treatment of hearing loss. For example, we know from both behavioural and physiological studies that hearing sensitivity to air-conducted stimuli improves with maturation (Balfour, Pillon & Gaskin, 1998; Berg & Smith, 1983; Eisele, Berry & Shriner, 1975; Franklin, Johnson, Smith-Olinde, & Nicholson, 2009; Herdman & Stapells, 2001; Klein, 1984; Parry, Hacking, Bamford, & Davy, 2003; Rance & Tomlin, 2006; Sininger, Abdala, & Cone-Wesson, 1997; Van Maanen & Stapells, 2009; Weir, 1979; Werner, Folsom & Mancl, 1994; Werner & Mancl, 1993; Werner-Olsho, Folsom, & Mancl, 1993). In contrast to air-conduction studies, an  2  ASSR study conducted by Small and Stapells (2008a) suggests that sensitivity to bone-conducted stimuli becomes worse with maturation for the low frequencies (500 & 1000 Hz), improves slightly at 2000 Hz, and does not change at 4000 Hz. It is necessary to take these maturational trends into consideration when diagnosing and quantifying hearing loss in infants. One difficulty in diagnosing hearing loss in young infants that has not been studied in detail is the ability to obtain reliable ear-specific bone-conduction thresholds. Typically, in older children and adults, the use of a noise, referred to as a masking noise, is utilized in the opposite or contralateral nontest ear to ensure that the behavioural response to the tone is coming from stimulation of the ear of interest or the test ear. The importance of using a masking noise clinically can be illustrated in cases of unilateral sensorineural hearing loss and bilateral conductive hearing loss (e.g.,Campbell, Harris, Hendricks & Sirimanna, 2004; Hatanaka, Yasuhara, Hon & Kobayahi, 1990; Weber, 1983). In the study conducted by Hatanaka and colleagues (1990), the importance of masking was depicted by examining the ABR of a young infant with total unilateral hearing loss, with and without contralateral masking. For this infant, crossover responses were observed without contralateral masking, but were completely eliminated with the introduction of 45 dB ilL of contralateral masking when air-conducted 85 dB HL clicks were presented to the deaf ear. These results suggest that such cross-over responses will contribute to the ipsi- and contralateral recorded ABR waveform when an ABR recording is carried out without contralateral masking (Hatanaka et a!., 1990). Methods that limit the response to bone-conducted stimuli from the non-test ear are required for all stimulus intensities for adults because their inter-aural attenuation for a bone conducted stimulus is at least 0-10 dB (Hood, 1960; Nolan & Lyon, 1981; Sanders &  3  Rintelmann, 1964; Studebaker, 1967). In contrast to adults, bone-conduction stimulation in normal-hearing infants is not necessarily binaural. A bone-conducted signal may or may not cross over to stimulate the opposite cochlea depending on the presentation level. Previous studies have estimated at least 10-30 dB of inter-aural attenuation for bone-conducted stimuli in infants using indirect measures (Small & Stapells, 2008b; Stuart, Yang, & Botea, 1996; Yang, Rupert and Moushegian, 1987). We know that, compared to adults, infants show greater ipsilateral/contralateral asymmetries for both air- and bone-conduction ABRs and ASSRs (Foxe & Stapells, 1993; Small and Stapells, 2008b; Stapells & Mosseri, 1991; Stapells & Ruben, 1989; Stuart et al., 1996). This phenomenon is utilized, at least for low presentation levels, to predict which cochlea has responded to the stimulus. In adults and older children, ear-specific masked bone-conduction thresholds are obtained using standard masking procedures. One example of a commonly used masking procedure is the “plateau” method (Hood, 1960).  The masking plateau method takes different principles of  masking into account. Undermasking occurs, when insufficient masking levels have been presented and the effective masking level (EML), the ability of a masking noise to mask a signal of known frequency and intensity, has not been reached. In the undermasking condition, the test signal can be heard in the non-test ear. The range of EMLs for which responses from the nontest ear have been eliminated and true thresholds of the test ear can be established is the plateau. The lower end of the plateau, “minimum masking level”, is defined as the lowest level of masking that will mask out contributions from the non-test ear. The upper end of the plateau, “maximum masking level”, is defined as the greatest level of masking noise that will prevent contributions from the non-test ear, but will not be loud enough to crossover and shift the threshold of the test ear. When the noise is loud enough to be heard in the test ear, the EML has  4  been surpassed resulting in a threshold shift of the signal in the test ear and this is referred to as “overmasking”. For adults, formulae have been developed to predict undermasking, minimum masking level, maximum masking level, and overmasking (Hood, 1960). Additionally, maskers generated by clinical audiometers are currently calibrated in dB effective masking (dB EM), a unit determined by the EMLs at different frequencies for a large group of normal hearing adults (ANSI S3.6, 1996). The calibration of maskers delivered by most clinical audiometers will be discussed in greater detail later. Formulae to predict the required masking levels for bone-conducted stimuli in young infants do not currently exist because sensitivity of infants to bone-conducted stimuli and how these levels change with age have not been fully described. The amount, frequency dependence, and maturation of inter-aural attenuation, is also not fully understood. Furthermore, EMLs for bone- and air-conducted stimuli in infants have been studied very little. This study will attempt to shed some light on this problem, by investigating the differences in EMLs for bone-conducted stimuli in adults and infants that are under six-months of age. The second challenge regarding the difficulties of behavioural testing have been partly overcome through the use of electrophysiological techniques, such as the Auditory Brainstem Response (ABR) and the Auditory Steady State Response (ASSR). Both ABR and ASSR testing techniques involve the recording of brainwaves in response to auditory stimuli and thus do not require subjective responses from the listener. Although the brief-tone ABR is currently the clinical gold standard for the estimation of frequency-specific hearing thresholds in young infants under five to six months of age (Stapells 2000a; Stapells, Herdman, Small, Dimitrijevic & Hatton, 2005), the ASSR recording techniques was utilized in the current study mainly because it is a more objective measure than the ABR. The ASSR technique is more objective  5  because, unlike the ABR technique that requires the subjective interpretation of waveforms by a clinician to determine presence/absence of a response, it utilizes statistical measures in the determination of the presence/absence of responses. Four key areas will be discussed in the following sections to provide background for the present study which investigates the maturational effects of EMLs for bone-conducted stimuli: (i) types of masking and the effects of maturation on masking, (ii) development of both the auditory system and the structures of the skull (iii) the mechanisms that underlie bone conduction hearing and (iv) an overview of the auditory steady-state response technique that will be used for determining the presence/absence to the signal in the unmasked and masked experimental conditions.  1.2 Masking Masking refers to the phenomenon whereby the sensitivity of one sound is affected by the presence of another sound. Often, the introduction of a new sound can affect the detection of an existing sound. For example, the threshold of detection for a sound may be only 10 dB SPL when presented on it’s own, but may rise to 30 dB SPL when presented in conjunction with a second sound. This increase in threshold is called a threshold shift. “Partial” masking occurs in situations where the introduction of a second sound does not lead to a change in the threshold of detection for an initial sound, but simply results in a reduction of the loudness to an initial sound (Scharf, 1971). Masked thresholds can be determined experimentally using one of two methods. In the first method, the difference between unmasked (baseline) and masked thresholds is found to 6  determine the masked threshold of the stimulus. In the second method, masked threshold is found by presenting the stimulus at a fixed presentation level and varying the masker until the stimulus is just audible (Gelfand, 2004). Typically, masking noise is delivered via airconduction. However, one method for testing bone-conduction thresholds, based on Rainville’s “sensorinural acuity level” (SAL) test (Dirks, 1973; Jerger & Tiliman, 1960), delivers masking noise via bone-conduction. For the SAL technique, the amount of bone-conduction noise required to mask out an air-conduction tone near threshold for a panel of normal hearing listeners is determined. The results of the SAL technique from individual patients is then compared to the group of normal hearing individuals in order to classify the type of hearing loss present. For conductive losses, the air-conduction threshold for the test tone will be elevated but the amount of bone-conduction masking noise will be the same as it is for individuals with normal hearing. For sensorineural losses, the amount of bone-conduction noise required to reach the EML will be elevated with the pure-tone threshold. Finally, for mixed losses, the amount of conductive loss is estimated by the difference between the air-conduction threshold and masking level, while the amount of the sensorineural loss is estimated from the level of bone-conduction masking. Because more is known about air-conduction masking and air-conduction masking was utilized in the current study, the following sections will mainly focus on air-conduction masking principles.  7  1.2.1 Theory of Signal Detection When examining the phenomenon of masking, it is important to have a basic understanding of the theory of signal detection. The theory of signal detection is based on two assumptions. The first assumption is that each decision made by a subject is based on a statistic that is derived from many characteristics of the event in question (Green & Swets, 1966). Thus, for the detection in noise task, the decision of whether or not a tone is present will depend on the sensitivity of the individual as well as other factors inherent to the individual such as their processing abilities and internal noise (e.g.,acoustic noise in the ear canal, vascular or other physiological noise, or normal “neural” noise). Hence, a detection task in absolute silence cannot exist. When the experimental masking stimulus is absent, both environmental and internal physiological noises are always present. As a result, the presentation of an experimental masking noise simply adds to the environmental and internal physiological noise that is already there (Egan, 1971). The second assumption of the theory of signal detection is that a subject will adopt a fixed criterion value. Thus, for a behavioural detection in noise task the response to a signal depends on a criterion signal-to-noise-ratio (SNR) in the neural domain of the auditory system. For a listener to maintain a certain level of performance in the detection threshold task (e.g.,50% on the psychometric function), the signal level must increase as the noise in the system increases. The overall intensity of the noise in the system will begin to increase as the external masking noise approaches the intensity of the internal noise of the listener. A shift upward in threshold indicates that the ear is sensitive to the masking stimulus, such that the masker has added to the internal noise and is depicted in the neural response. The point at which the masker intensity becomes sufficient to shift threshold is called the minimum masking level (Nozza & Flenson, 8  1999). Given that auditory thresholds are dependent on both an individual’s sensitivity and other factors (e.g.,internal noise, inferior processing) there is no such thing as absolute threshold.  1.2.2 Properties of Simultaneous Maskers The masking that is produced by a particular masking sound (masker) is dependent to a great extent on the maskers’ intensity (level), spectrum, and bandwidth. The following discussion will focus mainly on simultaneous masking because simultaneous masking is the type of masking most often utilized in the clinic, and is the type of masking used for the current study. Some common simultaneous maskers include: tonal maskers, narrowband noise maskers, broadband noise maskers, and speech noise maskers. Of these simultaneous maskers, most clinical audiometers come equipped with narrowband noise, white noise, and speech noise maskers (Yacullo, 2009). Narrowband noise maskers are maskers which are focused around a particular tone. White noise makers, on the other-hand, are composed of equal energy at all frequencies. Finally, speech noise maskers consist of a white noise that has been filtered to simulate the long-term average spectrum of speech (Yacullo, 2009). Much insight regarding the effects of intensity and spectrum of a masker were gained from early experiments conducted with tonal and narrowband noise maskers. As early as 1894, Mayer reported that although low-frequency tones effectively masked out higher-frequency tones, higher-frequency tones were poor maskers of low-frequency tones. Since 1894, Mayer’s original report has been repeatedly demonstrated for tonal maskers (Ehmer, 1 959a; Finck, 1961; Small, 1959; Wegel and Lane, 1924). One tonal masker study conducted by Ehmer (1959a), examined monaural masking patterns at intensity levels of 20-100 dB Sensation Level (SL) from 250 to 4000 Hz in three  9  listeners, and at 8000 Hz for 100 dB SL in one listener. The monaural masking patterns found in the Ehmer (1959a) study are depicted in Figure 1.1. Looking at Figure 1.1, the following observations can be made. First, the strongest masking occurs at the “centre” frequency and diminishes on either side. Second, higher levels of masking are observed with increases in intensity. Additionally, results from Hawkins and Stevens (1950) suggest that a 10 dB increase in masker intensity will produce a 10 dB increase in masked thresholds, independent of frequency. This 10 dB increase in masked thresholds, with a 10 dB increase in masker intensity, holds for speech stimuli as well as pure tones. Furthermore, this invariance in masking level holds for all signal frequencies ranging from 300 to 5000 Hz and for all intensities except for extremely low-level maskers (Hawkins & Stevens, 1950). Also, when maskers are presented continuously rather than gated, the invariance holds for both shorter and longer duration stimuli (Carolyn & Moore, 1986). Third, masking tends to be symmetric at lower intensities, becoming asymmetrically wider with increasing intensity. In addition, the greatest masking tends to occur for tones that are higher than the masker frequency, with little masking occurring at lower frequencies. In other words, as masking intensity is raised, there is substantial spread of the masking effect upward in frequency, yet only a minor effect downward in frequency. This phenomenon is known as the upward spread of masking. Finally, masking patterns tend to be wide for low frequencies but narrow for high frequencies. Subsequent studies utilizing narrowband masking have essentially confirmed the masking patterns generated in tonal masking studies (Egan and Hake, 1950; Ehmer, 1959b; Greenwood, 1961). The non-linear masking patterns that are observed in Figure 1.1 can be accounted for by basilar membrane tuning mechanism. Outer hair cells have been shown to play an important role in basilar membrane tuning. The ability of the outer hair cells to change length in response to changes in electrical  10  potential and their subsequent ability to relay this information to the inner hair cells through mechanical linkages leads to the enhancement of the travelling wave. The basilar membrane tuning mechanism essentially produces a non-linear basilar membrane response, an enhanced sensitivity at low stimulus levels and a more highly tuned response to narrowband acoustic simulation (e.g.,Dallos, 1992; Dallos & Evans, 1995; Dallos, Evans, & Hallworth, 1991; Dallos, He, Lin, Sziklai, Mehta & Burt, 1997; Galambos, 1956).  I1eqtj Fp  r  Figure 1.1. Masking patterns produced by various pure-tone maskers. The masker frequency in Hz is depicted in a box in each frame. The intensity of the masker in dB SL is indicated by the numbers on the curves. (Adapted with permission from Ehmer, 1959a).  11  The impact of the bandwidth of a noise for masking purposes was initially examined in a study conducted by Fletcher in 1940. Fletcher conducted a centered masking experiment where the bandwidths of a masking noise were adjusted and discovered that masked pure-tone thresholds increased with increasing bandwidths of the masking noise, but only to a certain point. In other words, Fletcher observed that as the bandwidth of the noise increased, masked pure-tone threshold also increased. Once a particular noise band was reached, additional masking of the pure-tone signal did not occur. The point beyond which an increase in masking bandwidth no long leads to additional masking of the pure-tone signal is known as the critical bandwidth (Fletcher, 1940). The concept of the critical bandwidth described by Fletcher (1940) consists of two components. First, when masking a pure tone with a broadband noise, the only components of that noise which have a masking effect on the tone are those frequencies included within a narrow-band centered around the frequency of the tone. Second, when a pure-tone is just audible in the presence of the noise, the total noise power present in the narrow-band of frequencies is equal to the power of the tone. Based on the findings of Fletcher’s study, it can be seen that an efficient masker is one which generates a given EML with the least overall sound pressure level (SPL). Thus, for tonal stimuli, narrowband noises are more efficient maskers than broadband noises because they result in an EML, with the lowest overall SPL. Thus, for the purposes of the current study, narrowband noises from a clinical audiometer were utilized. One potential problem that is associated with narrowband noise maskers is that they may be perceived as tones resulting in confusion of the masker with the signal. In order to reduce this perception of tonality, the bands specified by ANSI are wider to some extent than the critical bands for effective masking (ANSI S3.6-1996).  12  1.2.3 Effective Masking (EM) and Calibration of the Audiometer Effective masking (EM) refers to the SPL of a noise required to mask a signal to 50% probability of detection (ANSI S3.6, 1996). For a normal-hearing ear the EML is equivalent to the degree of decibels that a given band of noise shifts a pure-tone threshold. For instance, when the noise and pure tone are presented simultaneously to the same ear, 40 dB EM would produce a threshold shift of 40-dB HL signal (ANSI S 3.6, 1996). In general, the different maskers generated by clinical audiometers are calibrated in dB EM. Usually narrow-band noise is used to mask out pure tones. When narrow-band noise is used clinically, it is centered geometrically around the audiometric test frequencies. As well, SPL of the noise outside of the tabulated limits specified, should fall at the rate of 12 dB per octave for at least three octaves, and thereafter, not rise above -36 dB relative to the level at the center of the band (ANSI S3.6, 1996). The AM/FM stimuli used in the present study have centre frequencies of 1000.92 and 4199.52, and bandwidths of 518.71 and 1110.65, respectively (Figure 1.2). Although the stimuli being masked out were not pure-tones, the centre frequencies and bandwidths of the 1000 Hz narrowband noise and the 4000 Hz narrowband noise were sufficient maskers of the AM/FM stimuli.  13  AM/FM  100  1000  10000  Frequency (Hz) Figure 1.2. Acoustic spectra of bone-conducted stimuli (AM/FM) used in this study for carrier frequencies of 1000 and 4000 Hz. Y axis ticks represent 10 dB intervals.  1.2.4 Types of Masking Overall, masking can be divided into four basic types: simultaneous masking, nonsimultaneous masking, central masking and informational masking. Simultaneous masking and non-simultaneous masking, with the exception of backward masking (Duiffluis, 1973; Elliott, 1962), are due mainly to interactions of the signal and masker within the cochlea, and thus are often referred to as peripheral or energetic masking (Gardner, 1947; Harris, 1959; Lusher & Zwisloki, 1949; Munson & Gardner, 1950; Samoloiva, 1961; Smiarowski & Carhart, 1975). In contrast to simultaneous and non-simultaneous masking, central and informational masking  14  affects are due to higher level or central processes (e.g.,Kidd, Mason, Deliwala, Woods, & Colborn, 1994; Kidd, Mason, & Rohtla, 1995; Kidd, Mason, Rohtla, and Deliwala, 1998; Leek, Brown, & Dorman, 1991; Liden, Nilsson, & Anderson, 1959; Watson, Kelly, & Wronton, 1976). Simultaneous masking, as discussed earlier, occurs when the test signal and the masker overlap in time. In contrast to simultaneous masking, temporal or non-simultaneous masking occurs when the masker and the test signal do not overlap in time. In general, there are three kinds of temporal or non-simultaneous masking including backward masking, forward masking or a combination of forward and backward masking. Backward masking, also known as pre masking, takes place when the masker is preceded by the signal, with the masking effect occurring backward in time. Forward masking or post-masking, occurs when the masker is presented first followed by the signal, with masking taking place forward in time. Finally, forward and backward masking can be combined through the placement of the signal between two maskers. Many studies have shown that more masking occurs with the combination of forward and backward masking than would result if the individual contributions of forward and backward masking were simply added together (Cokely & Humes, 1993; Elliott, 1969; Oxenham & Moore, 1994, 1995; Pastore, Harris, & Goldstein, 1980; Penner, 1980; Pollack, 1964; Robinson and Pollack, 1973; Wilson and Carhart, 1971). These findings suggest that forward and backward masking, are dependent on different underlying mechanisms. Various studies have attributed forward masking to peripheral mechanisms involving properties of the inner hair cell / auditory nerve fiber synapse and neural adaptation (Duan & Canlon,1996a, 1996b; Duifhuis, 1973; Javel, 1986; Salvi, Saunders, Ahroon, Shivapuja, & Arehole, 1986; Smith,  15  1979). Mechanisms underlying backward masking however are not well understood, with several studies suggesting both peripheral and central factors (Duiffluis, 1973; Elliott, 1962). Central masking occurs when the introduction of a masking noise to one ear produces a small threshold shift in the opposite ear for a test signal. This change in threshold occurs, even if the masking levels used are insufficient to produce over-masking. In other words, a masker presented to one ear can cause a threshold shift for a signal at the other ear even when the masker level is too low for it to cross over to the signal ear (Dirks and Malmquist, 1964; Dirks and Norris, 1966; Ingham 1959; Sherrick & Mangabeira-Albarnez, 1961; Zwislocki, Buining, & Glantz, 1968; Zwislocki, Damianopoulos, & Buining, 1967). According to Zwislocki (1972), the central masking effect is probably the result of an interaction of the masker and the test signal within the central nervous system, likely taking place at the level of the superior olivary complex, where bilateral representation is available. The threshold shift produced by central masking is generally considered to be approximately 5 dB (Konkle and Berry, 1983; Martin, 1966); however, variable results have been reported across studies and subjects (Yacullo, 2009). In addition, there is some evidence that the effects of central masking increase with masking level (Dirks and Malmquist, 1964; Studebaker, 1962). As the typical central masking effect is only about 5 dB, which is considered to be within good test-retest reliability, and the given variability of central masking across subjects makes it difficult to determine an appropriate correction factor, it is not recommended that the effect of central masking be subtracted from clinical masked thresholds (Yacullo, 2009). Finally, informational masking or distraction masking refers to masking effects that are the result of higher-level (central) processes, rather than the interaction of the signal and masker in the cochlea. That is, informational masking can be attributed to the uncertainty introduced to  16  a listening task, through the randomization of masker components; thus interfering with the facility to perceive the signal, even when it has not been rendered inaudible at the periphery (Pollack, 1975; Watson & Kelly, 1981). Typically, informational masking experiments require the participant to detect a pure-tone signal in the presence of a multiple-frequency masker. A typical approach involves raising and lowering the signal to find its masked threshold using a two-interval forced-choice method, in which one interval contains the signal and the masker, and the other interval contains only the masker (e.g.,Green, 1964; Lutfi, 1994; Neff & Dethlefs, 1995; Watson & Kelly, 1981). In addition, information masking experiments use a critical-band-  wide protected region around the signal frequency that does not contain any masker components, so as to avoid or at least minimize any confounding effects resulting from peripheral masking (Neff, Dethlefs, & Jesteadt, 1993). For adults, informational maskers can produce as much as 10 dB of masking when presented sequentially to a tone (Kidd & Watson, 1992; Watson, Foyle, & Kidd, 1990; Watson & Kelly, 1981; Watson et al., 1976; Watson, Wronton, Kelly, & Benbassat, 1975) and as much as 50 dB of masking when presented simultaneously to a tone (Neff & Callaghan, 1987; 1988; Neff & Green, 1987; Spiegel, Picardi, & Green, 1981; Spiegel & Watson, 1981). Also, the effect is greatest when the masker contains a relatively small number of components, usually about 20 or fewer (Neff & Callaghan, 1988; Neff & Green, 1987; Oh & Lutfi, 1997). In addition, there is a considerable amount of variability among subjects for informational masking and informational masking tends to be affected by a variety of factors (Neff and Callaghan, 1988; Neff and Dethlefts, 1995; Richards, Tang, & Kidd, 2002; Wright & Saberi, 1999). The concepts of informational masking should be kept in mind given that informational masking can affect infant and adult behavourial thresholds.  17  1.2.5 Adverse Effects of Tonal Maskers Overall, the following three factors can adversely affect tone-on-tone masking: beats, combination tones, and off-frequency listening. When a subject is presented with two tones differing in frequency by only a few cycles per second, such as 1000 and 1003 Hz, simultaneously, audible fluctuations called beats occur. Thus, when the masker and signal are very close in frequency, it is difficult, if not impossible, to determine whether the subject is responding to the beats or to the test tone (Wegel & Lane, 1924). In fact, when the masker and signal are close in frequency, such audible beats can result in notches at the peaks of the masking patterns (Wegel & Lane, 1924). Replacing tonal maskers with narrowband noise maskers, which are centered on specified given frequencies, may partially but not completely eliminate beats (Patterson & Moore, 1986). In addition to beats, when two or more tones are presented together, combination tones are produced. Combination tones are the result of nonlinear distortion in the ear, which leads to the presence of tones in the output that were not initially in the input. Thus combination tones are produced at frequencies equal to numerical combinations of the two original tones (f 1 and f ), 2 such as 1 -f or 2 2 f -f (Greenwood, 1971). Unlike beats, the elimination of combination tones 1 2f requires more sophisticated manipulations (Patterson and Moore, 1986). Finally, a phenomenon called off-frequency listening, can have adverse effects on tone on-tone masking. To understand the off-frequency phenomenon, it is important to have solid knowledge of auditory filters. Auditory filters refer to the ear’s filtering capabilities that allow the ear to analyze sound in such a way that an individual can separate one frequency from the other (Gelfand, 2004). A person’s ability to distinguish different frequencies from each other is called frequency selectivity, and it is dependent on the width of the ear’s auditory filters 18  (Gelfand, 2004). Findings from a study conducted by Hawkins and Stevens (1950) have shown that the critical band or width of the auditory filter becomes wider as the centre frequency of a tone increases. In addition, critical bands can be thought of as bandwidths around any particular frequency, thus critical bandwidths are not discrete contiguous filters but overlap with one another (Scharf, 1970). Off-frequency listening occurs when there is a shift to another auditory frequency that includes the test frequency, but is not centered there. For example, if a tone is presented in background noise, a neighboring filter centered at a slightly higher frequency may increase the audibility of that particular tone, thus increasing the SNR between the test tone and the masking noise (Patterson, 1976). According to O’Laoughlin and Moore (1981), an effective approach to minimize the confounding effects of off-frequency listening, is to present the signal of interest along with additional noise(s), which will mask out the frequencies above and below the range of interest. These additional noises that mask out frequencies above and below the range of interest are called notched noise maskers. Typically, for ASSRs this phenomenon does not significantly affect thresholds so notched-noise masking is not routinely required (e.g.,Herdman, Picton, & Stapells, 2002).  1.2.6 Psychophysical Tuning Curves A good representation of the ear’s frequency selectivity can be provided with psychophysical or psychoacoustic tuning curves (PTCs). PTCs are tracings of the ears auditory filters that are obtained utilizing masking procedures developed by Chistovich (1957), Small (1959), and Zwicker (1974), wherein the signal is held at a fixed level and the level of the masker is adjusted until it just masks the test tone. The above-mentioned procedure is repeated  19  at several different frequencies of the masker, resulting in PTCs, such as those shown in Figure 1.2. As can be seen in Figure 1.3, the level of masker needed to just mask out the test signal decreases as the masker gets closer to the frequency of the test signal.  100 infans • n odu?s  80  -J  a -o > 1 I  0  0  0  Mosker Frequency fkHzl Figure 1.3. Composite tuning curves for infants and adults, derived from averaged masked thresholds from different subjects at different masker frequencies. Absolute (unmasked) thresholds are plotted with open symbols (Taken with permission from Olsho, 1985).  20  1.2.7 Masking Level Difference The manner in which the signal and masking noise is presented to an individual can differ significantly and the signal and masker can be presented monotically, diotically, or dichotically. Monotic presentation refers to the presentation of the stimulus (eg. signal and masker) to only one ear (Hirsh, 1948a; Jeffress, Blodgett & Deatherage, 1952). Diotic presentation refers to the presentation of an identical stimulus (eg. signal and masker) to both ears (Sever & Small, 1979). Finally, dichotic presentation refers to the presentation of stimuli that in some way differs between the two ears (Hirsh, 1948a; Jeffress, Blodgett & Deatherage, 1952). Stimuli may differ in phase of the signal, phase of the masking noise, presentation of the signal to only one ear with masking noise to both and in presentation rate. By convention, the signal is denoted by 5, the masking noise is denoted by N, monotic presentation is denoted by m, diotic presentation is denoted by o, and a phase difference in the stimulus between the two ears is denoted by it (Hirsh, 1948a; Jeffress, Blodgett & Deatherage, 1952; Sever & Small, 1979). Surprisingly enough, early studies found that a binaural advantage occurs only under dichotic conditions, for both tonal stimuli (Hirsh, 1948a) and speech stimuli (Licklider, 1948). Thus, the term masking level difference also known as binaural unmasking, binaural release from masking, or the binaural masking level difference (BMLD), refers to a difference in masked thresholds between dichotically presented stimuli and signals or stimuli that are presented monotically or diotically (Sever & Small, 1979). Given that auditory input from both cochleae reaches the superior olivary complexes through ipsilateral and contralateral connections, it is reasonable to surmise that the BMLD is generated at and/or above the brainstem level (Moore, 1991). Evidence for the BMLD being processed at the level of the brainstem has been provided through previous research in humans 21  with central auditory processing disorders (Ferguson, Cook, Hall, Grose, & Pillsbury, 1998; Gravel, Wallace, Ruben, 1996; Hannley, Jerger, Riveria, 1983; Noffsinger, Martinez, & Schaefer, 1982) and on single cells in guinea pigs (Caird, Palmer, & Rees, 1991; Jiang, McAlpine, & Palmer, 1997a, 1997b; Palmer, Jiang, & McAlpine, 1999, 2000) and chinchillas (Mandava, Rupert, & Moushegian, 1996). Although the above evidence suggests that the BMLD is generated within the brainstem, in adults, the BMLD is only obtained for slow cortical auditory evoked potentials (waves N1-P2) and is not obtained for the 80 Hz ASSR, ABR, or middle latency response (Fowler & Mikami, 1992a, 1992b, 1995, 1996; Kevanishvili & Lagidze, 1987; Wong & Stapells, 2004) suggesting that the BMLD is generated at the cortical level. For the present study, masking noise was presented in phase to both ears and the signal was presented via bone conduction. Given that the inter-aural attenuation in adults for boneconduction ranges from 0-10 dB (Hood, 1960; Nolan & Lyon, 1981; Sanders & Rintelmann, 1964; Studebaker, 1967), the presentation of the stimuli in the current study is essentially diotic and can be represented as SoNo. In contrast to adults, the inter-aural attenuation in young infants is greater than 0 dB. Consequently, the status of the signal at each ear will be different and the exact nature of this difference in signal at each ear remains unknown.  1.2.8 Behavioural Masking Studies in Infants Only a small handful of behavioural masking studies have been conducted in infants. The majority of these masking studies have utilized a form of behavioural testing in young infants called Visual Reinforcement Audiometry (VRA). For VRA, a head turn response to repeated auditory stimulation is maintained or reinforced through the presentation of a visual reinforcer, such as an animated toy. The VRA technique works well for determining thresholds  22  in infants who are six months through twenty-four months of age (Moore et al., 1977; Thompson & Wilson, 1984), and tends to be inconsistent for testing infants that are under six months of age (Moore et al, 1977; Muir et al, 1989). Although VRA works well for threshold determination in infants six months of age to twenty-four months of age, many infants find the introduction of masking using the VRA procedure confusing. The difficulty infants experience with successfully completing the VRA task in the presence of masking levels is reflected by the high drop-out rates during the training phase of VRA in various studies (e.g.,Berg & Boswell, 1995, 1999; Schneider, Morrongiello, &Trehub, 1990). Masked thresholds for both tone and noise stimuli tend to be elevated by approximately 5-15 dB in six month old infants, gradually decreasing to adult levels by about 10 years of age  (Allen & Wightmen, 1994; Nozza & Wilson, 1984; Schneider & Trehub, 1992; Schneider, Trehub, Morrongeillo, & Thorpe, 1989). In addition, these infant-adult differences in masked thresholds seem to vary as a function of frequency, with the difference being greatest in the lower frequencies (about 15 to 25 dB at 500 Hz), and least in the higher frequencies, at least over the range of 500 to 4000 Hz (Nozza and Henson, 1999). In general, masked thresholds are determined by filter bandwidth, which limits the amount of noise passing through the auditory filter, and processing efficiency (Patterson, Nimmo-Smith, Weber, & Milroy, 1982). Processing efficiency refers to the SNR at the filter output that is required to detect the presence of a signal (Patterson et al., 1982). Both sensory and nonsenosry factors can contribute to processing efficiency. Schneider, Morrongiello, & Trehub (1990), utilizing a VRA procedure, found that masked thresholds for all age groups increased with bandwidth up to a critical width, beyond which further increases in bandwidth were ineffective in increasing threshold. These critical  23  widths did not change substantially with age; despite substantial changes in masked thresholds with age. Thus, changes in auditory filter width cannot account for developmental changes in masked or absolute thresholds (Schneider et al, 1990). In addition, as shown in Figure 1.3, an earlier study conducted by Olsho (1985) demonstrated that infants and adults did not differ significantly from each other in psychophysical tuning-curve width. Because developmental changes in masked thresholds cannot be accounted for by changes in auditory filter width, differences in masked thresholds are generally credited to reduced auditory processing efficiency in young listeners (Werner & Marean, 1996). Both the sensory and non-sensory components of processing efficiency in young listeners have been investigated. Nonsensory components of processing efficiency, such as attention and motivation have been widely recognized as variables that may contribute to age-related differences in masked thresholds (Werner & Marean, 1996; Wightman & Allen, 1992). Werner and Bargones (1991) conducted one such study that investigated the nonsensory component of attention in infants and adults, through two experiments. In their first experiment, Werner and B argones (1991), estimated the detection thresholds of six-month-old infants and adults for a 1000 Hz tone for three different conditions: in quiet, and in the presence of a 4- to 10- kHz band-pass noise at 40 dB SPL and at 50 dB SPL, respectively. The band-pass noise presented to both the infants and the adults was not expected to produce any peripheral interference with the tone. Thus, any differences observed in thresholds between the three conditions, would be due to informational masking. Werner and Bargones (1991) found adult thresholds to be the same under all three conditions, indicating that little or no sensory masking took place in the presence of the noise. In contrast to adults, infant thresholds were approximately 10 dB higher in the presence of the noise, suggesting the presence of distraction masking.  24  Their second experiment investigated the effect of gating the noise at trial onset. Thresholds for the same tone were estimated both in quiet and in the presence of a continuous band-pass noise, at 40 dB SPL. As found in the first experiment, distraction masking was still observed for infants in the presence of a continuous masker. Thus, these experiments suggest that masking can have nonsensory effects on infants’ performance in a psychoacoustic task. Werner and Bargones (1991) attributed the distraction masking observed for infants to attentional factors. In other words, to detect a signal in noise, an individual not only has to physically isolate the relevant spectral information, but they also must be able to selectively process that information. As discussed earlier, even adults may have difficulty selectively processing information under conditions of uncertainty. From the results of the Werner and Bargones (1991) study, it can be observed that infants may be less able than adults to selectively attend to information representing the probe or signal. A study conducted by Berg and Boswell (1999), investigated one aspect of the sensory component of processing efficiency. As mentioned previously, the SNR at masked threshold in adults remains constant over a wide range of stimulus conditions (Hawkins & Stevens, 1950). Berg and Boswell (1999), examined this same sensory component of processing efficiency in adults and in seven-month old infants, by obtaining masked thresholds for 500 and 4000-Hz tones, presented in four levels of continuous masking noise, for stimuli of both 10- and 100-ms in duration. Masker spectrum levels ranged from 5 to 35 dB/Hz and from -5 to25 dB/Hz, for 500 and 4000 Hz stimuli, respectively. As can be seen in Figure 1.4, for seven-month-old infants, the relationship between the ratio of signal energy to spectrum level of the masker (E/N ) 0 or the SNR at masked threshold and level of the masker, was dependent on both signal frequency and duration. Infant performance was most “adult like” at 4000 Hz, with thresholds only being  25  about 7 dB higher in infants and EIN 0 at threshold remaining essentially constant for maskers ranging from -5 to25 dBfHz, for both 10- and 100-ms stimuli durations. In contrast, SNRs at 500 Hz were exceptionally large at lower levels of the masker, and decreased significantly as masker level increased from 5 to 35 dB/Hz. Given that auditory filter bandwidths are comparable in infants and adults, these findings suggest that the processing efficiency of seven-month-old infants is significantly poorer for 500- than for 4000-Hz tones.  40  so 20 10 0 5  15  25  35  -5  5  Noise Spectrum Level (dB/Hz)  15  25  Figure 1.4. Mean infant and adult threholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Tone durations were 10 and 100 msec. Values plotted at points labeled “Q” are absolute thresholds. Error bars represent SD from the mean. (Taken with permission from Berg and Boswell, 1999).  26  In general, for adults, auditory thresholds decrease exponentially with increases in stimulus duration, up to a maximum duration of 200-300 ms (Gerken, Bhat, & Hutchinson Clutter, 1990; Plomp & Bouman, 1959; Watson & Gengel, 1969). This decrease in threshold is believed to be a result of the summation of neural activity within the auditory system over brief periods of time, and is typically referred to as temporal integration or temporal summation (Gerken, Bhat, & Hutchmson-Clutter, 1990; Plomp & Bouman, 1959; Watson & Gengel, 1969). In agreement with a previous study carried out by Berg (1991) who found that temporal summation is adult-like for 4000 Hz tones, but not for octave-band noise bursts centered at 4000 Hz, no age difference for temporal summation was found for the 4000-Hz tone. Also, in agreement with previous studies (Berg & Boswell, 1995), an effect of masker level on temporal summation of low-frequency tones during infancy was revealed. Temporal summation of 500Hz tones, measured as the difference between thresholds for 10- and 100- ms stimuli, was significantly greater for infants than for adults at low levels of the masker. However, when the masker spectrum level was raised to 35 dB/Hz age, differences in temporal summation were no longer significant because infant thresholds improved more rapidly with level for 10- than for 100-ms tones. It is unlikely that nonsensory factors on their own can account for the frequency specific differences in thresholds displayed in Figure 1.3. As a result, various sensory explanations for differences in processing efficiency, with age have been proposed. Moore, Peters, and Glasberg (1990) suggested that reduction in processing efficiency might be due to the narrowing of auditory filter bandwidths, with decreasing centre frequency. Narrowing of auditory filter bandwidths in turn makes random fluctuations at the filter output more perceptible, and thus interferes more with performance at low than at high frequencies. In addition, data reported by  27  Grose, Hall, and Gibbs (1993), indicates that these effects may be more discernible in infants than adults, particularly when durations are short and relatively few cycles of the signal are available. Age differences in masking may also be accounted for by immaturities in the coding of intensity. Schneider et al. (1989) suggested two possible intensity coding immaturities which may contribute to age related differences in masking. First, neural excitation could grow more slowly with increasing intensity for infants and young children, compared to adults, and such differences in excitation could have affects on the perception of loudness. Second, there may be greater variability in the neural representation of intensity in younger listeners, thus limiting their ability to detect changes in intensity. In addition, different mechanisms could be involved in the coding of loudness at low and high frequencies (Zeng & Shannon, 1994). Based on loudness judgments gathered from auditory implant patients, Zeng and Shannon (1994) concluded that the compression of auditory input evident in the loudness function is mediated predominantly by neural mechanisms located in the cochlear nucleus for low-frequency stimuli, and by mechanical processes in the cochlea for high frequencies. As a result, the differences and similarities in adult versus infant masked thresholds observed at 500- and 4000- Hz, respectively, may be attributed to differences in maturity of the two frequency-specific systems. Behavioural Masking Level Difference in Infants  Although the BMLD cannot be recorded in adults using the ABR and 80-Hz ASSR, there are currently no published studies that have investigated a similar phenomenon in infants using ABRs or ASSRs. To date, only one behavioural VRA study conducted by Schneider, Bull and Trehub (1987) has investigated the BMLD in infants utilizing a modified BMLD procedure  28  where noise signals presented through two loudspeakers were either coherent (from the same noise generator) or incoherent (from an independent noise generator). Schneider and colleagues (1987) discovered that it was easier for both adults and 12-month-old infants to locate the incoherent signals even when the two types of signals were adjusted to produce equal increments in power, thus suggesting that the mechanisms responsible for the BMLD are in place by 12 months of age. If the BMLD is functional in infants younger than 12 months of age, it should be easier for infants (i.e., dichotic) to detect a signal in noise presented via bone-conduction than it is for adults (i.e., diotic). Clearly, more research investigating the BMLD in infants is required to better interpret the EMLs obtained in infants and adults when the signal is presented via boneconduction.  1.2.9 Electrophysiological Masking Studies in Infants  A relatively small number of studies have investigated the electrophysiological affects of simultaneous masking in infants. To my knowledge, with the exception of the SAL technique, all of the infant electrophysiological masking studies to date have been done with ABR. Most of these studies have investigated air-conduction masking affects on air-conducted stimuli. However, one study has examined the effects of air-conduction masking on bone-conducted stimuli. In addition, all of these studies focused strictly on the affects that simultaneous masking has on wave latencies and amplitudes, with no electrophysiological study to date focusing on the effects of simultaneous masking on threshold. The SAL technique was initially adapted to bone-conduction ABR tests by Hicks (1980), who demonstrated its’ use in adults and infants with simulated conductive losses. Ysunza and Cone-Wesson (1987) later demonstrated the sensitivity and specificity of the adapted ABR SAL  29  technique in a group of infants and children with congenital ear anomalies. The SAL technique was also utilized by Webb and Greenberg (1983) to estimate bone-conduction thresholds and by Janssen, Borcaar, and Van Zanten (1993) for the development of correction factors for the estimation of the air-bone gap in infants. Finally Cone-Wesson, Rickards, Poulis, Parker, Tan and Pollard (2002) utilized an adapted ASSR SAL technique to estimate bone-conduction thresholds at 1000 Hz for a group of 39 infants with risk factors for hearing loss (e.g. familial history, pinna or other ear anomalies, sydromes etc.). Even though adapted ABR and ASSR SAL techniques show some promise in their ability to differentiate between conductive, sensorineural, and mixed hearing losses, issues with the accuracy of the technique limits it’s clinical use. First, the accuracy of the technique for infants needs to be verified with behavioural methods (Cone-Wesson et al., 2002). Second, bone-conduction threshold measurements for the SAL method are made in the occluded state. In contrast, conventional bone-conduction audiometry is typically carried out in the unoccluded state. Thus direct comparisons between the two testing methods are complicated. The studies that have investigated simultaneous air-conduction masking effects on airconducted stimuli utilized masking as a means of providing more information on infant-adult differences to the neural contributions of the ABR, including: (i) Wave V latencies and amplitudes (Folsom, 1984, 1985; Folsom & Wynne, 1986; Hecox, 1975; Klein, 1986), ii) Wave I latencies and amplitudes (Klein, 1986), and (iii) Wave V tuning curves (Folsom & Wynne, 1987). Overall, in comparison to adults, the studies revealed a greater low-frequency contribution to the responses of infants. Because the present study is interested in determining EMLs for clinical applications of masking of bone-conducted stimuli (i. e. the separation of the  30  non-test cochlea from the test cochlea) the studies that have investigated the effects of airconduction masking on air-conducted stimuli will not be discussed in further detail. Finally the electrophysiological masking study conducted by Yang, Rupert, and Moushegian (1987) looked at the effects of ipsilateral and contralateral masking on the ABR latencies of bone-conducted click stimuli in neonates, one-year-old infants, and adults, for three different conditions. In the first condition, the stimulus level was 35 dB nHL, the white noise masker level was 40 dB nHL, and the SNR was -5 dB. In the second condition, the stimulus level was 35 dB nHL, the masker level was 50 dB nilL, and the SNR was -15 dB. Finally, in the last condition, the stimulus level was 25 dB nHL, the masker level was 40 dB nHL, and the SNR was -15 dB nHL. Overall, Yang and colleagues (1987) found that ipsilateral masking resulted in longer wave V latencies than contralateral masking, and these prolongations in latency were greater in neonates than in one-year-old infants and adults. In the Yang et al. (1987) study, an increase in masker level from 40 to 50 dB nHL resulted in a significant increase of wave V latencies in neonates, a moderate increase in latencies for one-year-old infants, and only a small latency increase for adults. When the SNRs were fixed at -15 dB, a decrease of both signal and masker levels by 10 dB nHL produced a significant increase of wave V latencies in adults, a moderate increase for one year olds, and almost no latency change for neonates. Although the Yang et al (1987) study demonstrated that differences in wave V latencies to masker levels exists in neonates, one-year-old infants, and adults, they did not investigative the effects that masker levels have on threshold, nor did they establish EMLs for neonates.  31  1.3 Maturation of the Auditory System Although the cochlea is anatomically mature at birth (Eby & Nadol, 1986), and most of the structures of the external and middle ear are in place by 21 weeks gestational age and close to adult-like by 32 weeks gestational age, the remaining structures of the outer ear, middle ear, brainstem, auditory cortex, and skull continue to develop well beyond the neonatal period (Moore & Linthicum, 2008). The maturation of the auditory system, and the manner in which structural auditory immaturities influence the ways infants perceive sounds, will be discussed in the following sections.  1.3.1 Maturation of the Cochlea The functional maturity of the cochlea at different ages was assessed by Abdala, Sininger, Ekelid, and Zeng (1996) through the examination of distortion product otoacoustic emission (DPOAE) suppression in normal hearing adults and infants (36-41 weeks gestation). For DPOAE suppression to occur, a tone, known as a suppressor tone, must be presented simultaneously in the presence of two stimulating tones. In the Abdala et al. (1996) study, a DPOAE iso-suppression curve was obtained through varying the level of the suppressor tone until the DPOAE amplitude was reduced by a criterion amount. These suppression tuning curves (STC) are representative of cochlear frequency resolution in the area surrounding the stimulating tone (Abdala et al, 1996). Abdala et al. (1996) discovered that adult and infant STCs are similar in shape, width, slope, and tip frequency. The similarities between infant and adult STCs suggest that the cochlea is adult-like at birth for at least the mid and high frequencies. Thus it is  32  feasible that the active mechanisms in the cochlea responsible for the regulation and sharpening of frequency resolution are mature at birth (Abdala et al, 1996).  1.3.2 Maturation of the Outer and Middle Ear Over the first two years of postnatal development, the ear canal and middle ear undergo various structural changes that subsequently affect the mechano-acoustical properties through which sound is transferred into the cochlea (Keefe, Bulen, Arehart, & Burns, 1993). For instance, studies investigating infant-adult differences in SPL at the tympanic membrane using insert earphones have consistently demonstrated larger SPL in the infant ear relative to the adult ear, with the difference increasing with frequency up to at least 6000 Hz (Bagatto, Moodie, Scollie, Seewald, Moodie, Pumford, & Lui, 2005; Rance & Tomlin, 2006; Sininger, Abdala, & Cone-Wesson, 1997). Real-ear-to-coupler-difference (RECD) measurements were taken, for both ears, of all participants in the current study because the SPL at the eardrum is known to differ significantly between infants and adults. Also, in comparison to adults, infants have higher middle-ear resistance and lower middle-ear compliance. That is, relative to adults, the energy transfer into the middle ear of infants is much less (Keefe et a!, 1993). The abovementioned differences in energy transfer, although not completely understood, are thought to directly influence hearing sensitivity to both air- and bone-conduction stimuli. Although the manner in which infants and adults differ from each other in the transfer of acoustic energy is not well understood, there are physical changes to the outer ear/tympanic membrane and middle ear which could affect the transfer of sound energy to the cochlea. Due to the formation of the bony canal wall up until about one year of age, there is an increase in the size and diameter of the ear canal and a decrease in the compliance of the ear canal walls (Anson  33  & Donaldson, 1981). Based on the absorption of sound energy, Small and Hu (in prep) suggested that the more compliant ear canal walls of the infant can contribute to a power transfer loss, thus reducing the ear canal energy reflectance at low frequencies when it is in motion. Consequently, this leads to a reduction in the effectiveness of the external ear in coupling power into the middle ear. In addition, there is a fusing of the tympanic ring and thus a change in orientation of the tympanic membrane from a more horizontal to vertical position up to three or four years of age (Eby & Nadol, 1986). In the first six months of life, there is growth of the middle-ear cavity from the tympanic membrane to the stapes footplate (Eby & Nadol, 1986), and an increase in the pneumatization of mastoid air cells (Anson & Donaldson, 1981). At approximately five months postnatally, there is a loss of amniotic fluid and mesenchyme in the middle ear cavity; leading to a decrease in the overall mass of the middle ear (Paparella, Shea, Meyerhoff, & Goycoolea, 1980). A reduction in the density of the stapes due to internal bone erosion could result in a reduction in mass (Anson & Donaldson, 1981). Finally, tightening of the ossicular joints and stapes footplate attachment to the oval window may decrease the resistive component (Saunders, Doan, & Cohen, 1993). Clearly these maturational changes to the outer and middle ear may affect the transmission of sound to the cochlea. Given that masking levels in the present study were delivered through air conduction any influences that maturational changes to the outer and middle ear may have on the transmission of sound to the cohlea(e) should be considered.  34  1.3.3 Maturation of the Skull  At birth, the human skull consists of forty-five bony elements, separated by cartilage or connective tissue (Carlson, 1999). The majority of these bony elements gradually fuse together with growth, forming the twenty-two solid bones of the rigid adult skull (Carlson, 1999). In the infant, the bones of the roof of the skull are initially separated by five regions of dense connective tissue, called cranial sutures. In addition, there are also six larger regions of connective tissue, where multiple sutures meet, called fontanels. At birth, the cranial sutures and fontanels are fibrous and moveable. Over time, as growth and ossification progress, the connective tissue of the fontanels is invaded and replaced by bone. As a result, the posterior fontanel usually closes by 8 weeks of age (Moore, 1973). The anterior fontanel can remain open up to 18 months of age (Langman, 2003). Clearly, these developmental changes of the human skull, could potentially account for some of the differences observed between infants and adults in physiological bone-conduction thresholds and response characteristics. According to Stuart, Yang, and Stentrom (1990), better click-ABR thresholds to boneversus air-conducted stimuli in neonates may be accounted for by the flexible sutures of the infants skull, which result in less energy dissipating to the rest of the skull, causing the temporal bone to oscillate more in isolation; thus, resulting in a more effective bone-conducted stimulus across frequencies in infants. In addition, Foxe and Stapells (1993) estimated that the bone conduction stimulus was 9-17 dB and 12.8 dB more effective at 500 Hz and 2000 Hz, respectively, for infants compared to adults. Foxe and Stapells (1993) suggested that the smaller mass of the temporal bone in infants results in a more intense signal activating the cochlea. Although, the structural properties of the infant skull results in sounds being more efficiently conducted to the cochlea in comparison to the adult skull, the presence of the 35  fontanels, as well as neurophysiological differences in the structure and positioning of neural generators in infants compared to adults appears to contribute to significant inter-aural attenuation of bone-conducted signals. When looking at two-channel bone-conduction ASSRs, the mean bone-conduction ASSR thresholds in infants were 13 to 15 dB poorer (or higher) in the contralateral EEG channel relative to the ipsilateral EEG channel, for a frequency range of 500 to 4000 Hz (Small & Stapells, 2008b). In contrast, there were no large differences between ipsilateral and contralateral ASSR thresholds found for adults. Based on these ipsilateral/contralateral differences, interaural attenuation, was estimated to be at least 10 to 30 dB for most infants, and negligible in adults. Thus, in contrast to adults, in infants a boneconducted signal may or may not cross over to stimulate the opposite cochlea, depending on the stimulus presentation level. In addition to the Small and Stapells (2008b) ASSR study, previous two-channel recordings of the bone-conduction ABR have also revealed maturational differences between infants and adults (Foxe & Stapells, 1993; Stapells & Ruben, 1989; Stuart, Yang, & Botea, 1996). Stapells and Mosseri (1991) found that latency and amplitude asymmetries for air conducted sitmuli in infants are present at all different intensity levels and persist until at least 1 to 2 years of age. The smaller wave V amplitudes and longer wave V latencies in the contralateral EEG channel compared to the ipsilateral EEG channel of young infant’s contrasts with similar wave V amplitudes and latencies found in the two channels for adults (Edwards, Durieux-Smith, & Picton, 1985).  36  1.4 Mechanisms of Bone-Conduction A review of the literature reveals three classical osseous mechanisms of bone-conduction and one non-osseous mechanism of bone-conduction (Vento & Durrant, 2009). The three classical modes of bone-conduction include the compression, the inertial, and the external canal mechanism. The manner in which maturation of the auditory system interacts with and affects bone-conduction mechanisms in infants versus adults will be discussed.  1.4.1 The Compression Mechanism  The compression mechanism, also known as the distortion or osseous mechanism, was first proposed by Herzog and Krainz in 1926, and was later modified by Tonndorf in 1968. The compression mechanism theory is dependent on a flexible round window and states that vibrations of the bones of the skull will result in stimulation of the basilar membrane due to the compression of the cochlear skull. Tonndorf (1968) added to this theory by suggesting an even greater pressure differential, and therefore a greater deflection of the cochlear partition are produced by the involvement of the volume of fluid in the semicircular canals. In fact, the inertia of the cochlear fluid in the compression mechanism seems to be the most important contributor to low-frequency bone-conduction in unoccluded adult ears (Kucuk, Abe, Ushiki, Inuyama, Fukuda, & Ishikawa, 1991; Rosowski, Songer, Nakajima, Brinsko & Merchant, 2004; Stenfelt & Goode, 2005; Yoshida & Uemura, 1991). As discussed previously, immaturities of the infant skull will affect the vibrations of the skull and will, in turn, affect the manner in which the compression mechanism operates in adults versus infants.  37  1.4.2 The Inertial Mechanism The inertial mechanism, proposed by Barany in 1938, is based on the idea that inertial bone-conduction derives from the mass of the ossicles and occurs most efficiently in the low frequencies where the skull will vibrate from side-to-side as a whole, thereby setting up relative motion between the ossicular chain and the skull resulting in vibration of the stapes footplate in the oval window. Because the ossicular chain does not constitute merely a dangling mass, it follows that the exact contribution of inertial bone-conduction to the total bone-conduction response, is determined further by the impedance characteristics of the ossicular chain, including effects of the rest of the middle-ear system, and the external canal, to which it is connected via the tympanic membrane. As mentioned earlier, both the middle ear system and the external canal to which it is connected undergo much change in the early years of life, thus, the manner in which the inertial mechanism functions in infants and adults should differ substantially. 1.4.3 The External Canal Mechanism  The external canal mechanism, also known as the osseotympanic mechanism, states that once the cartilaginous canal is vibrating, the sound waves created in the canal should excite hearing through the normal air-conduction route (Naunton, 1963; Stenfelt, Wild, Hato, & Goode, 2003). When the ear canal is unocciuded, the energy derived from this mode of bone conduction, is effectively high-pass filtered; wherein low-frequency energy “leaks” out or escapes from the ear canal. The osseotympanic mechanism is responsible for a common phenomenon, called the occlusion effect. The occlusion effect, accounts for the significant improvement of bone-conduction thresholds that occurs whenever the ear canals are occluded. Although the occlusion effect is observed in adults, Small, Hatton and Stapells (2007) have 38  demonstrated that the same effect is not present in young infants. That is, for young infants, there was no significant difference in pure-tone bone-conduction ASSR thresholds, when the ears were occluded versus unoccluded. In addition, Small and Hu (in prep) have found similar results in a larger group of young infants; with only a slight trend, indicating an emerging occlusion effect, in older infants at 1000 Hz. Although it appears young infants do not have an occlusion effect, more research is needed to confirm at what age in infancy the occlusion effect begins to occur, and when this phenomenon becomes adult-like. Because the maturation of the occlusion effect is still relatively poorly understood, for the purposes of the current study, both infant and adult ears were occluded during the ASSR recordings.  1.4.4 The Non-Osseous Mechanism Finally, the non-osseous mechanism of bone conduction concludes that placement of the bone vibrator on the skull, leads to the stimulation of a fluid pathway, in addition to the activation of the classical bone-conduction pathways. Specifically, the vibrating bone oscillator initiates audio-frequency pressure waves in the cerebral spinal fluid (CSF) which stimulates the cochlear fluids resulting in activation of the basilar membrane (Freeman, Sichel, & Sohmer, 2000). This non-osseous mechanism of bone conduction has been demonstrated recently in both animals and humans (Freeman et al., 2000; Sohmer, Freeman, Gael-Dor, Adelman, & Savio, 2000), but requires further investigation to determine its’ contribution to bone-conduction hearing sensitivity.  39  1.5 Auditory Steady State Responses (ASSRs) According to Regan (1989), a steady state response is a repetitive evoked potential that can be thought of in terms of its’ frequency components rather than its’ time-domain waveform. The first steady-state responses were utilized to assess the visual modality (Regan, 1966). Following the use of steady-state responses for the visual modality, steady-state responses for the auditory modality were recorded with the use of regularly repeating auditory stimuli (Picton, Stapells, Perrault, Baribeau-Braun & Stuss, 1984). Several types of auditory stimuli can be used to evoke ASSRs, and each type of stimulus comes with it’s own advantages and disadvantages. Brief tones, clicks, sinusoidally amplitude modulated (AM) tones, exponential AM stimuli, frequency modulated tones (FM), and a combination of AM and FM stimuli, known as mixedmodulation (AM/FM) tones, are some commonly found stimuli types used to evoke ASSRs. In general, less frequency-specific stimuli, such as clicks, tend to elicit larger responses than more frequency-specific stimuli, such as AM tones (Picton, John, Dimitrijevic, & Purcell, 2003). Sinusoidally AM tones consist of spectral energy at a carrier frequency and at two sidebands located above and below the carrier frequency. The two sidebands are separated from the carrier frequency by the modulation frequency (Picton et al., 2003). When AM and FM tones are presented similutaneously, the AM and FM responses are largely independent of one another for modulation rates of 80 to 100 Hz (John, Dimitrijevic, van Roon, & Picton, 2001; Dimitrijevic, John, van Roon, & Picton, 2001). In general, AM and FM are mediated by neurons with higher and lower characteristic frequencies than the carrier frequency of the modulated tone, respectively (Zwicker & Fast!, 1990). Cohen, Rickard, and Clark (1991) found that response were larger for AM and FM stimuli than AM stimuli only when the phase of the AM and FM components were adjusted such that the highest frequency and highest amplitude occurred at the 40  same time. In other words FM can be used to enhance the amplitudes of the AM response. Because sinusoidally AM and FM bone-conducted tones have been shown to have good frequency specificity and the combination of the two can result in larger responses than AM stimuli on its’ own, AM and FM bone-conducted tones were utilized in the current study.  1.5.1 Stimulus Rate and EEG Noise When recording ASSRs, the stimulus rate can be manipulated to assess the functioning of different levels within the auditory pathway. For ASSRs, when the stimulus rates are high enough, the resulting response will resemble a sinusoidal waveform whose fundamental frequency is the same as the stimulation rate, although it may be more complex. The effects of stimulus rate (or modulation frequency) on the adult ASSR are complex. In general, two basic concepts govern the effects of rate. First, responses generally tend to decrease with increasing stimulus rate. Second, the response is enhanced against this general decline in certain regions, such as in the 30-50 Hz range, known as the 40 Hz ASSR, and in the 70-110 Hz range, known as the 80 Hz ASSR (Picton et al, 2003). As well, there may also be an enhancement of the response near 5 Hz (Arnold, 2007). In addition, the background EEG noise, against which the response is recorded, also decreases in amplitude with increasing stimulus rate; thus, although the response amplitude decreases with increasing stimulus rate, the SNR may actually increase. The background EEG noise could be related to non-physiologic interference in the surrounding environment, such as electrical interference from other equipment, physiological activity, such as eye blinks, crying or muscle activity that is unrelated to the signal, or a combination of the two. The majority of ASSR statistical measures are reliant on the SNR for response detection because  41  response amplitude and EEG noise level are crucial for response detection, and both vary as a function of rate. In infants, the typical pattern of the ASSR response in which there is an enhancement at 40 Hz, is not seen and thus cannot be reliably recorded (Levi, Folsom, & Dobie, 1993, 1995; Maurizi, Almadori, Paludetti, Ottaviani, Fosignoli, & Luciano, 1990; Stapells, Galambos, Costello, & Makeig, 1988). The infant response may differ from the adult response for two reasons. First, in infants, the cortex is still immature and is thus probably unable to support a sustained rhythmic response at rapid rates. Second, it is difficult to record from an infant that is awake, thus the infant must be asleep for recording to be made. Because sleep attenuates the 40 Hz response even in adults; it makes sense that this frequency cannot be reliably recorded in infants. In contrast to the 40-Hz ASSR, the 80-Hz ASSR does not change significantly with age (Pethe, Muhler, Siewert, & von Specht, 2004) and as a result can be reliably recorded in both adults and infants. Thus, for this current study, the 80 Hz response was recorded.  1.5.2 ASSR Generators ASSRs have multiple generators including a brainstem response which follows a broad range of frequencies (e.g.,Herdman, Lins, Van Roon, Stapells, Scherg, & Picton, 2002), a thalamocortical response that responds to frequencies up to 70 Hz (e.g.,Johnson, Weinberg, Ribary, Cheyne, & Ancill, 1988; Kuwada, Anderson, Batra, Fitzpatrick, Teissier, & D’Angelo, 2002) and a cortical response that detects changes at slower frequencies (e.g.,Herdman et al., 2002; Johnson et al., 2002; Kuwada et al., 2002). The contribution of each generator is dependent on the modulation frequency utilized (Herdman et al., 2002). Evidence from numerous human and animal studies indicated the 80-Hz ASSR originates primarily from  42  brainstem structures (Herdman et al., 2002; John & Picton, 2000a; Kuwada et aL, 2002; Mauer & Doring. 1999). A brain-source analysis of the 88-Hz ASSR conducted by Herdman and colleagues (2002) suggested a midline brainstem source with minor cortical contributions. It may also be the case that 80-Hz ASSRs are actually ABR waves V to rapidly presented stimuli; however this still needs to be confirmed (Lins, Picton, Picton, Champagne, & Durieux-Smith, 1995; Stapells, Herdman, Small, Dimitrijevic, & Hatton, 2005).  1.5.3 Analysis of the Responses  As mentioned previously, one of the main advantages of ASSRs over ABRs is that ASSRs are not reliant on subjective peak selection by a clinician. ASSRs can be measured both in the time domain and the frequency domain. Through their analysis in the frequency domain, ASSRs can overcome the subjective peak selection that is characteristic of analysis done solely in the time domain (i.e., what is typically done for the ABR). The Fast Fourier Transform (FFT) (Rickards & Clark, 1984) is one common analysis method that can be utilized in the frequency domain. The FFT allows one to look at the amplitude spectra for many discrete frequencies. As a result, the amplitude of the response(s) can be observed at each modulation frequency and compared to adjacent frequency bins which can provide an estimate of background noise level (Picton et al., 2003). ASSR recordings made by the mu1tiMASTER system are measured in the time domain and then analyzed online in the frequency domain with the use of a FFT. This allows for easy determination of response presence/absence (John & Picton, 2000b).  43  1.5.4 Signal Averaging, Artifact Rejection and Weighted Averaging As mentioned earlier, ASSRs are reflective of a combination of EEG activity related to the presentation of the auditory signal of interest and background noise activity which is unrelated to the signal. Objective ASSR detection is dependent on the ability to distinguish the response to the auditory signal from the “background noise”; thus, high levels of background EEG noise are major contributors to sources of unreliability in the recording of averaged evoked potentials (Picton, Linden, Hamel, & Maru, 1983). To determine ASSR response absence, when no significant result (p.  >  05) is present, a mean EEG noise criterion level of 11 nV is  recommended (Cone-Wesson & Dimitrijevic, 2009; Herdman & Stapells, 2003; Picton et al., 2003; Van Maanen & Stapells, 2005). The multiMaster system records the EEG noise and the circle radius (CR). The CR is representative of the 95% confidence interval for response detection, rather than the background noise level per se. That is, if response amplitude(s) are larger than the CR value(s), one can be 95% certain that a response at the modulation rate of interest differs significantly than the surrounding background noise. Using multiMaster, a criterion CR value of < 20 nV to determine a no response (p.  >  05), is equivalent to a mean EEG  noise level of 11 nV. The effects of noise levels on the detection of ASSRs can be decreased using numerous techniques. In order to increase the SNR, background noise must be decreased and/or ASSR amplitudes must be increased. ASSR amplitudes may be increased, by increasing stimulus intensity and/or modulation depth. One way to decrease noise is to increase the number of averages in a recording by increasing the recording time (John, Lins, B oucher, & Picton, 1998; Luts & Woulters, 2004; Picton et al., 2003). If the background noise is assumed to be random, than averaging the response over time will reduce the amplitude of the noise by the square root 44  of the number of sweeps in the average (Picton et al., 1983). In the multiMaster system, the recording sweeps are averaged together and each sweep is formed by a group of smaller segments, called “epochs”, which typically last around one to two seconds. When noise varies from one trial to the next, the ability of averaging to reduce noise levels becomes less effective. The multiMaster system addresses this limitation of averaging through the use of weighted averaging and artifact rejection (John, Dimitrijevic & Picton, 2001). Weighted averaging is a technique that places greater emphasis on epochs with lower noise levels than on epochs containing more noise (e.g.,Lutkenhoner, Hoke, & Pantev, 1985; John, Dimitrij evic, & Picton, 2001 a; John, Dimitrijevic, & Picton, 2003). In contrast to weighted averaging, artifact rejection is an older technique that entails the rejection of any epoch(s) in place of an entire sweep that contains noise levels above a criterion noise level. Epochs that are rejected within a recording sweep are subsequently filled with the next data recorded. In some instances, however, this could result in too few accepted trials.  1.5.5 Objective Response Detection Two common statistical methods that have been developed to objectively determine the absence or presence of ASSRs amongst background EEG noise include phase coherence and the F-test. Objective response presence or absence using the multiMaster system is determined through the use of the F-test. The multiMaster F-test works by measuring whether a response (plus noise) at the modulation frequency is significantly larger than the noise in its’ adjacent 120 frequency bins, where 60 bins are located above and 60 bins are located below the ASSR frequency (John & Picton, 2000b). The multiMaster system determines that no response is present if the amplitude of the signal at its modulation frequency is not statistically different  45  from the amplitude in adjacent frequencies in the spectrum. MultiMaster determines that a response is present if the amplitude at the modulation rate (signal) exceeds that at other frequencies (noise) within 120 frequency bins with a probability of p  <  .05. Normally a  minimum of at least two consecutive sweeps at significance are required in order to determine the presence of a response. Additionally, Luts, Van Dun, Alaerts, and Wouters (2008) have proposed a minimum presentation of eight sweeps in order to decrease the error rate due to variable recording lengths. To determine the absence of a response (p ? .05), recordings continue until a predetermined noise criterion is reached. Stricter noise criterions will result in more accurate thresholds; however more testing time is needed (John, Purcell, Dimtrejevic, & Picton, 2002; Picton et al., 1983). The noise criterion for 80 Hz threshold studies is typically set at  <  20 nV  (CR; Dimitrijevic, van Roon, Purcell, Adamonis, Ostroff, Nedzelski, & Picton, 2002; Herdman & Stapells, 2003; Picton, Dimitrijevic, Perez-Abalo, & van Roon, 2005; Small & Stapells, 2005; Van Maanen & Stapells, 2005).  1.5.6 Electrode Montage A study conducted by Herdman, Lins, Van Roon, Stapells, Scherg, and Picton (2002) examined different ASSR electrode montages for a 1000 Hz carrier frequency at 88 Hz AM in adult subjects. Herdman et al. (2002) found that the optimal polarity inversions occurred between the mid-frontal or vertex position (non-inverting) and the inion or the neck position (inverting), indicating that ASSRs in the 80  —  100 Hz AM range may be more efficiently  recorded at these positions. To identify EEG derivations that result in high SNRs of the adult ASSR, Van der Reijden, Mens, and Snik (2004) utilized a 90 to 94 Hz AM stimuli and applied a  46  set of 10 EEG electrodes to positions that had been used in many previous studies on 80 to 100 Hz AM stimuli. They found that the following subset of three EEG derivations: Cz-inion, Cz left mastoid and Cz- right mastoid (a two-channel recording) produced significantly larger SNRs than a conventional single-channel recording with a 500 or 2000 Hz carrier frequency. A similar EEG derivation study conducted by Van der Reijden and colleagues (2005) in infants (0 5 -  months) revealed that the preferred derivations (e.g.,largest SNRs) are both mastoids ipsilateral to the stimulated ear with Cz as common reference. Unlike adults, high SNRs were not found at the inion-Cz derivation. For the current study the non-inverting electrode was placed in a high and medial position on the forehead, the ground electrode was placed in a high and lateral forehead position and the inverting electrodes were placed on the low mastoids of the left and right ear. The following electrode array was chosen to increase the SNRs of the ASSR recordings in the young infants and adults tested. As mentioned previously asymmetrical differences observed in the ipsilateral/contralateral recordings of infants can be utilized to predict which cochlea has responded to the stimulus. By using a two-channel recording, responses to stimuli ipsilateral and contralateral to the test ear can be obtained to investigate the effects of masking on the responses in these two channels. Such investigation may provide another indirect measure of the inter aural attenuation values of young infants. Thus, although most ASSR recordings are conducted using a single channel (Small & Stapells, 2008a; van der Reijden, Mens & Snik, 2005; Van Maanen & Stapells, 2009), the current study made use of a two-channel recording.  47  1.5.7 Electrophysiological Thresholds Maturational differences in the properties of the human skull, outer ear, middle ear, and neural development between infants and adults likely effect air and bone-conduction thresholds. This section will focus on the differences in electrophysiological air- and bone-conduction thresholds commonly seen in infants and adults.  Electrophysiological Bone-Conduction thresholds Bone-conduction ASSR thresholds have been recorded in normal-hearing adults and infants. Adult data reported by Small and Stapells (2008a), as shown in Figure 1.5, indicated that bone-conduction ASSR thresholds at 500 Hz were significantly poorer compared to thresholds at all other frequencies. No significant differences were found for 1000 to 4000 Hz (Small & Stapells, 2008a).  48  500 Hz  -J I  -A-  1000 Hz  -a,-  2000 Hz  —)(-  4000 Hz  40 .  30  .3  .C Cl)  20  a)  I. 0 0-11 months  12-24 months  adult  Age Group Figure 1.5. Mean bone-conduction ASSR thresholds (1 SD) at each carrier frequency for 35 young infants, 13 older infants, and 18 adults with normal hearing (Taken with permission from Small & Stapells, 2008a). In addition, Small and Stapells (2008a) reported findings regarding ASSR thresholds for a large group of infants over a large age range (Small et al., 2007; Small & Stapells, 2005, 2006). In all, bone-conduction ASSRs were obtained for a group of 13 “older” infants, in the age range of 12-24 months (mean age: 18.3 months), and 35 “younger” infants, in the age range of 0-11 months (mean age: 16.0 weeks). In contrast to adults, young infants had lower (better) boneconduction ASSR thresholds in the low frequencies compared to the high frequencies (Figure 1.4). Older infants on the other hand, had better thresholds for 1000 compared to 2000 Hz, with there being no differences in thresholds at 500, 1000, and 4000 Hz (Figure 1.5). A comparison of adult, older infant, and younger infant thresholds, revealed that low-frequency (e.g.,500 and 49  1000 Hz) bone-conduction ASSR thresholds increased with age (up to 24 months), whereas high-frequency (e.g.,4000 Hz) bone-conduction ASSR thresholds were unaffected by age, except for a slight improvement at 2000 Hz. Additionally, previous ABR findings revealed a similar trend, whereby maturation involves a worsening in low-frequency thresholds (e.g.,500 Hz) and an improvement in high-frequency thresholds (e.g.,4000 Hz) (Cone-Wesson & Ramirez, 1997; Foxe & Stapells, 1993; Stapells & Ruben, 1989). The current study focused on bone-conduction ASSR thresholds at 1000 and 4000 Hz, because the greatest and least maturational boneconduction ASSR differences are observed at these two frequencies, respectively. In the Small & Stapells (2008a) study, it was found that relative to adults, infants had larger response amplitudes at 1000 Hz, smaller response amplitudes at 2000 Hz, and no difference at 500 Hz. For 4000 Hz, only the younger infant group was found to have significantly smaller response amplitudes compared to adults. Among all three age groups, in the Small and Stapells (2008a) study, there were no significant differences in phase delays at 1000 and 4000 Hz. In general, as frequency increased from 1000 to 4000 Hz, the ASSR phase delays became shorter for all age groups. This trend supports the traveling wave theory of cochlear mechanics. Phase measurements were not investigated in the present study and will not be discussed further.  Electrophysiological Air-Conduction Thresholds Although, many studies have investigated air-conduction ASSR thresholds in infants, there is great variability in the literature amongst the 80-Hz ASSR thresholds in infants. The amount of variability that exists in the literature is attributed to variability amongst the type of  50  stimuli used, equipment, subject age, test environment, monaural versus binaural testing, and response/noise criterion levels (Hatton, 2008). While much variation has been observed for air-conduction ASSR thresholds in infants, general air-conduction ASSR threshold trends in infants have been observed. Overall, with maturation, air-conduction ASSR thresholds improve and amplitudes become larger at all frequencies, with larger changes seen for the higher frequencies (Rance & Tomlin, 2006). Rance and Tomlin (2006) and Sininger et al (1997) concluded that infant-adult differences in the neonatal period are the result of neural (auditory brainstem) development. Van Maanen and Stapells (2009) suggested that responses present at 36, 30, 24, and 15 dB ilL at 500, 1000, 2000, and 4000 Hz, respectively (criterion of 50% of responses present), were consistent with normal hearing. Thus, thresholds were highest for 500 Hz, decreasing as carrier frequency increased. Additionally, most of the infants (90%) showed present ASSRs at 49, 45, 36, and 32 dB ilL at 500, 1000, 2000, and 4000 Hz, respectively, with no differences in the results of younger versus older infants (Van Maanen & Stapells, 2009). The above results indicated that infant thresholds were poorer than those of adults and older children, with the exception of 4000 Hz for which thresholds were the same for infants, older children, and adults. As well, very young infants and premature infants show elevated ASSR thresholds, which may be due to neural immaturity, vemix in the ear canal, or changes to the acoustics of the ear canal or middle ear. In addition, earlier ABR studies have shown that thresholds at all frequencies improve with age, with the high-frequency thresholds tending to improve earlier in life than the low-frequency thresholds (Balfour, Pillon & Gaskin, 1998; Klein, 1984; Sininger et al, 1997; Werner, Folsom & Manci, 1994). These trends are opposite from those observed in the maturation of bone-conduction responses.  51  1.6 Rationale for Thesis The purpose of the current study is to compare effective masking levels for boneconducted stimuli for normal-hearing young infants, under six-months of age, to adults, using ASSRs. Although a handful of electrophysiological masking studies in young infants have been done for air-conduction, only one study has been done for bone-conduction, and all of these studies have focused strictly on wave latencies and amplitudes. To date, masking threshold studies in infants have only been carried out through the use of visual-reinforcement behavioural procedures for older infants. The EMLs obtained in this study for bone-conduction ASSRs, through the presentation of bilateral simultaneous air-conducted masking noise, will provide further information on: (i) the effective intensities of bone-conducted stimuli in young infants at different frequencies (ii) knowledge that will help determine the appropriate amount of masking to use clinically when obtaining ear and frequency-specific bone-conduction ASSR thresholds and (iii) information about infant-adult differences in masking effects that will contribute to our understanding of the maturation of the auditory system.  52  CHAPTER 2: Effective Masking Levels for Bone-Conduction Auditory Steady State Response Thresholds in Infants  53  2.1 Introduction With the rise of newborn hearing screening intervention programs, such as the British Columbia Early Hearing Program, children with permanent sensorineural, conductive, and mixed hearing losses, are now being identified by three months of age and treated by six months of age (Joint Committee on Infant Hearing, 2007). To generate hearing-aid gain targets that will better suit the audibility needs of infants with permanent hearing losses, it is important to obtain earand frequency-specific bone-conduction information to establish the type of hearing loss present in each ear. At present, clinicians obtain unmasked bone-conduction thresholds for each ear, but cannot be sure whether only one or both cochlea(e) are contributing to the response. To obtain ear-specific bone-conduction information early on in the diagnostic and intervention process, we must first establish effective masking levels (EMLs) (i.e., the ability of a masking noise to mask a signal of known frequency and intensity) for bone-conducted stimuli at different frequencies when thresholds are obtained using objective physiological measures which are suitable for young infants. Physiological measures are preferable as it is extremely difficult to conduct masking procedures in young children using behavioural measures, such as visual reinforcement (VRA) audiometry and play audiometry, due to the complexity of the task (Moore, 1997). Many young children are confused by the presentation of masking noise (Moore, 1997); thus masking procedures are not generally incorporated into a pediatric audiometric test battery. Because it is difficult to carry out standard masking procedures in young children, detailed ear specific bone-conduction information often cannot be obtained until the child is approximately 8 years of age, when they are old enough to participate in standard behavioural audiometric procedures (Hartley, Wright, Hogan, & Moore, 2000). As a result, the audiologist must use indirect methods to predict which cochlea is responding to the bone-conducted test signal. 54  For adults, methods that limit the response to bone-conducted stimuli from the ear that is not being tested are needed for all stimulus intensities because the inter-aural attenuation for a bone-conducted stimulus is at least 0-10 dB (Hood, 1960; Nolan & Lyon, 1981; Sanders & Rintelmann, 1964; Studebaker, 1967). For adults and older children, ear-specific masked boneconduction thresholds are obtained using standard masking procedures. One example of a commonly used masking procedure is the “plateau” method (Hood, 1960). The “plateau” refers to a range of masker levels that effectively eliminate responses from the non-test so that true thresholds of the test ear can be established. The masking plateau method takes different principles of masking into account. The lower end of the plateau, the “minimum masking level”, is defined as the lowest level of masking that will mask out contributions from the non-test ear. The upper end of the plateau, the “maximum masking level”, is defined as the greatest level of masking noise that will prevent contributions from the non-test ear but will not be loud enough to crossover and shift the threshold of the test ear. When the noise is loud enough to be heard in the test ear, the EML has been surpassed resulting in a threshold shift in the test ear and this is referred to as “overmasking”. For adults, formulae have been developed to predict undermasking, minimum masking level, maximum masking level and overmasking (Hood, 1960). Moreover, maskers generated by clinical audiometers are calibrated in dB effective masking (dB EM), which is based on levels of a masker needed to mask a signal to 50% probability of detection for a large group of normal hearing adults (ANSI S3.6, 1996). Similar formulae and dB EM levels have not been developed for establishing masked bone-conduction thresholds for infants. Bone-conduction stimulation in normal-hearing infants is not necessarily binaural as it is for adults. A bone-conducted signal may or may not cross over to stimulate the opposite cochlea  55  depending on the presentation level. Previous studies have estimated at least 10-30 dB of inter arual attenuation for bone-conducted stimuli in infants using indirect measures (Small & Stapells, 2008b; Stuart, Yang, & Botea, 1996; Yang, Rupert and Moushegian, 1987). We know that, compared to adults, infants show greater ipsilateral/contralateral asymmetries for both airand bone-conduction ABRs and ASSRs (Foxe & Stapells, 1993; Small and Stapells, 2008b; Stapells & Mosseri, 1991; Stap.ells & Ruben, 1989; Stuart et al., 1996). We take advantage of this phenomenon when using ABR or ASSR to predict which cochlea has responded to the stimulus. The greater asymmetries observed in infants are due to both neuro-maturational changes in the auditory brainstem (Edwards, Durieux-Smith, & Picton, 1985) and to structural differences in the infant skull (Foxe & Stapells, 1993). It is likely that the nuclei and pathways responsible for generating ABRs/ASSRs have a slightly different spatial organization for infants compared to adults (Edwards et al., 1985). Thus, it is expected that as the neuro-generators reach adult anatomical geometry, infant ipsilateral/contralateral asymmetries should become less evident. The infant skull is smaller in size relative to that of an adult (Eby & Nadol, 1986); it contains fontanels and the bone itself is also more pliable/flexible (Anson & Donaldson, 1981). ABR studies have suggested that for infant skulls, the fontanels likely results in inter-aural attenuation of bone-conducted signals (Foxe & Stapells, 1993; Stapells & Ruben, 1989; Stuart et al., 1996; Stapells & Mosseri, 1991), and the smaller mass of the temporal bone results in more intense signals activating the cochlea (Stuart, Yang, & Stenstrom, 1990; Foxe & Stapells, 1993). Small and Stapells (2008a) have also found that low-frequency bone-conduction ASSR thresholds tend to increase with age, whereas high-frequency bone-conduction ASSR thresholds are unaffected by age except for a slight improvement at 2000 Hz. Even though the mechanisms are not well  56  understood, the differences between infant and adult skulls are believed to contribute to the greater effectiveness of bone-conduction stimulation for infants. Due to an estimated inter-aural attenuation of at least 10-30 dB ilL, infants do not require methods to limit the response of the non-test ear from bone-conducted stimuli at all intensities. However, they still may require methods to limit the response of the non-test ear for intensities that exceed this inter-aural attenuation (Small & Stapells, 2008b; Stuart et al, 1996). Given that there is wide individual variability for the inter-aural attenuation of both adults and infants, conservative estimates of the lower limit of inter-aural attenuation are recommended to ensure that crossover does not occur (Studebaker, 1967). Despite the importance of obtaining ear-specific bone-conducted thresholds in young infants, adapting the plateau procedure to ASSR or ABR testing in infants is not currently a possibility because the required levels of masking noise for young infants are unknown. The establishment of an appropriate masking procedure for infants is difficult for the following reasons: (i) an infant’s sensitivity to bone-conducted stimuli and how this changes with age is not completely understood, (ii) the amount, frequency dependence, and maturation of the inter-aural attenuation of a bone-conducted stimulus for infants of different ages has not been fully described, (iii) infants may not sleep long enough to allow for an efficient assessment of masked bone-conduction thresholds, and (iv) the EMLs for bone-conducted stimuli in young infants have not been investigated. Only a handful of masking studies have been conducted in young infants, and all of these studies have only been carried out for air-conducted stimuli via VRA procedures. In general, masked thresholds, for both air-conducted tone and noise stimuli, tend to be elevated by approximately 5-15 dB in six-month-old infants, gradually decreasing to adult levels by about  57  ten years of age (Allen & Wightman, 1994; Nozza & Wilson, 1984; Schneider & Trehub, 1992; Schneider, Trehub, Morrongeillo, & Thorpe, 1989). In addition, these infant-adult differences in masked thresholds seem to vary as a function of frequency, with the difference being greatest in the lower frequencies (about 15 to 25 dB at 500 Hz), and least in the higher frequencies, at least over the range of 500 to 4000 Hz (Nozza & Henson, 1990). Generally speaking, masked thresholds are determined by filter bandwidth and processing efficiency (Patterson, Nimmo-Smith, Weber, & Milroy, 1982). The filter bandwidth limits the amount of noise passing through the auditory filter and has been shown to be similar in infants and adults (Olsho, 1985; Schneider, Morrongiello, & Trehub, 1990). Because filter bandwidth is similar in infants and adults, it cannot account for the developmental changes observed in masked thresholds. As a result, maturational differences in masked thresholds are often attributed to processing efficiency (Werner & Marean, 1996). Processing efficiency refers to the signal-to-noise-ratio (SNR) at the filter output that is required to detect the presence of a signal and it is affected by both sensory and nonsensory factors (Patterson et al., 1982). Berg and Boswell (1999) have provided evidence for a strong sensory component to processing efficiency. They found that seven-month-old infants have similar SNRs across different masker levels to adults at 4000 Hz but larger SNRs than adults for lower compared to higher masker levels at 500 Hz. The electrophysiological studies that have been done in young infants have all utilized the ABR and have focused on the effects that maskers have on wave latencies and amplitudes. Of these studies, only one study conducted by Yang and colleagues (1987) has focused on the effects of masking on bone-conducted stimuli. Yang et al. (1987) looked at the effects of ipsi and contralateral masking on the ABR latencies of bone-conducted click stimuli. Overall, Yang  58  et al (1987) found that ipsilateral masking resulted in longer wave V latencies than contralateral masking and these prolongations in latency were greater in neonates than one-year-olds and adults. An increase in masker level from 40 to 50 dB nHL resulted in a significant increase of wave V latencies in neonates, a moderate increase in latencies for one-year olds, and only a small latency increase for adults. When the SNRs were fixed at -15 dB, a decrease of both signal and masker levels by 10 dB produced a significant increase of wave V latencies in adults, a moderate increase for one-year-olds, and almost no latency change for neonates. After examining the latency-intensity results from different combinations of signal and noise levels, Yang and colleagues (1987) estimated the inter-aural attenuation of a bone-conducted click to be 0-10, 1525, and 25-35 dB in adults, one-year old infants, and neonates respectively. Based on these results, Yang and colleagues (1987) concluded that masking of the non-test ear of neonates is probably not needed for low stimulus levels (e.g.,below 25 dB nHL) but is recommended for high stimulus levels (e.g.,35 dB nHL). The purpose of this study was to determine EMLs for ASSRs elicited by bone-conducted stimuli for a group of normal-hearing infants (under six-months of age) and adults. The EMLs obtained in this study may contribute to: (i) establishing EMLs that could be used for the accurate assessment of ear- and frequency-specific bone-conduction thresholds in young infants, (ii) our understanding of effective intensities of bone-conducted stimuli in infants, and (iii) our knowledge of the maturation of the auditory system.  59  2.2 Methods and Materials 2.2.1 Experiment 1A- Infant and Adult Effective Masking Levels (ASSRs) 2.2.1.1 Participants Fifteen infants (mean age: 17 weeks; range: 4-27 weeks; 7 female) and 15 adults (mean age: 25.5 years; range: 21-3 1 years; 9 female) with normal hearing participated in this study. Infants who passed a transient evoked otoacoustic emissions (TEOAE) screening test in both ears on the day of testing were considered to be at low risk for hearing loss and were included in the study. Thirteen infants passed the TEOAE screening in both ears. One infant passed only in the left ear because screening could not be completed in the right ear due to excessive movement (Note: the left ear was used as the test ear). One infant did not pass the TEOAE screening in either ear, but was reported to have passed an automated auditory brainstem response screening at birth. Both of these infants were included in the study’. For adults to be considered “normal”, pure-tone bone- and air-conduction thresholds needed to be  25 dB ilL for 500, 1000, 2000 and  4000 Hz, with no significant ( 10 dB ilL) air-bone gaps.  1  The masking levels required to mask out the responses to the bone-conducted stimuli for these two infants were similar to those of infants who passed the hearing screening. 60  2.2.1.2 Stimuli and Maskers Stimuli  The acoustic spectra of the bone-conducted stimuli used in this study are depicted in Figure 2.1. The bone-conducted ASSR stimuli were presented as single sinusoidal tones with carrier frequencies of 1000 and 4000 Hz. These single stimuli were 100% amplitude and 25% frequency modulated at 85.449 and 101.3 18 Hz for 1000 and 4000 Hz, respectively. The AM/FM stimuli used in the present study have centre frequencies of 1000.92 and 4199.52 Hz, and bandwidths of 518.71 and 1110.65 Hz, respectively (Figure 1.2). The bone-conducted stimuli were presented using the Radioear B-71 bone oscillator which was placed on the temporal bone slightly posterior to the upper part of the pinna for all participants. The bone oscillator was either coupled to the head by an elastic headband for adults and some infants, or held in place by hand for infants, with approximately 400-450 g of force (Small, Hatton, & Stapells, 2007).  61  AMIFM  100  1000  10000  Frequency (Hz) Figure 2.1. Acoustic spectra of bone-conducted stimuli (AM/FM) used in this study for carrier frequencies of 1000 and 4000 Hz. Y axis ticks represent 10 dB intervals.  A previous study conducted by Small and Stapells (2008a) showed that infant thresholds to bone-conducted stimuli in dB ilL are better compared to adults at 1000 Hz, but are comparable at 4000 Hz. To account for infant-adult frequency-dependent differences in sensitivity, the stimulus level in this study were expressed in dB relative to the mean bone conduction ASSR thresholds at 1000 and 4000 Hz for young infants and adults as reported in Small and Stapells (2008a). The units “dB mSL” will be used throughout the study to describe the stimulus level. The equivalent adult and infant dB HL values for dB mSL are shown in Table 2.1.  62  Table 2.1 Conversion of bone-conduction stimulus levels in dB mSL to dB ilL for adult and infant participants. Stimulus Level  1000 Hz  4000 Hz  indBmSL  -10  0  10  20  10  20  AdultindBllL  15  25  35  45  25  35  ---  ---  15  25  25  35  Infant in dB HL  Bone-conducted stimuli at 1000 and 4000 Hz were presented at two levels, 10 and 20 dB mSL for infants and adults. For adults, an increase in masker intensity should produce a linear increase in masked thresholds independent of frequency, for all but extremely low levels of a masker (Hawkins & Stevens, 1950). In order to determine whether the relationship for the EMLs required for low-level bone-conducted stimuli was linear, and to directly compare the masking required to eliminate infant and adult ASSRs to bone-conducted stimuli in dB HL, adults were also tested using stimuli at -10 and 0 dB dB mSL for 1000 Hz. The bone-conducted stimuli were generated by a two-channel version of the MultiMASTER Research System (John & Picton, 2000a), using a digital-to-analog rate of 31, 250 Hz. The stimuli were routed through a Stanford Research Systems Model SR 650 amplifier that increased the gain of the stimulus by 10 dB. The stimuli were then attenuated through the Tucker-Davis Technologies PA5 and HB7 modules. Maskers  Air-conduction masking noise was presented to both ears simultaneously using ER-3A insert earphones. The air-conduction masking noise consisted of 1 and 4 kHz narrow-band noise maskers presented via the Madsen OB 802 clinical audiometer. The bandwidths of the 1 and 4 kHz narrow-band noise maskers are depicted in Figure 2.2.  63  100  80 -J 0. Co .  40  20 100  1000  10000  Frequency (Hz) Figure 2.2. Spectral content for narrow-band noise maskers presented at 90 dB effective masking level via an OB 802 clinical audiometer.  2.2.1.3 Calibration Stimuli  The bone-conducted ASSR and pure-tone screening stimuli were calibrated in Reference Equivalent Force Levels (RETFL) in dB re: 1iN corresponding to 0 dB HL for the mastoid (ANSI S3.6-1996, 1996) via the Larson and Davis system 824 sound level meter and LD AMC 493 artificial mastoid. The artificial mastoid was mounted on an AEC 1000 NBS-9A 6cc coupler. The weight on top of the artificial mastoid was adjusted to provide a force of 4-5 Newton on the oscillator. The air-conducted pure-tone screening stimuli were calibrated in Reference Equivalent Threshold Sound Pressure Levels (RETSPL) in dB SPL corresponding to 0  64  dB HL via the Larson and Davis system 824. The air- and bone-conduction pure-tone screening stimuli were calibrated in dB HL. The bone-conducted ASSR stimuli were calibrated in dB HL then converted to dB mSL. Maskers The air-conducted narrow-band maskers were calibrated using the 1/3 dB octave band correction to RETSPL in dB SPL via the Larson and Davis system 824 model that was coupled to a G.R.A.S. Sound & Vibration RAO1 13 2-cc coupler. To calibrate the narrow-band maskers for the ear canal of each adult and infant participant, real-ear-to-coupler difference measures (RECDs) were measured in a double-walled sound-attenuated booth, using the Fonix 6500-CX real-ear analyzer. Real-ear measures were made for each participant’s right and left ear 2 with a probe-tube inserted into the ear canal 11, 28, and 32 mm from the tragal notch for infant, adult female, and adult male participants, respectively (Bagatto, 2001; Bagatto, Seewald, Scollie, & Tharpe, 2006).  These real-ear measurements were then compared to measurements taken in a  Fonix HA-2 coupler. The RECD measurements were used to convert the masker dB SPL measured in the coupler to dB SPL in the ear canal for each individual participant.  2.2.1.4 Recording ASSRs were recorded using the two-channel Multi-MASTER Research system (John & Picton, 2000a). Four gold-plated electrodes were used to record the electrophysiological responses: the non-inverting and ground electrode were placed on the high forehead with the non-inverting electrode placed in a high and medial position on the forehead and the ground electrode placed in a high and lateral forehead position. The inverting electrodes were placed on 2  one infant and two adults, RECDs were obtained for the right ear only. Also for one adult, RECDs were not obtained.  65  the low mastoids of the left and right ear. The skin underneath the electrodes was lightly abraded in order to obtain inter-electrode impedances of less than 3 kOhms at 10 Hz. The EEG was filtered by a 30-Hz high-pass filter (-12 dB/octave) and a 300-Hz low pass filter (-24 dB/octave), and amplified 100, 000 times [l0,000X in GRASS LP5 11 AC Amplifier; lox in National Instruments DAQmx (USB-6259)]. The EEG was further filtered using a 400Hz low-pass anti-aliasing filter via the Stanford Research Systems Model SR650 (115 dB/ octave). The EEG was then processed using a 1250-Hz analog-to-digital conversion rate (Small & Stapells, 2004). Each EEG recording sweep was made up of 16 epochs of 1024 data points (0.8 19 seconds per epoch) and lasted a total of 13.107 seconds. The artifact rejection was set to eliminate epochs of electrophysiologic activity that exceeded 80 iV in amplitude to reduce contributions to the EEG from muscle artifact. ASSRs were averaged in the time domain then analyzed online in the frequency domain using a Fast Fourier Transform (FFT). Weighted averaging was used over a frequency range of 70-110 Hz (John, Dimitrijevic & Picton, 2001). The FFT resolution, over a range of 0 to 625 Hz, was 0.076 Hz. Amplitudes were measured baseline-to-peak and expressed in nano Volts (nV). An F-ratio was calculated by Multi-MASTER to estimate the probability that the amplitude of the ASSR at the modulation frequency for each carrier frequency was significantly different from the average amplitude of the background noise in adjacent frequencies within 60 bins of the modulation frequency (“noise”) (John & Picton, 2000a). ASSRs were recorded for a minimum of 10 consecutive sweeps, either until a response was detected (p< .05) or until the EEG noise criterion was reached (circle radius of < 20 nV) and no response detected (p  .05). A response was considered to be present if the F ratio, compared  to the critical values for F (2, 240), was significant at a level of p  <  0.05 for at least two  66  consecutive sweeps. Alternatively, for adult participants a stopping criterion of amplitude <10 nV and p value .30 was used to indicate that no response was detected.  2.2.1.5 Procedure This study involved one or two recording sessions, which lasted for approximately two hours. Adult participants and the parent(s)/legal guardian(s) of infant participants signed a consent form before initiation of testing and were paid an honorarium at the end of the session. All of the procedures in this study were approved by the UBC Clinical Research Ethics Board. ASSR measurements took place in a double-walled sound-attenuated booth. Infant participants were asleep, and adult participants were instructed to relax or sleep in a reclining chair for the ASSR recordings. The presentation order for the stimuli and initial masking level was randomized for all participants. Unmasked and masked ASSR responses were obtained for 1000- and 4000-Hz bone-conducted stimuli. All experimental conditions were attempted for each individual participant; however, not all participants completed all conditions . 3 For the 1000- and 4000-Hz BC stimuli, masking was presented binaurally and the initial masking level was randomized. For 1000-Hz, the masking levels were: 18, 28, 38, 48, 58, and 68 dB SPL and for 4000 Hz the masking levels were: 20, 30, 40, 50, 60, and 70 dB SPL. If the ASSR was present with the initial masking level, the masker was increased in 10 dB increments until the response was no longer detected, then the masker was decreased in 5 dB increments. If the ASSR was absent, the masker was decreased in 10 dB increments until a significant response  Seven infants did not sleep long enough to complete all conditions. Six adults did not complete all conditions due either to running out of test time or not having responses present in the unmasked condition. The experimental conditions that were completed for each individual infant and adult participant are listed in Appendix B and Appendix C, respectively. 67  was found, than the masker was increased in 5 dB increments. The lowest level of the masker that resulted in an absent bone-conduction ASSR response was taken as the EML.  2.3.1 Data and Statistical Analyses Mean ASSR EMLs calibrated in the coupler, RECDs, EMLs calibrated in the ear canal and ASSR amplitude values were averaged across participants for each age group, including ASSR amplitudes for responses that were not significant. Results were only reported if at least five participants contributed to the mean. ASSR EMLs calibrated in the coupler were compared between age groups and frequencies for bone-conducted stimuli presented at 25 dB HL. Also, ASSR EMLs calibrated in the coupler and in the ear canal were compared between age groups, bone-conducted stimulus levels (dB mSL) and frequencies. Real-ear-to-coupler-difference measures were compared between age groups, frequency, and the ear tested. Unmasked bone-conducted ASSR amplitudes were compared between age group, EEG (response vs. noise), recording channel (ipsi- vs. contralateral), bone-conducted stimulus level, and frequency. For 1000 Hz, bone-conducted stimuli presented at 10 and 20 dB mSL were compared between age-groups for the unmasked and 48 dB SPL masking level condition. For 4000 Hz, ASSR amplitudes for bone-conducted stimuli presented at 10 and 20 dB mSL were compared between age groups for the unmasked and 30 dB SPL masking level condition. These conditions were selected for comparison because the greatest number of infants and adults completed these conditions. ASSR EML comparisons between age-group, and frequency for bone-conducted stimuli presented at 25 dB HL were made for the masker calibrated in the coupler, using a two-way mixed-model analysis of variance (ANOVA). ASSR EML comparisons between age-group,  68  frequency, and bone-conducted stimuli level were made for each masker calibration method (coupler versus ear-canal), using a three-way mixed-model ANOVA. Real-ear measure comparisons across age group, ear tested, and frequency were made using a two-way mixedmodel ANOVA. Additionally, ASSR EML comparisons (dB SPL in the ear canal) between agegroups were made within each frequency using a two-way mixed-model ANOVA. Unmasked ASSR amplitude comparisons between age group, frequency, EEG noise, recording channel and bone-conducted stimulus level were made using a five-way mixed-model ANOVA. Finally, ASSR amplitude comparisons across age group, bone-conducted stimuli, masking level and channel within each frequency were made using a four-way mixed-model ANOVA. Newman Keuls post-hoc comparisons were performed for significant main effects and interactions. Results for all analyses were considered statistically significant if p  <  .05.  2.2.2 Experiment 1B- Adult Effective Masking Levels (Behavioural) Seventeen adults (10 female; mean age for adult subjects: 27.6 years; range: 20-33 years), with normal hearing participated in this study. Thirteen of these adults also participated in Experiment 1A. The stimuli and maskers used were the same as described in Experiment 1A. RECDs were obtained for a total of seventeen adults for Experiments 1A & B. RECDs were obtained in the right ear only for two adults. Adult participants were instructed to keep their hand up in the air as long as they heard the bone-conducted signal. If the participant could not hear the bone-conducted signal and could only hear the masker, they were instructed to lower their hand. The EMLs for the 1000- and 4000-Hz bone-conducted stimuli, were determined using a bracketing technique, adjusting the  69  level of the masker in 5 dB steps. The lowest masking level at which the ASSR behavioural response was absent 3 out of 5 presentations was considered the EML. Mean EMLs calibrated in the coupler and RECD values were averaged across participants. EMLs were compared across testing method (ASSR versus Behavioural), frequency (1000 versus 4000 Hz), and bone-conducted signal level (10 versus 20 dB mSL). Comparisons were made by a three-way repeated measures model ANOVA. Neuman-keuls post-hoc comparisons were performed for significant main effects and interactions. Results for  all analyses were considered statistically significant if p  <  .05.  2.4 Results 2.4.1 Experiment 1A- Infant and Adult Effective Masking Levels (ASSRs) Effective Masking Levels in the Coupler: Stimuli in dB HL  Figure 2.3 shows representative ASSR amplitudes elicited by 1000- and 4000-Hz boneconducted stimuli presented at 25 dB ilL for an unmasked condition and different levels of simultaneous masking for a typical infant and adult. As expected, the infant and adult ASSR amplitudes decreased with increased masker level. For both the infant and adult, less masking was needed to eliminate the ASSR response for a 4000-Hz stimulus compared to a 1000-Hz stimulus. The EMLs for the infant were higher compared to the adult at 1000 Hz but were lower compared to the adult at 4000 Hz. For this infant, 15 dB more and 20 dB less masking was needed at 1000 and 4000 Hz, respectively, compared to the adult to eliminate the bone conduction ASSR response for a stimulus presented at 25 dB HL.  70  CONDITION Frequency (Hz)  1000  INFANT  ADULT  Masker Intensity (dB SPL)  20 nV I-  Unmasked  .  .1  28 V  33  —_-  V  38 V  48 53  4000  I—---  —--------.  it__a.  Unmasked  Ia__  15  —  s_ns_  —I  I  V  L_..  V  —  —  30  ...!._iI•  35  -I-.--------VI  70  I  I  80  11111  90  100  I  110  liii  70  80  II  90  I  100  I  I  110  Figure 2.3 Representative ASSRs for 1000- and 4000-Hz bone-conducted stimuli presented at 20 dB HL with different levels of air-conducted narrow-band noise masker for an individual infant and adult. Masker levels were calibrated in the coupler. Shown are amplitude spectra resulting from FFT analyses (75-105 Hz) of the ASSRs. Filled triangles indicate responses that differ significantly from the background noise (p < .05). Open triangles indicate no response (p ? .05 and EEG noise 1 mV). Effective masking level is defined as the lowest intensity of the masker that resulted in an absent response. The effects of age group on the mean EMLs calibrated in the coupler for 1000- and 4000Hz bone-conducted stimuli in dB HL are shown in Figure 2.4. For adults, the EMLs for 1000 Hz were 34.8 (range: 27.8-42.8 dB SPL) and 44.6 dB SPL (range: 37.8-52.8 dB SPL) at 15 and 25 dB HL, respectively. For adults, the EMLs for 4000 Hz were 34.9 (range: 25.3-45.3 dB SPL) and 43.9 dB SPL (range: 25.3-55.3 dB SPL) at 25 and 35 dB HL, respectively. For infants, the  71  EMLs for 1000 Hz were 41.6 (range: 27.8-47.8 dB SPL) and 51.2 dB SPL (range: 42.8-57.8 dB SPL) at 15 and 25 dB HL, respectively. The EMLs for infants for 4000 Hz were 27.2 (range: 15.3-40.3 dB SPL) and 33.8 dB SPL (range: 25.3-45.3 dB SPL) at 25 and 35 dB HL, respectively.  60 ---S  0-i >0 -J  .-‘--  50 40  .‘-  Cad)  30 20 10 0 15  25  25  35  Bone Conducted Signal (dB HL)  Figure 2.4. Effective masking levels calibrated in the coupler for 1000- and 4000-Hz boneconducted signals (dB HL) in adults (N= 10-14) and infants (N= 10-13).  When adult EMLs were compared between frequencies, the 4000-Hz bone-conducted stimulus required approximately 10 dB less masking than the 1000-Hz bone-conducted stimulus. When infant EMLs were compared between frequencies, the 4000-Hz bone-conducted stimulus required approximately 24 dB less masking on average than the 1000-Hz bone-conducted stimulus. Comparisons between infant and adult masking levels calibrated in the coupler indicated that 7 dB more masking was required for infants at 1000 Hz; however, 8-10 dB less masking was required for infants at 4000 Hz. Results of an ANOVA comparing mean ASSR EMLs in the coupler for a bone conducted stimulus presented at 25 dB HL for 1000 and 4000 Hz between age groups revealed a 72  significant effect of frequency [F (1, 19) (1, 19) p  =  =  0.14, p  =  =  71.98, p  <  .00011, but no significant effect of age [F  0.711]. However, a significant age x frequency interaction [F (1, 19)  =  12.72,  0.002] was found. Post hoc comparisons revealed that infants required more masking at  1000 Hz (p  =  0.033) and less masking at 4000 Hz (p  =  0.009) compared to adults. These  comparisons also revealed that less masking was required at 4000 compared to 1000 Hz for both age groups. Effective Masking Levels in the Coupler: Stimuli in dB mSL Infants have better sensitivity to bone-conducted stimuli at 1000 Hz in comparison to adults; therefore the remainder of the comparisons between infant-adult bone-conducted stimuli will be expressed in dB mSL. This will account for infant-adult differences in the signal reaching the cochlea. Figure 2.4 shows representative ASSR amplitudes elicited by 1000- and 4000-Hz bone-conducted stimuli presented at 20 dB mSL for an unmasked condition and different levels of simultaneous masking for a typical infant and adult. When bone-conduction sensitivity was taken into consideration the EMLs for the infant were lower compared to the adult at both frequencies. For this infant, 5 and 20 dB less masking was needed at 1000- and 4000-Hz, respectively, compared to the adult to eliminate the bone-conduction ASSR response for a stimulus presented at 20 dB mSL.  73  CONDITION Frequency (Hz)  1000  INFANT  Masker Intensity (dB SPL)  ADULT 2OnV1  ‘V  Unmasked  48  53  -----  -_-__y”  L...  .Jg.L.J_  58  4000  Unmasked 20 30  ..J______  —--  ——  k...  --•-  ..Lb...L.  40 45  —  50  •—-—-• 1 . 1 -—---•x’—.• 1 I  70  •  80  90  •  100  •  110  ib  ido  Figure 2.5 Representative ASSRs for 1000- and 4000-Hz bone-conducted stimuli presented at 20 dB mSL with different levels of air-conducted narrow-band noise masker for an individual infant and adult. Masker levels were calibrated in the coupler. Shown are amplitude spectra resulting from FFT analyses (75-105 Hz) of the ASSRs. Filled triangles indicate responses that differ significantly from the background noise (p < .05). Open triangles indicate no response (p .05 and EEG noise 1 mV). Effective masking level is defined as the lowest intensity of the masker that resulted in an absent response.  The effects of age group on EMLs calibrated in the coupler and in the ear-canal for 1000and 4000-Hz bone-conducted stimuli (in dB mSL) are shown in Figure 2.5. For adults, there was a linear relationship between the 1000-Hz bone-conducted stimulus levels and the amount of 74  masking required to eliminate the bone-conduction ASSR, even at the lowest stimulus level. The relationship between the two bone-conducted stimulus levels and the EMLs calibrated in the coupler at 4000 Hz was also linear for the two stimulus levels tested. When adult EMLs were compared between frequencies, the 4000-Hz bone-conducted stimulus required approximately 20 dB less masking on average, than the 1000-Hz bone-conducted stimulus.  280 >0  60  280  iso cl) —I  60  (0)  40  .? 4-  20  0  4-  0 -10  0  10  20  10  20  Bone Conducted Signal (dB mSL)  Figure 2.6. Effective masking levels calibrated in the coupler and in the ear canal for 1000- and 4000-Hz bone-conducted signals in adults (N= 10-15) and infants (N= 10-13).  75  Similar to adults, there was a linear relationship between the bone-conducted stimulus level and the EML calibrated in the coupler at 1000 Hz for infants. However, for 4000-Hz, an increase in bone-conducted stimulus level by 10 dB resulted in only a 6 dB increase in EML. Similar to adults, when EMLs in infants are compared between frequencies less masking (14-18 dB) was required at 4000 compared to 1000 Hz. Comparisons between infant and adult masking levels calibrated in the coupler indicated that approximately 7-10 and 12 dB less masking was required for infants at 1000 and 4000 Hz, respectively. Results of an ANOVA comparing mean ASSR EMLs in the coupler at 1000 and 4000 Hz across age group and bone-conduction stimulus level revealed a significant effect of age, frequency, and bone-conduction level as shown in Table 2.2. No significant interactions were found between factors. Table 2.2. ASSR EMLs measured in the coupler and in the ear canal: Three-way mixed-model ANOVAs showing comparisons between bone-conduction stimulus level (10 & 20 dB mSL), across age group (infants and adults) and carrier frequency (1000 & 4000 Hz)  Cou p ler  EarCanal  Source Age Frequency Stimulus Level Age x Stimulus Level Stimulus Leveix Frequency Agex Frequency Age x Stimulus Level x Frequency  df 1, 17 1, 17 1, 17 1, 17 1,17 1,17 1, 17  F 26.42 50.58 126.59 1.34 2.16 0.23 1.57  p <.0001* <.0001* <.0001* 0.264 0.160 0.640 0.228  Age Frequency Stimulus Level Age x Stimulus Level Stimulus Level x Frequency Agex Frequency Age x Stimulus Level x Frequency  1,17 1, 17 1, 17 1, 17 1, 17 1,17 1, 17  3.14 23.91 103.57 1.97 1.27 1.14 0.84  0.094 .0001* <.0001* 0.179 0.275 0.300 0.373  *significant (p <0.05)  76  Real Ear Measures  As shown in Table 2.3 the mean RECDs of the maskers are about 6-7 dB greater in infants compared to adults at both frequencies and the mean RECDs at 1000 Hz are about 5-6 dB smaller compared to 4000 Hz, for both age-groups. Results of an ANOVA revealed a significant main effect of age [F (1, 27)  =  134.35, p  <  .00011 and frequency [F (1, 27)  but no significant age x frequency interaction [F (1, 27) was no significant effect of test ear [F (1, 27)  =  0.13, p  =  =  0.55, p  =  155.77, p  <  .00011  0.466]. In addition, there  0.726].  Table 2.3. Comparison of the mean real-ear-to-coupler differences (dB) between a group of 15 young infants (3-27 wk) and a group of 17 adults for 1000 and 4000 Hz. RECD values represent the average for the left and right ear.  1000 Hz RECDs  4000Hz  Mean  SD  Mean  SD  Infant  10.47  2.50  16.02  2.94  Adult  3.99  1.47  8.93  2.37  Difference  6.48  7.09  Effective Masking Levels in the Ear Canal: Stimuli in dB HL Because RECDs are age- and frequency dependent, corrections for ear canal volume changed the EMLs needed to eliminate the bone-conduction ASSR at both frequencies and for each age group. When adult EMLs were compared between frequencies, the 4000-Hz bone conducted stimulus required 6 dB less masking than the 1000-Hz bone-conducted stimulus. When infant EMLs were compared between frequencies, the 4000-Hz bone-conducted stimulus required 18 dB less masking on average than the 1000-Hz bone-conducted stimulus. Comparisons between infant and adult masking levels calibrated in the ear canal indicated that 77  12-13 dB more masking was required for infants at 1000 Hz; however, the same amount of masking (maturational difference was only 0-3 dB) was required for infants at 4000 Hz. Results of an ANOVA comparing mean ASSR EMLs in the ear canal for a boneconducted stimulus presented at 25 dB HL for 1000 and 4000 Hz between age groups revealed a significant effect of frequency [F (1, 19)  =  37.48, p  <  .0001] and age [F (1, 19)  0.037]. Also a significant age x frequency interaction [F (1, 19)  =  10.57, p  =  =  =  =  0.004] was found.  Post hoc comparisons revealed that infants required more masking at 1000 Hz (p  similar amounts of masking at 4000 Hz (p  5.05, p  =  0.0007) and  0.892) compared to adults. These comparisons also  revealed that less masking was required at 4000 compared to 1000 Hz for both age groups. Effective Masking Levels in the Ear Canal: Stimuli in dB mSL For adults with a 10 dB mSL bone-conducted signal, EMLs calibrated in the ear-canal were 58 and 42 dB SPL at 1000 and 4000 Hz, respectively. For a 20 dB mSL bone-conducted signal, EMLs calibrated in the ear canal were 70 and 55 dB SPL at 1000 and 4000 Hz, respectively. When EMLs calibrated in the ear canal were compared between frequencies, 14-16 dB less masking was required at 4000 compared to 1000 Hz (Figure 2.5). For infants with a 10 dB mSL bone-conducted signal, EMLs calibrated in the ear canal were 55 and 42 dB SPL at 1000 and 4000 Hz, respectively. With a 20 dB mSL bone-conducted signal, EMLs calibrated in the ear canal were 62 and 54 dB SPL at 1000 and 4000 Hz, respectively. When the EMLs calibrated in the ear canal were compared between frequencies, approximately 9-12 dB less masking was required at 4000 compared to 1000 Hz. When the EMLs calibrated in the ear canal were compared between age groups, infants required approximately 5-7 dB less masking than adults at 1000 Hz; however, infants required approximately the same amount of masking as adults at 4000 Hz (Figure 2.5). This contrasts  78  with the findings for EMLs calibrated in the coupler, where less masking at 4000 Hz was required for infants compared to adults. When an ANOVA was conducted for masker levels calibrated in the ear canal, main effects of frequency and bone-conduction level remained significant; however the main effect of age was no longer significant (Table 2.2). Similar to EMLs calibrated in the coupler, none of the interactions between factors for EMLs calibrated in the ear canal were significant (Table 2.2). Separate ANOVAs for each frequency were also carried out because the number of infant and adult participants completing either the 1000- or the 4000-Hz condition was greater than the number of infant and adult participants who completed both conditions. Results of an ANOVA comparing mean ASSR EMLs in the ear canal for 1000 Hz between age group and boneconduction level revealed a significant main effect of signal level [F(1, 24) and age [F(1, 24)  =  5.10, p  =  =  74.43, p  <  .000 1]  0.034], but no significant interactions between age and signal level.  In contrast to 1000 Hz, an ANOVA comparing mean ASSR EMLs in the ear canal for 4000 Hz between age group and bone-conduction level revealed no significant effect of age [F( 1, 1 8)= 0.40, p  =  0.536]. In summary, infants require significantly less masking than adults in the ear  canal at 1000 Hz and approximately the same amount of masking as adults in the ear canal at 4000 Hz when SPL of the masker in the ear canal is considered.  79  Amplitude: Stimuli in dB mSL Ipsilateral Recording Channel  ASSR mean amplitudes and EEG noise in the ipsilateral and contralateral recording channels, for each age group, carrier frequency, and bone-conducted stimulus level are shown in Figure 2.6. For adults, as the masker level at 1000 Hz was raised from 0 to 48 dB SPL, the ASSR amplitudes are reduced by 54 and 44 % for stimuli presented at 10 & 20 dB mSL, respectively. As the masker level at 4000-Hz is raised from 0 to 30 dB SPL, the ASSR amplitudes are reduced by 51 and 55 % for stimuli presented at 10 & 20 dB dB mSL, respectively. For adults, as the bone-conduction stimulus level is raised by 10 dB, the ASSR amplitudes increased by 31 and 15 % at 1000 and 4000 Hz, respectively. In adults, when ASSR amplitudes were compared between frequencies, the ASSR amplitudes for 1000-Hz were 46-56 % larger than the ASSR amplitudes for 4000 Hz.  80  A. 1000 Hz  Ipsilateral  140  •  120  —o-—  100  Contralateral  Infant Amplitudes Adult Amplitudes  10 dB mSL  80 60 —  0  *  40 20  z  a  0 140 120 100 80  I  60 40 20 0 20  0  40  60  0  20  40  60  Masking Level (dB SPL)  B. 4000 Hz  Ipsilateral  Contralateral  80  1OdBmSL  60 40 > C  20  0 V 0.  I  I ‘kkL  w  0 80  E  60 40 20  2OdBmSL  I I  0 0  20  40  60  0  20  40  60  Masking Level (dB SPL)  Figure 2.7. Mean ASSR amplitudes for 1000 Hz (A.) and 4000 Hz (B.) bone-conducted stimuli, presented at 10 and 20 dB mSL, at different masking levels, in the ipsi- and contralateral recording channel for infants (Nr= 5-14) and adults (N=5-14). Results were only reported if at least five participants contributed to the mean.  81  For infants, as the masker level at 1000-Hz was raised from 0 to 48 dB SPL, ASSR amplitudes were reduced by 79 and 72 % for stimuli presented at 10 & 20 dB mSL, respectively. This reduction in amplitude with the 1 kHz masker is 25-28 % greater than the reduction observed in adults. As the masker level at 4000-Hz was raised from 0 to 30 dB SPL, the ASSR amplitudes were reduced by 40 and 49 % for stimuli presented at 10 & 20 dB mSL, which is similar to the adult. For infants, as the bone-conduction stimulus level is raised by 10 dB, the ASSR amplitudes increased by 22 and 15 % at 1000 and 4000 Hz, respectively. The increase in amplitude with an increase in bone-conduction level is 9 % less and similar to that observed in adults at 1000 and 4000 Hz, respectively. Finally, when ASSR amplitudes are compared between infants and adults at 1000 Hz, ASSR amplitudes for infants are 36 and 44 % smaller compared to adults for 10 and 20 dB mSL bone-conducted stimulus levels, respectively. Similarly, ASSR amplitudes for 4000-Hz stimuli in infants are 54 and 52 % smaller than that of adults for stimuli presented at 10 and 20 dB mSL, respectively. Results of an ANOVA for the unmasked conditions revealed that the smaller amplitudes observed for infants compared to adults [F (1, 15)  =  12.58, p  =  0.003] reached statistical  significane. The smaller amplitudes at 4000 compared to 1000 Hz [F (1, 15)  =  20.11, p  0.00041, and at 10 compared to 20 dB mSL bone-conducted stimuli levels [F (1, 15)  =  =  31.20, p  <  .0001], also reached statistical significance. In addition, the significant frequency x stimulus level [F (1, 15)  =  9.61, p  =  0.0073] interaction revealed that the difference in amplitude with  increases in stimulus level was statistically significant for 1000 Hz (p Hz (p  =  =  0.0002) but not for 4000  0.146).  ANOVAs within each frequency were also carried out to compare across masker level because there were not enough infant and adult participants that required the same masking  82  levels for both 1000 and 4000 Hz. As expected, an increase in masker resulted in a statistically significant decrease in ASSR amplitudes for both 1000 [F (1, 16) Hz [F (1, 9)  =  15.17, p  interaction [F (1, 16) interaction [F (1, 9)  =  =  =  =  47.28, p  <  .0001] and 4000  0.004]. In addition, for 1000 Hz, a significant age x stimulus level  12.80, p  15.17, p  =  =  0.003] was found. For 4000 Hz, the masker level x age  0.004] was also significant. Post hoc comparisons for the age x  stimulus level interaction for 1000-Hz stimulus revealed significantly larger amplitudes for stimuli presented at 20 dB mSL for adults (p  =  0.0002) but not for infants (p  =  0.089). Post hoc  comparisons for the masker level x age interaction for 4000 Hz revealed significantly larger amplitudes for adults compared to infants in the unmasked condition (p  =  0.007).  Contralateral Recording We know that infants, compared to adults, show greater ipsilaterallcontralateral asymmetries for both air- and bone-conduction ABRs and ASSRs (Foxe & Stapells, 1993; Small and Stapells, 2008b; Stapells & Mosseri, 1991; Stapells & Ruben, 1989; Stuart et al., 1996). For low-intensity levels, this phenomenon is utilized to predict which cochlea has responded to the stimulus. Because ipsilateral/contralateral asymmetries are utilized in infants to predict which cochlea has responded, it is important to consider the effects masking has on amplitudes in both channels. In addition, by taking the dB difference in masking required to eliminate responses in the ipsilateral compared to the contralateral channel, an estimate of inter-aural attenuation can be obtained. At 1000 Hz, in the unmasked condition, adult ASSR amplitudes in the contralateral channel were smaller by 38 and 42 % compared to ASSR amplitudes in the ipsilateral channel for 10- and 20- dB mSL bone-conducted stimulus levels, respectively (Figure 2.6). As the 1 kllz masker level was increased, the amplitudes in the ipsilateral channel were eliminated with 58 and  83  68 dB SPL of masking at 10 and 20 dB mSL, respectively. Similarly, amplitudes in the contralateral channel were eliminated with 58 and 63 dB SPL of masking at 10 and 20 dB mSL, respectively.  Thus for adults, the estimated inter-aural attenuation at 1000 Hz is at least 0-5 dB  (Figure 2.6). At 4000 Hz, in the unmasked condition, adult ASSR amplitudes in the contralateral channel were approximately 38 and 36 % smaller than ASSR amplitudes in the ipsilateral channel for 10 and 20 dB mSL bone-conducted stimuli levels, respectively. As the 4 kllz masker level was increased, the amplitudes in the ipsilateral channel were eliminated with about 40 and 55 dB SPL of masking at 10 and 20 dB mSL, respectively. Similarly amplitudes in the contralateral channel were eliminated with 40 and 45 dB SPL of masking at 10 and 20 dB mSL, respectively.  Thus for adults, the estimated inter-aural attenuation at 4000 Hz is at least 0-10  dB (Figure 2.6). In the unmasked condition at 1000 Hz, infant ASSR amplitudes in the contralateral channel were approximately 65 % smaller than ASSR amplitudes in the ipsilateral channel for a stimulus presentation level of 20 dB mSL. Similarly, 1000-Hz stimuli presented at 10 dB mSL resulted in ASSR amplitudes that were 72 % smaller in the contralateral channel, which was barely above the noise floor. As the 1 kHz masker level was increased, the amplitudes in the ipsilateral channel were eliminated with 48 and 58 dB SPL of masking at 10 and 20 dB mSL, respectively. In contrast, amplitudes in the contralateral channel were eliminated with 38 and 48 dB SPL of masking at 10 and 20 dB mSL, respectively.  Thus for infants, the estimated inter  aural attenuation at 1000 Hz is at least 10 dB (Figure 2.6). Unlike adults, infant ASSR amplitudes in the contralateral channel were absent in the unmasked condition at 4000 Hz for both stimuli presentation levels. Given that the maximum presentation level was 35 dB HL at 4000 Hz, the inter-aural attenuation at this frequency has to be at least 35 dB.  84  Results of an ANOVA for the unmasked conditions revealed that the smaller amplitudes in the ipsi- compared to the contralateral recording channel [F (1, 16)  =  31.30, p  <  .00011  reached statistical significance. Post hoc comparisons for the significant channel x EEG interaction [F (1, 16)  35.54, p  =  <  .00011 revealed that there was statistically more noise in the  ipsilateral recording channel than in the contralateral recording channel (p = 0.025). The significant bone-conduction level x channel interaction [F (1, 16)  =  6.30, p  =  0.024] showed that  the smaller amplitudes in the ipsilateral channel for the 10 dB mSL bone-conducted stimuli compared to the 20 dB mSL stimuli reached statistical significance. The significant frequency x channel interaction [F (1, 16)  =  5.85, p  =  0.029] showed that the amplitudes in the contralateral  channel were significantly smaller compared to the ipsilateral channel for 1000-Hz (p but not for 4000-Hz (p  =  =  0.0002)  0.0007). Finally, post hoc comparisons for the frequency x stimulus  level x channel interaction [F (1, 16)  =  8.64, p  =  0.0 10] which was significant was due to larger  response amplitudes in the ipsilateral channel for the 20 dB mSL bone-conducted stimuli compared to the 10 dB mSL bone-conducted stimuli for 1000 Hz (p Hz (p  =  =  0.0002) but not for 4000  0.159). The degree of asymmetry observed between the ipsi- and contralateral channels  for both adults and infants did not reach statistical significance in the unmasked condition. For the masked responses, a significant masker level x channel interaction was observed for both 1000 [F (1, 16)  =  33.14, p  <  .0001] and 4000 Hz [F (1, 9)  =  7.89, p  =  0.020] and was  due to a lager difference between amplitudes in the ipsi- and contralateral channels for the unmasked condition than for the masked condition. Finally, for 1000 Hz, a significant masker level x channel x age interaction [F (1, 19)  =  4.67, p  =  0.046] was observed. Post hoc  comparisons for the masker level x channel x age interaction at 1000-Hz indicated that the significant effect of masker level and channel pooled across age was due to significantly larger  85  amplitudes in the contralateral channel for the 48 dB SPL masker level in adults (p compared to infants (p  =  =  0.0002)  0.120).  2.4.2 Experiment 1B- Adult Effective Masking Levels (Behavioural) Previous research investigating adult and infant air- and bone-condution thresholds thresholds utilizing physiological and behavioural testing have revealed that although strong correlations exist between the two testing methods, the thresholds obtained from each method are not directly comparable. In order to assess if similar correlations between behavioural and physiological testing methods are present for different masker intensities, EMLs calibrated in the coupler at 1000 and 4000 Hz were obtained using both physiological and behavioural testing methods for a group of adults. When tested behaviourally, EMLs calibrated in the coupler at 1000 and 4000 Hz increased significantly with increases in the bone-conducted stimulus level. Similarly, for the ASSR procedure, EMLs calibrated in the coupler at 1000 and 4000 Hz increased significantly with increases in stimulus level. The EMLs for the behavioural method were approximately 15 dB higher than the EMLs for the ASSR method at all stimulus levels for both 1000 and 4000 Hz. Results of an ANOVA comparing the EMLs for method of testing, bone-conduction level, and frequency revealed significant main effects of signal level [F(1, 9) 0.00011, method [F(1, 9)  =  24.41, p  <  .0001], and frequency [F(1, 9)  =  =  31.47, p  significant interaction between bone-conduction level and method [F(1, 9)  =  47.139, p <  =  .00011, and a  5.52, p  =  0.043].  The significant bone-conduction level x method interaction is mainly driven by the larger difference in EMLs observed for a bone-conducted stimulus level of 10 compared to 20 dB mSL.  86  CHAPTER 3: Discussion and Conclusion  87  3.1 Discussion 3.1.1 Experiment 1A- Infant and Adult Effective Masking Levels (ASSRs) Effective Masking Levels This is the first study to examine the EMLs of bilateral simultaneous masking noise on ASSR to bone-conducted stimuli in infants and adults. The results of this study clearly show that EMLs calibrated in the coupler for ASSRs elicited to bone-conducted stimuli presented in dB HL in young infants are significantly higher than that of adults for 1000 Hz, but are significantly lower than that of adults for 4000 Hz. In this study, infant-adult differences in the SPL of the air-conducted narrow-band noise in the ear canal were accounted for by taking RECD measurements. When these differences in the ear-canal were taken into account, infants were found to have even higher EMLs (in dB HL) than adults for 1000 Hz, which is consistent with a bone-conducted stimulus being a more effective stimulus for infants at this frequency (Small & Stapells, 2008a; Stapells & Ruben, 1989). In contrast, approximately the same EMLs (in dB HL) were required for adults and infants at 4000 Hz, which is consistent with similar boneconduction sensitivity for infants and adults (Small & Stapells, 2008a). Based on the masking levels obtained in the ear canal at each frequency, a boneconducted stimulus was estimated to be 12 dB more effective for infants compared to adults at 1000 Hz. For 4000 Hz, infant-adult differences in required masking levels in the ear canal ranged from 0-3 dB and were neither statistically nor practically significant. Foxe and Stapells (1993) predicted that a 500-Hz bone-conducted stimulus was 17 dB more effective for infants compared to adults based on expected ABR latency differences between infant and adult responses derived from air-conducted findings (Suzuki, Kodera, & Yamada, 1984). In addition, 88  Small and Stapells (2008a) found that infants had better bone-conduction thresholds at 1000 Hz (i.e., mean bone-conduction threshold of 7 and 24 dB HL in infants and adults, respectively) and similar bone-conduction thresholds at 4000 Hz (i.e., mean bone-conduction thresholds of 12 and 17 dB ilL in infants and adults, respectively). Thus, the estimates of the effectiveness of boneconducted stimuli in infants and adults obtained from this study fall within the same range as previous research. In order to better isolate the factors that contribute to these infant-adult differences in masking levels, the current study also accounted for the frequency-dependent differences in bone-conduction hearing sensitivity by expressing the bone-conducted stimuli in dB mSL. EMLs calibrated in the coupler for ASSRs to bone-conducted stimuli expressed in dB mSL in young infants were found to be significantly lower than that of adults for both 1000 and 4000 Hz. When infant-adult differences in the ear canal are accounted for, infants were still found to have lower EMLs in dB mSL than adults at 1000 Hz; however, at 4000 Hz, there was no longer any difference in EMLs in dB mSL between infants and adults. Previous studies examining masked thresholds in infants have only been conducted for air-conduction stimuli using behavioural visual reinforcement audiometry procedures. Due to inherent differences between behavioural and physiologic studies, direct comparisons between physiological and behavioural results must be interpreted with caution for several reasons. One reason is that the age of the infant participants in the current physiological study are younger than infant participants in past behavioural studies because behavioural visual reinforcement studies can only be reliably conducted in infants that are six months of age and older. Additionally, the hearing sensitivity values obtained using physiological and behavioural methods are not directly comparable; typically, behavioural thresholds are lower in dB HL  89  compared to physiological thresholds (Herdman & Stapells, 2001; Lins & Picton, 1995; Lins, Picton, & Picton, 1995; Lins, Picton, Boucher, Durieux-Smith, Champagne, Moran, Perez Abalo, Martin, & Savio, 1996; Luts, Desloovere, Kumar, Vandermeersch, & Woulters, 2004; Rance, Roper, Symons, Moody, Poulis, Dourlay, & Kelly, 2005; Rance & Tomlin, 2006). Furthermore, the type of stimuli used to elicit a response often varies between behavioural and physiologic studies. Although direct comparisons between behavioural and physiological studies are complicated for the reasons mentioned above, indirect comparisons focusing on general trends that arise between infants and adults can still be made. Results from previous behavioural masked threshold studies suggest that less masking is required to eliminate an infant’s response to an air-conducted signal compared to adults. In general, behavioural masked thresholds tend to be elevated by 5-15 dB in infants, gradually decreasing to adult levels by about ten years of age (Allen & Wightmen, 1994; Nozza & Wilson, 1984; Schneider & Trehub, 1992; Schneider et al, 1989). Given that masked thresholds in mfants are elevated compared to adults, if a signal is held constant, it is expected that 5-15 dB less masking will be required to disrupt the infant’s response to that signal. In agreement with behavioural masked threshold studies, the EMLs calibrated in the coupler for bone conduction ASSR stimuli (dB mSL) were found to be lower for infants than for adults by about 7-12 dB. The fact that masked physiologic results do not differ substantially from masked behavioural results suggests that the maturational differences observed for masked thresholds are largely due to sensory rather than non-sensory factors (e.g.,processing efficiency). Nozza and Henson (1999) also reported frequency-dependent differences for behavioural masked thresholds to air-conducted tonal stimuli. In comparison to lower frequencies, they found that masked thresholds at higher frequencies for infants and adults are more similar to each other.  90  In contrast to Nozza and Henson (1999), the current study did not reveal any frequencydependent differences for EMLs calibrated in the coupler, with differences for the low and high frequencies being no more than 2 dB. However, when SPLs in the ear canal were accounted for using individual RECDs, infants required 5-7 dB less masking at 1000 Hz and the same amount of masking at 4000 Hz compared to adults (i.e., frequency-dependent differences for physiological masked thresholds begin to emerge). To date, none of the maturational masking threshold studies have accounted for RECDs. Thus, in order to make direct comparisons in the ear canal to the existing masked threshold literature, RECD correction factors must be applied to the coupler values. Overall, adult RECDs tend to increase with increasing frequency. For example, Hawkins, Cooper, and Thompson (1990) reported mean real-ear SPLs that were 2, 7 and 12 dB higher at 1000, 2000, and 6000 Hz, respectively, compared to coupler values. The adult RECDs obtained for this study follow this general trend with mean real ear SPLs being 4 dB higher at 1000 Hz and 9 dB higher at 4000 Hz. Unlike adults, little published information regarding RECDs is available for infants and young children. The few studies that have been done show that RECDs are larger for children than adults, with children under 12 months showing the greatest RECDs (Feigin, Kopun, Stelmachowicz, & Gorga, 1989). One study conducted by Feigin and colleagues (1989) revealed RECDs to be 11.3 and 17.5 dB greater at 1000 and 4000 Hz, respectively. In comparison to Feigm et al. (1989), a study conducted by Westwood and Bamford (1995) revealed RECDs to be 6.4 and 8.2 dB at 1000 and 4000 Hz, respectively. The RECDs obtained for the current study were closer to the values obtained by Feigin et al. (1989) with RECDs being 10 and 16 dB at 1000 and 4000 Hz, respectively. In general, the RECDs found for infants in this study help to confirm previous findings that RECDs are feasible to obtain with young infants and  91  that there are large RECD values for infants in the first year of life, with large between-subject variability. Due to the inherent between-subject variability, individual RECDs should be obtained when possible and the use of mean RECDs to correct coupler measurements should be used with caution. Although the differences in EMLs between infants and adults found in the current study are similar to the trends observed for air-conducted masked behavioural studies, the signal to noise ratio (SNR) trends for infants compared to adults differ slightly from the findings of a behavioural visual reinforcement study conducted by Berg and Boswell (1999). In the Berg and Boswell (1999) study, SNRs for four different masking levels were determined at 500 and 4000 Hz by finding the masked thresholds for each individual masker. They found that seven-month old infants have similar SNRs to adults across different masker levels at 4000 Hz but larger SNRs than adults for lower compared to higher masker levels at 500 Hz. In the present study, SNRs can be inferred by looking at the EMLs required for different bone-conduction stimulus levels (calibrated in dB mSL) at 1000 and 4000 Hz. If the SNRs for the EMLs differ with the intensity of the signal at a particular frequency, than age and boneconduction level interactions should be observed. Similar to the Berg and Boswell (1999) study, SNRs for different levels of a 4000 Hz bone-conducted stimulus remained consistent (i.e., SNRs did not change across different noise spectrum levels) across infant and adult participants. That is, at 4000 Hz there was no significant interaction between age and bone-conduction level. However, in contrast to the larger SNRs for low intensity maskers found in infants at 500 Hz by Berg and Boswell (1999), the present study found that SNRs were consistent across 1000-Hz bone-conducted stimulus intensity levels (dB mSL).  92  In addition to the complications that arise in comparing behavioural and physiologic results, comparisons between the current physiological and Berg and Boswell (1999) study are further complicated by the differences in stimulus frequency and intensity between studies. In the Berg and Boswell (1999) study, masked thresholds were obtained for 500, but not for 1000 Hz. It is possible that if Berg and Boswell (1999) tested 1000 Hz, no differences in SNRs for differing masker intensity levels in infants would be found. Also, Berg and Boswell (1999) examined each frequency at four different masker intensity levels, whereas the current physiologic study, due to time restraints, only looked at two bone-conduction levels that differed by 10 dB for each frequency. Thus, it is possible that if a difference in SNRs does exist at 1000 Hz, it may only become apparent with greater disparities in bone-conducted stimulus levels. To gain further insight into the frequency-dependent maturational differences in processing efficiency, future behavioural and physiologic studies should find SNRs for a larger range of stimulus levels at more than two frequencies for infants of different ages. Comparisons of the current physiologic study to previous behavioural masking studies are further complicated by differences in the method of presentation of the signal. Previous studies used air-conducted stimuli and masking. The current study used bone-conducted stimuli and air-conducted binaural masking. These differences in experimental design make direct comparisons with previous infant masking studies difficult for at least two reasons. First, the differences in the maturation of the middle ear will have different effects on air- and bone conducted signals. Second, maturational differences in the infant skull and neurophysiology will result in different testing paradigms for infants and adults when they are tested with bone compared to air-conducted test stimuli (i.e., is the bone-conducted signal going to one ear or both for infants?).  93  In comparison to adults, infants have higher middle-ear resistance and lower middle-ear compliance. That is, relative to adults, the energy transfer into the middle ear of infants from an air-conducted signal is much less (Keefe, Bulen, Arehart, & Burns, 1993, 1994) and, in turn, the signal that actually reaches the cochlea should also be less. To obtain an estimate of the amount of power absorbed by the middle ear of infants 1-24 months of age, Keefe et al. (1993) compared input conductance for an air-conducted stimulus, in order to measure total middle-ear conductance. For very young infants (less than one month of age), Keefe et al (1993) found that, compared to adults, the middle ear absorbs 5 and 11 dB less power at 1000 and 4000 Hz, respectively. In contrast, the amount of power absorbed by older infants (6-24 months of age) was only 2-4 dB less than adults for 500-4000 Hz (Keefe et al, 1993). It is important to consider the implications of these differences in the current study for two reasons. First the majority of the infant participants in the study were younger than six months of age and as mentioned above, in comparison to adults, middle-ear differences are greater for very young infants. Secondly, the signal in the current study was presented via bone conduction, whereas the masker was presented through air conduction. In previously mentioned behavourial EML studies, the signal and masker were both presented through air conduction, thus any impact the middle ear may have on infants compared to adults would affect the signal and the masker to the same degree. In contrast, for the current physiologic study, the middle ear likely affects the signal presented via bone-conduction to a different degree than it affects the masker that is presented via air-conduction. Currently, methods do not exist to measure the exact signal level reaching the cochlea for an individual when a stimulus is presented via bone conduction. One method for accurately measuring and calibrating the bone-conductor for individual participants was proposed by  94  Purcell, Kunov, and Clerghorn (1999) after they demonstrated that distortion product otoacoustic emissions could be elicited with bone-conducted stimuli (Purcell, Kunov, Madsen, & Clerghom, 1998). The method involves utilizing distortion product otoacoustic emissions to calibrate the bone-oscillator for individual participants, and although it has various clinical limitations, Purcell et al (1999) suggested that it may be a useful tool for determining inter-aural attenuation in individuals with normal hearing (Purcell et al, 1999; Purcell, Kunov, & Clerghorn, 2003). Future bone-conduction studies in infants could make use of the distortion product otoacoustic emissions calibration procedure mentioned to further investigate how much energy is reaching the cochlea via bone-conduction in infants and adults. There are also obvious differences between an infant and adult skull that could affect the transmission of a bone-conducted signal. Compared to the adults, the infant skull is much smaller in size. Furthermore, the dimensions of the mastoid bone in terms of width, length and depth increase rapidly in the first two years of life (Eby and Nadol, 1986). Foxe and Stapells (1993) have suggested that in comparison to adults a more intense signal may be transmitted to the cochlea due to the smaller temporal bone in infants, thus strengthening the energy of boneconducted stimuli. Additional studies have hypothesized that immaturaities in the infant skull might enhance the intensity and efficiency of energy tramsmission to the cochlea, thus contributing to the observed infant-adult differences in electrophysiological thresholds and latencies (Foxe & Stapells, 1993; Sohmer, Freeman, Geal-Dor, Adelman & Savion, 2000; Stuart et al, 1990; Yang et al., 1987). Another difference between infants and adults is the connective tissue that separates the bones of the skull. In infants the temporal bone of the skull is connected to the other bones of the cranium through flexible sutures (e.g.,fontanels) that gradually become bony sutures at  95  around one year of age. According to Stuart et al. (1990), better click-ABR thresholds to boneversus air-conducted stimuli in neonates may be accounted for by the flexible sutures of the infant skull, which result in less energy dissipating to the rest of the skull. They proposed that this causes the temporal bone to oscillate more in isolation, resulting in a more effective boneconducted stimulus across frequencies in infants. Finally, the fontanels in infants (Foxe & Stapells, 1993), along with maturational differences in the anatomical location of neuro-generators relative to the skull (Edwards et a!., 1985), are believed to contribute to maturational differences in inter-aural attenuation.  Maturational differences in inter-aural attenuation can be estimated by examining ipsilateral/contralateral asymmetries in latencies and amplitudes of the ASSR or ABR. The infant-adult differences for amplitudes found in the present study will be discussed in greater detail in the following section.  Amplitudes  The present study was the first study to examine the affects of masking noise on ASSR amplitudes for infants and adults in both the ipsi- and contralateral recording channels. This study found the following trends for ASSR amplitudes in the ipsilateral recording channel (stimuli presented in dB mSL): (i) amplitudes were reduced by 44-54% and 72-79% for adults and infants, respectively at 1000 Hz with 48 dB SPL of masking, while amplitudes were reduced by 5 1-55% and 40-49% for adults and infants, respectively at 4000 Hz with 30 dB SPL of masking, (ii) with a 10 dB increase in bone-conducted stimulus level, amplitudes increased by 31 and 22 % at 1000 Hz for infants and adults, respectively, while amplitudes increased by 15 % at 4000 Hz for both age-groups, (iii) amplitudes were larger at 1000 compared to 4000 Hz for both  96  infants and adults, (iv) mean amplitudes for adults were larger than the those for infants, and (v) the combined effects of varying bone-conducted stimulus and masker levels on ASSR amplitudes resulted in consistent SNRs (i.e., SNRs did not change across different noise spectrum levels) for infants and adults at 4000 Hz and for adults at 1000 Hz, but inconsistent SNRs (i.e.,SNRs were dependent on the noise spectrum level) for infants at 1000 Hz. For ASSR amplitudes in the contralateral channel, the following trends were observed: (i) overall amplitudes were smaller in the contralateral channel compared to the ipsilateral channel, (ii) infants had fewer significant responses compared to adults, (iii) fewer significant responses were present for 4000 than for 1000 Hz for both infants and adults, and (iv) for 1000 Hz in adults, similar masking levels were required in the ipsi- compared to the contralateral channel to eliminate ASSR responses; however, for infants less masking was needed in the ipsilateral channel than in the contralateral channel to eliminate ASSR responses. With the exception of the adult amplitudes at 1000 Hz, the mean amplitudes found in the present study are similar to the amplitudes obtained in the Small and Stapells (2008a) study, differing by no more than 4-23 %. Even though the amplitudes found in this study for adults at 1000 Hz were about 35-38 % larger than those found by Small and Stapells (2008a), the trend in which adult amplitudes are larger than that of infants is the same. Although the SNRs for the EMLs in the current study appear consistent between the two 1000-Hz bone-conducted stimulus levels; the SNRs from the ASSR amplitudes do not appear to be as consistent. If the SNR is consistent in infants, when the same change in level for either the bone-conducted stimulus or masker is made, one would expect the amplitudes to be affected similarly. Given that adults are known to have consistent SNRs across different intensity levels at both 1000- and 4000- Hz, it is also expected that the manner in which infants differ from  97  adults should be similar for changes made to both bone-conducted stimulus and masker levels. However, at 1000 Hz this is not the case. Similar to the consistent SNRs for the EMLs observed in infants at 4000 Hz, the ASSR amplitudes at 4000 Hz also appear to have consistent SNRs. That is, at 4000 Hz changes to either the stimulus level or masker level resulted in similar changes to the amplitudes of both age groups. In contrast, when a similar change in stimulus and masker level is applied to both agegroups at 1000 Hz, infants experienced only a 9% greater increase in amplitudes with stimulus level but a 25-28% greater reduction in amplitudes with masker level when compared to adults. Thus, the ASSR amplitude results suggest that the processing efficiency of infants at 1000 Hz may not be fully mature. Overall, the infant and adult ASSR amplitudes obtained from the ipsi- versus contralateral channel are similar to an earlier study conducted by Small and Stapells (2008b), where greater degrees of asymmetry were noted for infants compared to adults. The current study contributes additional information to this previous study by demonstrating what occurs in the presence of bilateral masking noise for both recording channels for infants and adults. In the current study, the minimum inter-aural attenuation for 1000 Hz in infants and adults was estimated by comparing the amount of masking noise required to eliminate ASSRs in the ipsi versus the contralateral recording channel. The difference in masking required to eliminate the ASSRs provides a rough estimate of the minimum inter-aural attenuation. In agreement with previous literature, the estimated inter-aural attenuation for adults is at least 0-5 and 0-10 dB at 1000 and 4000 Hz, respectively. This finding suggests that essentially both cochleae in adults are responding. In contrast, the estimated inter-aural attenuation at 1000 Hz for infants in this study was at least 10 dB. This estimate of minimum inter-aural attenuation  98  overlaps with Small and Stapells (2008b) estimate of 10-30 dB for a frequency range of 5004000 Hz. For 4000 Hz, infants had absent responses in the contralateral channel for the highest stimulus level used in the study which was 35 dB HL. This suggests that for infants the minimum inter-aural attenuation at 4000 Hz is greater than 35 dB. Due to the limited amount of testing time, EMLs were not specifically obtained for the contralateral channel. Future research which compares responses to bone-conducted stimuli that are clearly present in both recording channels in the unmasked condition, for simultaneous binaural, ipsilateral, and contralteral masking should be conducted. Such research may provide further information regarding interaural attenuation values in young infants. The difference in inter-aural attenuation between infants and adults essentially leads to two different testing paradigms whereby the same signal and masker are presented to both cochleae for adults, and the same masker but a different signal is presented to the cochleae of infants. That is, the presentation of the signal and masker are essentially diotic and dichotic for adults and infants, respectively. The importance of this difference becomes apparent when the psychoacoustic phenomenon of the binaural masking level difference (BMLD) is considered. The BMLD phenomenon refers to the binaural advantage for masked thresholds that occurs under dichotic conditions for pure-tone, complex-tone, click, and speech stimuli (Hirsh, 1948a; Licklider, 1948; Moore, 1997). Although many studies suggest that the BMLD is generated within the brainstem (e.g.,Jiang, McAlpine, & Palmer, 1997a, 1997b; Mandava, Rupert, & Moushegian, 1996; Martinez & Schaefer, 1982), for adults, the BMLD is only obtained for slow cortical auditory evoked potentials (waves N1-P2) and is not obtained for the 80 Hz ASSR, ABR, or middle latency response (Fowler & Mikami, 1992a, 1992b, 1995, 1996; Kevanishvili & Lagidze, 1987; Wong & Stapells, 2004). There are currently no published physiological studies  99  that have investigated a similar phenomenon in infants using ABRs or ASSRs. To date, only one behavioural visual reinforcement audiometry study conducted by Schneider, Bull and Trehub (1987) has investigated the BMLD in infants and their findings suggest that the mechanisms responsible for the BMLD are in place by 12 months of age. Thus, it should be easier for infants (i.e.,dichotic) to detect a signal in noise presented via bone-conduction than it is for adults (i.e.,diotic). Clearly, more research investigating the BMLD in infants is required to better interpret the EMLs obtained in infants and adults when the signal is presented via boneconduction. The probability of whether one or both cochlea(e) of the infant and adult are contributing to the ASSRs will increase as the level of the bone-conducted stimulus is raised. Given that ASSR recordings do not differentiate which cochlea the response is coming from, the increase in the magnitude of the ASSRs may be even greater if the bone-conducted signal surpasses the infants estimated inter-aural attenuation levels of at least 10 dB and  35 dB at 1000 and 4000  Hz, respectively, because the recordings will represent responses from two cochleae as opposed to only one cochlea. This, in turn, may affect the EMLs and amplitude SNRs found for infants at different bone-conduction stimuli levels. In adults, for whom the inter-aural attenuation is at least 0-5 and 0-10 dB at 1000 and 4000 Hz, respectively, it is reasonable to surmise that differences in bone-conducted stimuli levels may have less of an effect on the EML and amplitude SNRs. Additionally, in comparison to infants, the larger amplitudes for the unmasked condition found for adults in this study is consistent with the responses of two cochleae in adults as opposed to only one cochlea in infants.  100  3.1.2 Experiment 1B- Adult Effective Masking Levels (Behavioural) In the present study, EMLs obtained behaviourally and physiologically in adults were compared. The EMLs utilizing 10 and 20 dB mSL bone-conducted stimuli at 1000- and 4000 Hz for the behavioural testing method were approximately 15 dB greater than the EMLs obtained using the same stimuli for the ASSR testing method. Previous research assessing behavioural versus ASSR air- and bone- conduction thresholds to amplitude-modulated ASSR stimuli in adults have revealed strong linear relationships between ASSR and behavioural thresholds, where behavioural thresholds are approximately 7-17.5 dB lower (better) than ASSR thresholds (Herdman & Stapells, 2001; Lins & Picton, 1995; Lins, Picton, & Picton, 1995; Lins et al., 1996;). Considering that the stimulus levels in the present study were the same for both the behavioural and ASSR methods, the lower masking levels required to eliminate the ASSRs compared to the behavioural response agrees with previous literature because the behavioural stimuli were being presented at 10 and 20 dB mSL supra-threshold, which would be more compared to ASSR stimuli also presented at 10 and 20 dB mSL. In other words, for the present study, the bone-conducted stimuli were closer to the noise floor for ASSRs than for behavioural responses. Therefore, less masking should be required to eliminate ASSRs in comparison to behavioural responses. Similarly, in infants, strong correlations between ASSR and behavioural thresholds have been reported (Luts et a!, 2004; Rance et a!, 2005; Rance & Tomlin, 2006) for both normal hearing infants and infants with sensorineural hearing losses. In comparison to adults, the difference between behavioural and ASSR hearing thresholds for infants are lower, ranging  101  between -2 to 4 dB, but are also more variable among individual participants (Luts et al, 2004). Given that the difference between behavioural and ASSR thresholds are smaller for infants than adults, one could speculate that there may be less of a difference in behavioural versus ASSR EMLs. Future studies should investigate the correlation between EMLs in infants found both behaviourally and physiologically to determine appropriate bone-conduction masking levels for each assessment method.  3.1.3 Clinical Implications The findings of the present study clearly indicate that EMLs calibrated in the coupler for ASSRs elicited to bone-conducted stimuli calibrated in dB HL for infants are 7 dB higher compared to adults for 1000 Hz and approximately 8-10 dB lower compared to adults for 4000 Hz. These findings suggest that in comparison to adults and older children, different clinical masking levels should be used for infants under six months of age.  Specifically, higher and  lower masking levels should be adopted for use in the clinic with young infants at 1000 and 4000 Hz, respectively. Further research is required to determine the age at which adult masking levels should be utilized and whether older infants require different masking levels than both younger infants and adults. Preliminary masking levels for 1000 and 4000 Hz were determined by taking into consideration the greatest amount of masking required for infants at each stimulus level. For 1000 Hz, masking levels of 48 and 58 dB SPL at 15 and 25 dB ilL, respectively, are recommended. For 4000 Hz, masking levels of 40 and 45 dB SPL at 25 and 35 dB HL, respectively, are recommended. Although these effective masking levels are a good starting point, it should be kept in mind that they were obtained with binaural masking and not with  102  contralateral masking of the non-test ear which is typically used clinically. Thus, the levels obtained in this study may be slightly lower than the true levels required due to binaural summation effects (e.g. ,Hirsh, 1 948b). The fmdings from this study also have important implications for the calibration of narrow-band noises on clinical audiometers when testing infants. Currently, maskers are calibrated in dB EM for adults (ANSI S.3, 1996). The fact that narrow-band noise is calibrated for the EMLs of adults should be kept in mind when testing infants for at least two reasons. First, the EMLs of infants measured in the coupler have been shown to be higher when compared to the EMLs of adults for 1000 Hz and much less when compared to the EMLs of adults for 4000 Hz. Second, ASSR amplitude results from the current study and those from an air-conduction behavioural masked threshold study conducted by Berg and Boswell (1999) suggest that, unlike adults, low-frequency SNRs of infants are not consistent across different levels of a stimulus. Clearly, maturational differences in EMLs will have an effect on the dB EML calibration that should be used for infants. Even though masking of the other ear is not required to obtain ear-specific information for low-stimulus presentation levels due to an estimated minimum inter-aural attenuation of 1030 dB (Small & Stapells, 2008b), masking may be beneficial for presentation levels that exceed an individual infant’s inter-aural attenuation. For example, in cases of unilateral sensorineural, bilateral conductive, and/or mixed hearing losses, high presentation levels are often needed to stimulate the test ear with the hearing loss. For these types of hearing losses, it is likely that the required presentation levels of the bone-conducted stimuli will exceed the infant’s inter-aural attenuation and masking will be needed, particularly if clear ipsilaterallcontralateral asymmetries are not present. The use of masking under such situations would allow ear-specific thresholds to  103  be obtained so that hearing-aid gain targets could be more accurate and best meet the audibility needs of the infant. Future research regarding EMLs should include frequencies that are commonly tested in early hearing screening programs, such as 500 and 2000 Hz. As a result of the limited sleep time for infants, various studies have investigated whether or not the simultaneous presentation of multiple stimuli leads to any interaction effects (Armstrong & Stapells, 2007; Luts & Woulter, 2004). They have found that when stimuli are ran simultaneously there are no significant interaction affects for multiple stimulus presentations. This is true for both air- and boneconducted stimuli (Armstrong & Stapells, 2007; Luts & Wouter, 2004). Due to the time saving benefits of multiple ASSRs, future research with EMLs should investigate whether or not there are any interaction effects when both bone-conducted stimulus and air-conducted masking levels are presented simultaneously for different carrier frequencies.  3.2 Conclusion The findings of this study have important clinical and theoretical implications for our understanding of EMLs for bone-conducted stimuli. To conclude, when EMLs are calibrated in the coupler and bone-conducted stimuli are calibrated in dB ilL which is typically done clinically, infants under six-months of age have significantly higher and lower EMLs at 1000 and 4000 Hz, respectively, compared to adults. When infant-adult differences in bone-conduction hearing sensitivity are taken into consideration by presenting stimuli in dB mSL, infants under six-months of age have lower EMLs (calibrated in the coupler) for both 1000 and 4000 Hz compared to adults. When ear canal size is taken into account and stimuli are presented in dB 104  mSL, infants tend to have lower EMLs than adults at 1000 Hz, but little difference in EMLs compared to adults at 4000 Hz. In addition, when SNRs are considered for infant and adult ASSR amplitudes, the SNRs for adults are found to be consistent at both 1000 and 4000 Hz, whereas the SNRs of infants are only consistent for 4000 Hz. The EMLs calibrated in the coupler for bone-conducted stimuli presented in dB mSL obtained for this study are consistent with the findings for masked thresholds found in previous air-conduction behavioural studies (Allen & Wightmen, 1994; Nozza & Wilson, 1984; Schneider & Trehub, 1992; Schneider, Trehub, Morrongeillo, & Thorpe, 1989). As well, the inconsistent SNRs at 1000 Hz for ASSR amplitudes are in agreement with a previous air-conduction behavioural study conducted by Berg & Boswell (1999). In order to fully understand the underlying maturational changes responsible for the lower EMLs for bone-conducted stimuli found for infants in comparison to adults, future research should focus on accurately measuring the signal that is delivered to an individual’s cochlea(e) via bone-conduction. Overall, the findings of this study suggest that a masking procedure could eventually be incorporated into the pediatric physiologic diagnostic test battery. However, before this could be implemented in the clinic, future research that focuses on the EMLs to bone-conducted stimuli for ipsi- and contralateral masking noise across a range of frequencies for infants of different ages with normal hearing and with hearing loss is needed. Future masking studies should also investigate whether or not any interaction effects occur when the signal and masker are assessed at different frequencies simultaneously.  105  References Abdala, C., Sininger, Y.S., Ekelid, M., & Zeng, F. G. (1996). 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Zwislocki, J. J., Burning, E., & Glantz, J. (1968). Frequency distribution of central masking. Journal of the Acoustical Society ofAmerica, 43: 1267. Zwislocki, J. J., Damianopoulos, E. N., & Buining, E. (1967). Central masking: some steadystate and transient effects. Perception and Psychophysics, 2(2): 59-64.  134  Appendix A: Certificate of Ethics Approval  135  ___________________MENDMENT  _________  1ui1  ________  The University of British Columbia Office of Research Services Clinical Research Ethics Board Room 210, 828 West 10th Avenue, Vancouver, BC V5Z 1L8 —  ETHICS CERTIFICATE OF EXPEDITED APPROVAL: AMENDMENT PRINCIPAL INVESTIGATOR:  %san A. Small  DEPARTMENT: UBC/Medicine, Faculty of/Audiology & Speech Sciences  UBC CREB NUMBER:  H09-00278  I  Institution  UBC  Site  Vancouver (excludes UBC Hospital) Other locations where the research will be conducted:  N/A INSTITUTION(S) WHERE RESEARCH WILL BE CARRIED OUT: CO-INVESTIGATOR(S):  Erin E. Hansen PROJECT TITLE: Effective masking levels for bone conduction auditory steady-state response thresholds in infants SPONSORING AGENCIES: UBC Faculty of Medicine “New faculty start-up grant: Effective masking levels for bone-conduction auditory teady-state resp.. -  REMINDER: The current UBC CREB approval for this study expires: April 7, 2010 APPROVAL AMENDMENT(S):  Addition of funding.  pATE: 1Deceme 21, 2009  CERTIFICATION: In respect of clinical trials: 1. The membership of this Research Ethics Board complies with the membership requirements for Research Ethics Boards defined in Division 5 of the Food and Drug Regulations. 2. The Research Ethics Board carries out its functions in a manner consistent with Good Clinical Practices. 3. This Research Ethics Board has reviewed and approved the clinical trial protocol and informed consent form for the trial which is to be conducted by the qualified investigator named above at the specified clinical trial site. This approval and the views of this Research Ethics Board have been documented in writing.  rhe amendment(s) for the above-named project has been reviewed by the Chair of the University of Britisl olumbia Clinical Research Ethics Board and the accompanying documentation was found to be acceptable oi thical grounds for research involving human subjects.  Approval of the Clinical Research Ethics Board by one of  Dr. Peter Loewen, Chair Dr. James McCormack, Associate Chair 136  Appendix B: Individual Infant Data Experiment 1A  137  Individual effective masking levels, hearing screening results on the day of testing and RECDs obtained for 3-27 week-old infants. Infant #  Age  1000 Hz Stimuli  4000 Hz Stimuli  (weeks)  10  20  1  27.3  42.75  57.75  2  21  47.75  47.75  35.3  3  4.6  42.75  57.75  40.3  4  9.3  42.75  57.75  35.3  5  19.9  32.75  47.75  30.3  6  21.4  42.75  47.75  30.3  7  24.7  47.75  52.75  8  21.9  47.75  9  23.7  10  12.4  27.75  52.75  25.3  11  22.6  42.75  47.75  15.3  12  19.9  32.75  42.75  25.3  13  3.7  47.75  52.75  14  16.1  15  10.9  10  20  Hearing  RECDs  Screening P  B  40.3  P  B  45.3  P  B  P  B  35.3  P  B  35.3  P  B  25.3  P  B  30.3  P,.  B  30.3  ---  B  25.3  P  B  P  B  30.3  P  B  15.3  25.3  P  B  47.75  30.3  40.3  P  B  42.75  52.75  15.3  30.3  P  R  Mean  41.60  51.21  27.22  33.80  SD  6.50  4.74  8.05  6.69  13  13  13  10  N  Note:  ‘---‘  ---  indicates that data was not obtained for this individual  ‘P’ indicates individual passed the hearing screening in both ears ‘PL’  indicates individual passed the hearing screening for the left ear only  ‘B’ indicates RECDs were obtained for both ears ‘R’ indicates RECDs were obtained only for the right ear  138  C  I  c’1  00  I  c  I  :  I  I  I  I  I  I  I  :  I  I  :  I  I  I  n  00  C  I  I  :  I  I  I  I  I  I  :  I  I  :  Q  I  I  I  I  I  I  N  I  I  I  —  c— e  —  I  I  :  :  I  I  I  :  :  —  —  -  r cNN  00  ef  00  Ij -  I  oc 00  I  :  c  I  C  I  I  r  :  C  ‘  I  :  00 —  c rI  I  I  :  00  N  I I  I  C  I  I  I  C N  -  :  II  I  I I  C I  —  I  I  C  -  I  :  i  :  —  N  Cl) -  I  : I  N  It  fl  C  I  0 — I  I  I  c  I  :  I  I  I  I  I  :  I  I  I  :  I  :  :  ‘  I  I  ‘ I  I  I  I I  Cl) I —  —  -a——-— °  C  I  I  I  c’  c’4  °  Cl)  Wd  II.)  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I  :  I  I  I  I  I  I  I  I  I  :  ‘1•  I  —  -  0  00  ©  I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  -  c  In  I  I  — —  I  I  I  I  I  I  I  I  I  I  I  CD  ©‘ .  lf  N  I  I  I I  I  a)  oo  -O  0 It  .  0  a?  —  a?  :  ‘  :  ‘  I  I  I  N  —  :  I  I  N a)  c)  —  00  in  —  —  00  c  0 —  oc  C  C  rf  —  z  -e  a)  C  z  Appendix C: Individual Adult Data Experiment 1A  143  Individual effective masking levels and RECDs obtained for Adults. Adult #  Age  1000 Hz Stimuli  4000 Hz Stimuli  (years)  -10  0  10  20  1  25  32.75  37.75  42.75  62.75  2  25  ---  ---  62.75  62.75  3  26  32.75  42.75  57.75  4  24  42.75  47.75  6  31  ---  7  26  8  RECDs  10  20  45.3  50.3  B  67.75  35.3  55.3  B  57.75  72.75  35.3  55.3  B  ---  52.75  57.75  40.3  55.3  R  ---  47.75  57.75  72.75  25.3  B  24  37.75  52.75  57.75  67.75  30.3  35.3  B  9  27  32.75  47.75  57.75  77.75  25.3  35.3  B  10  23  ---  42.75  47.75  30.3  40.3  B  11  24  ---  ---  52.75  67.75  50.3  B  12  26  37.75  37.75  52.75  57.75  30.3  40.3  B  13  24  37.75  52.75  52.75  67.75  40.3  45.3  B  14  21  27.75  37.75  47.75  57.75  35.3  50.3  B  15  30  37.75  42.75  57.75  52.75  ---  35.3  B  16  27  27.75  42.75  62.75  62.75  35.3  40.3  B  Mean  34.75  44.57  53.25  63.92  34.85  43.87  SD  4.83  5.60  7.08  7.84  5.68  9.29  10  11  15  15  11  14  N Note:  ‘---‘  ---  indicates that data was not obtained for this individual  ‘B’ indicates RECDs were obtained for both ears ‘R’ indicates RECDs were obtained only for the right ear  144  _ I’,  C C  C C C a:  I  00  :-QQ  :  C  :I  I  .  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U, c  I  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I  I  I  I  I I I  I I I  I I  I I I  I I I  00  I I I  I I  ‘%C  N  C N  N  C  N  ‘-4 I I  I  \C  N  C’  Inì  I I I  N  N  I I I  N  ‘I  I I I  I I I  00  I I  •-4  C’  .  -  00  ‘  c___  •oO  In  C  .z  :  :  I I  :  I I  I I I  I I I  I I I  I I I  I  I I I  I  I I I  I I I  I I I  I  In  h  In  — ————  I I I  I I I  —  I I I  I I I  I I I  I I I  I I  I I I  I  C  i I I  —  ce———— 00  In  —  ON  II I  In  00  .  I I  -  I  I  I  I I  I I I  I I I  I I I  I I I  ON  I I I  I  I  I I I  I  :  I I I  ON  ‘  C  I  I I I  -  ON  C  I  -  ©  ‘n  rì  c  1 In  .  I i  —4  I I I  I I  I  ì  I I I  I I  I I I  00  I I  I I  I I I  I I I  I  I I  -  N  c  I I I  I I I  N  N  ON  I I  II I  ‘‘  C  I II  c  00  I  In  I I I  N  In  ON  O  I I I  ON  C  I  ‘  00 .  ON  ,-4  00 In c’  c’  o  od  ON  Cl)  ,-4 1-4  in  ©  I  I  I  J U,  I  :  I  I  I I I  I  I  I  I  :  I  I I I  I  :  N  00  D  I  00 ,  I I  I  I I I  cì  •  ON  ‘-4  O  4  I  I  :  C  I  -  I  :  r  N .  Inì 00  ON  In In  I I i  I I i  I I i  I I I  I I I  I I i  I I I  I I I  I I I  I I I  I  N —  C C  ‘-4  C In  In C  ON In  C  ci) C’:  C  : I  I  :  00  N  I  I I i  I I i  J-  Cd)  —  ——  c’: 00 00 —  -  1-  ‘.C  00  C  N  :  I  c  ON  ON  :11: .: —  .  O  00  ON  C 0 —  —  .  In  Q  z  0  z  Appendix D: Individual Adult Data Experiment lB  151  Individual effective masking levels and RECDs obtained for Adults. Adult #  Age  1000 Hz Stimuli  4000 Hz Stimuli  -10  0  10  20  10  20  RECDs  1  25  52.75  52.75  67.75  67.75  75.3  75.3  B  3  26  47.75  57.75  67.75  77.75  55.3  70.3  B  4  24  57.75  62.75  87.75  87.75  60.3  80.3  B  5  27  67.75  77.75  60.3  70.3  R  6  31  ---  57.75  57.75  40.3  50.3  R  7  26  57.75  87.75  87.75  45.3  70.3  B  8  24  37.75  47.75  72.75  57.75  35.3  50.3  B  9  27  52.75  67.75  82.75  87.75  60.3  65.3  B  10  23  52.75  62.75  45.3  45.3  B  11  24  ---  ---  62.75  72.75  70.3  80.3  B  12  26  47.75  62.75  72.75  77.75  40.3  55.3  B  13  24  32.75  42.75  67.75  67.75  45.3  45.3  B  15  30  57.75  72.75  72.75  77.75  40.3  50.3  B  16  27  37.75  52.75  62.75  62.75  45.3  45.3  B  17  32  ---  67.75  77.75  35.3  45.3  B  18  33  ---  62.75  77.75  50.3  55.3  B  19  20  32.75  52.75  62.75  45.3  50.3  ---  Mean  47.19  55.48  67.75  73.93  50.01  59.12  SD  9.17  11.48  10.61  9.28  11.79  12.93  N  9  11  17  17  17  17  Note:  ‘---‘  indicates that data was not obtained for this individual  ‘B’ indicates RECDs were obtained for both ears ‘R’ indicates RECDs were obtained only for the right ear  152  

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