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Multiple brainstem auditory steady-state response interactions for different stimuli Wood, Lori Laraine 2009

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MULTIPLE BRAINSTEM AUDITORY STEADY-STATE RESPONSE INTERACTIONS FOR DIFFERENT STIMULI  by Lori Laraine Wood B.Sc., University of Alberta, 1995  A THESIS SUBMITTED IN PARTIAL FULFILMENT 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)  November, 2009 © Lori Laraine Wood, 2009  Abstract Auditory steady-state responses (ASSRs) have been shown to be accurate in predicting thresholds of individuals with hearing loss. Although new stimuli are being proposed and clinically implemented, there are no data to indicate whether response interactions would be adversely affected by their use. This study investigated the effects of three different stimuli (AM, AM/FM and AM2) at two different intensities (60 dB HL and 80 dB HL) on response amplitudes and interactions in normal-hearing adults. Stimuli were generated by the Rotman MultiMASTER research system and presented via air conduction through EAR-3A insert earphones. Carrier frequencies of 0.5, 1, 2, and 4 kHz were 80-Hz modulated in three conditions: individually (monotic single; MS), simultaneously in one ear (monotic multiple; MM), and simultaneously in both ears (dichotic multiple; DM). It was predicted that stimuli with broader spectra would result in greater amplitudes. This was demonstrated in the MS condition by the AM/FM stimulus, which evoked responses significantly larger than those to both AM and AM2 stimuli at all frequencies except 0.5 kHz at 60 and 80 dB HL. In the multiple (MM and DM) conditions, response amplitudes to AM2 were significantly larger than AM and AM/FM response amplitudes at both intensities. It was also predicted that more interactions would be found when using stimuli with broader spectra, even at moderate intensities. This was illustrated by the drop in amplitude by the AM/FM stimulus in the multiple conditions versus in the single condition, even at 60 dB HL. Relative efficiency values in the multiple conditions were never less than that found in the single condition at 60 dB HL; at 80 dB HL, the majority (83%) of comparisons were more efficient in the multiple conditions than the single condition. Based on these results, the optimal stimulus to use appears to be dependent on the chosen condition. In the single condition, AM/FM ii  stimuli result in the largest response amplitudes, however, in the multiple condition, AM2 stimuli provide the best combination of amplitude values and testing efficiency.  iii  Table of Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi List of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Chapter 1: Literature review: Auditory Steady-State Responses . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 A General Review of ASSRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Generators of the ASSR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Slow Cortical ASSR (<20 Hz MF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Early Cortical ASSR (40 Hz MF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Brainstem ASSR (80 Hz MF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Subject Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Maturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Arousal State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Measurement and Analysis of the ASSR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Stopping Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Stimulus Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Modulation rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Carrier Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Rise Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Multiple Simultaneous Stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Relative Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Physiology Underlying Interactions Between ASSRs to Multiple Simultaneous Stimuli 19 Cochlear Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Neural Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Frequency Specificity of Stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Acoustic Specificity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Cochlear Place Specificity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Specificity of Central Auditory Neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Stimuli Used in Current Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Sinusoidal AM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Mixed Modulation (AM/FM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Exponential Envelope Modulation (AM2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Summary and Rationale for the Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 iv  Chapter 2: Multiple brainstem auditory steady-state response interactions for different stimuli 35 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Subjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Auditory Stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Recordings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Data Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Relative Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 ASSR Amplitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Monotic Single Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Monotic Multiple Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Dichotic Multiple Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Monotic Multiple versus Dichotic Multiple Conditions . . . . . . . . . . . . . 54 Single vs Multiple Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Relative Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Amplitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Relative Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Clinical Implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Future Directions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Appendix A: Corrected amplitude (nV) data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Appendix B: Circle radius noise (Cr) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Appendix C: Phase (degrees) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .94 Appendix D: Relative efficiency data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Appendix E: Behavioural research ethic board (BREB) approval . . . . . . . . . . . . . . . . . . . . . . 108  v  List of Figures Figure 1.1. Acoustic spectra of stimuli used in this study (AM, AM/FM, and AM2) for carrier frequencies of 0.5, 1, 2, and 4 kHz. Y axis ticks represent 20 dB intervals. . . . . . . . . . . 26  Figure 2.1. Acoustic spectra of stimuli used in this study (AM, AM/FM, and AM2) for carrier frequencies of 0.5, 1, 2, and 4 kHz. Y axis ticks represent 20 dB intervals. . . . . . . . . . . 43  Figure 2.2. Mean amplitudes (±1 SD) for all stimuli (AM, AM/FM, and AM2) in all conditions (MS, MM, and DM) at both intensities (60 and 80 dB HL). . . . . . . . . . . . . . . . . . . . . . . 51  Figure 2.3. Mean (±1 SD) relative efficiency (RE) values for all stimuli in MM and DM conditions at both intensities. Double asterisks over individual bars indicate values are significantly higher or lower than 1; single asterisks indicate nearly significant trends. Dashed line represents RE of MS condition (i.e., 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . .59  vi  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  CR  Circle radius  dB  Decibel  dB HL  Decibels - hearing level  dB SL  Decibels - sensation level  dB SPL  Decibels - sound pressure level  df  Degrees of freedom  DM  Dichotic multiple  EEG  Electroencephalogram  F  Fisher’s F ratio  fc  Carrier frequency  FFT  Fast Fourier transform  fm  Modulation frequency  Hz  Hertz  kHz  kiloHertz (1000 Hz)  MF  Modulation Frequency vii  MM  Monotic multiple  ms  millisecond  MS  Monotic single  µV  microvolt  nV  nanovolt  p  Probability  SD  Standard deviation  0  Epsilon correction factor for degrees of freedom  viii  Acknowledgments I would like to thank my supervisor, Dr. David Stapells, for providing continued guidance and expertise throughout my research process, and for setting such a high standard for my work. My committee members, Susan Small and Anna Van Maanen, provided invaluable comments and suggestions - thank you both. I would also like to thank all members of the HAPLAB, with special thanks going out to Brielle Cuthbert, Ieda Ishida, and Susan Marynewich for their encouragement and helpfulness.  Lastly, I would like to thank my friends and family for their continued rallying and support; especially my parents for their emotional (and financial!) support over the last few years, and my sister Shannon for her generous and ingenious editing of this thesis. You guys are the bestest.  This research was supported by a Natural Sciences and Engineering Research Council (NSERC) of Canada Graduate Scholarship to Lori Wood, and by NSERC grants to Dr. David Stapells.  ix  Chapter 1: Literature review: Auditory Steady-State Responses  1  Introduction Audiology involves the assessment, diagnosis, and treatment of hearing and balancerelated issues. A key component of the assessment process is the attainment of hearing thresholds through behavioural pure-tone testing. However, as behavioural results depend on a subject’s ability and willingness to participate, circumstances arise when this methodology is either not feasible (e.g., infants and other difficult-to-test populations) or desirable (e.g., adults involved in legal or compensation cases). In such instances, physiological methods of estimating hearing thresholds are both essential and invaluable.  Several auditory evoked potential (AEP) methods are available for objectively estimating hearing thresholds in children and adults through the recording of brain waves in response to auditory stimuli. The tone-evoked auditory brainstem response (ABR) is the most widely used and current AEP of choice for children (Stapells, 2000), whereas the slow cortical potential (SCP) is the current AEP of choice for adults (Stapells, 2009). Although both the ABR and SCP generate responses in an objective manner, they typically require subjective response interpretation by an experienced clinician. An alternative AEP providing objectiveness both in response generation and response detection of threshold estimations for children and adults is the auditory steady-state response (ASSR). The ASSR implements computerized statistical methods to automatically detect the presence of evoked responses.  2  This literature review will highlight many of the studies researching ASSRs, specifically focusing on the area of stimulus factors. This will provide the background to my research in multiple brainstem ASSR interactions.  A General Review of ASSRs ASSRs have also been referred to as amplitude-modulated following responses (AMFRs), envelope following responses, and steady-state evoked potentials. Although steady-state auditory evoked potentials were initially recorded by Geisler in 1960 (Geisler, 1960), they were not suggested as an objective means to assess hearing thresholds for another two decades (Galambos, Makeig, & Talmachoff, 1981). A current focus of ASSR research is the clinical optimization of various recording parameters.  ASSRs provide an accurate means of objectively estimating hearing thresholds across the audiometric range (for a review, see: Picton, John, Dimitrijevic, & Purcell, 2003). The differences between ASSR thresholds and behavioural pure-tone thresholds in normal-hearing adults are generally between 5 and 15 dB (Dimitrijevic et al., 2002; Herdman & Stapells, 2001, 2003; Perez-Abalo, et al., 2001; Rance & Rickards, 2002). Physiological thresholds are higher than behavioural thresholds due to the neural synchrony required to detect these responses (Herdman & Stapells, 2001; Lins et al., 1996; Perez-Abalo et al., 2001; Picton et al., 1998).  In order to evoke ASSRs, repeating stimuli are presented at such a rate that the response to any one stimulus overlaps the response(s) to preceding stimuli, creating a periodic waveform  3  (John, Lins, Boucher, & Picton, 1998b; Lins & Picton, 1995). The discrete frequency components of this periodic waveform remain constant in amplitude and phase during stimulus presentation (Regan, 1989, p.35).  Many different stimuli may be used to elicit ASSRs, such as noise bursts, beats, clicks, brief tones, sinusoidal amplitude modulation (AM), sinusoidal frequency modulation (FM), mixed modulation (MM or AM/FM), and exponential envelope modulation (AM2). Until recently, sinusoidal AM was the most commonly used stimulus; however, there is growing interest in the use of AM/FM (a combination of sine-AM and frequency modulation) and AM2 (AM stimuli with a steeper rise and fall slope), as these more complex stimuli have been shown to enhance response amplitudes above that generated by the sinusoidal AM stimulus (Cohen, Rickards, & Clark, 1991; John, Dimitrijevic, & Picton, 2002a). These new stimuli both have broader frequency spectra than AM (especially AM/FM), however, there are relatively few data for them.  ASSR stimuli may be presented to subjects in one of three conditions: individually (monotic single; MS), multiple stimuli to one ear (monotic multiple; MM), or multiple stimuli to in both ears (dichotic multiple; DM). Carrier frequencies (CFs) of 0.5, 1, 2, and 4 kHz are commonly utilized. In order to evoke ASSRs, CFs are modulated in the amplitude domain, the frequency domain, or both (Cone-Wesson & Dimitrijevic, 2009). A wide range of modulation frequencies (MFs) may be used, although rates between 10-200 Hz have been most often investigated. Because of the tonotopic representation of the cochlea, CFs are processed by  4  representative regions of the cochlea; responses are initiated in these regions at their corresponding MFs (John & Purcell, 2002). In contrast to ABRs, ASSRs may be recorded to multiple simultaneous stimuli at different MFs to one or both ears, potentially speeding up testing time (Herdman & Stapells, 2001; John, Lins, Boucher, & Picton, 1998a).  Generators of the ASSR Knowledge of intracerebral sources of steady-state responses is necessary for interpreting ASSR results (Cone-Wesson & Dimitrijevic, 2009; Herdman et al., 2002a). ASSRs have multiple generators, the contribution of each being dependant on the MF utilized (Herdman et al., 2002a). The two most widely investigated MFs include those centering around 40 and 80 Hz, as these two regions exhibit an enhancement in amplitude compared to other MFs (Cohen et al., 1991; Galambos et al., 1981; Rickards & Clark, 1984). Additional research has focused on ASSRs to lower rates (i.e., <20 Hz) where there may also be an augmentation of the response (Campbell, Atkinson, Francis, & Green, 1977; Picton, et al., 1987; Wong & Stapells, 2004).  Slow Cortical ASSR (<20 Hz MF) Although this response may be difficult to separate from background noise, it is not impossible to record (Herdman et al., 2002a; Picton et al., 1987; Wong & Stapells, 2004). A study using brain electric source analysis (Herdman et al., 2002a) suggested responses to <20 Hz stimuli originated from the combined activation of both brainstem and auditory cortex sources.  5  Early Cortical ASSR (40 Hz MF) Herdman et al. (2002a) also investigated the neural sources of ASSRs to 40 Hz, and concluded that although the response has both brainstem and cortical generators, the auditory cortex is the primary source (Herdman et al., 2002a). Electrophysiological analyses of ASSRs to 40 Hz by Johnson et al. (Johnson, Weinberg, Ribary, Cheyne, & Ancill, 1988) and Kuwada et al. (Kuwada et al., 2002) suggested they might be generated in the auditory cortices and the thalmocortical circuits. Several magnetoencephalographic studies further identified these cortical generators as originating within the supratemporal gyrus (Gutschalk et al., 1999; Hari, Hamalainen & Joutsiniemi, 1989; Mäkelä & Hari, 1987; Pantev et al., 1993; Pantev, Roberts, Elbert, Ross, & Wienbruch, 1996).  Brainstem ASSR (80 Hz MF) Recent studies investigating human and animal neural sources of ASSRs to 80 Hz indicated they originate primarily from brainstem structures (Herdman et al., 2002a; Kuwada et al., 2002; John & Picton, 2000a; Mauer & Döring, 1999). A brain source analysis of the 88-Hz ASSR suggested a midline brainstem source, along with a minor cortical contribution (Herdman et al., 2002a). Although not yet confirmed, it is quite likely that the 80-Hz ASSRs are actually ABR waves V to rapidly presented stimuli (Lins, Picton, Picton, Champagne, & Durieux-Smith, 1995; Stapells, Herdman, Small, Dimitrijevic, & Hatton, 2005).  6  Subject Factors Maturation Although ASSR thresholds to 80-Hz air-conducted stimuli do not change significantly with age during adulthood (Boettcher, Poth, Mills, & Dubno, 2001; Johnson et al., 1988; Muchnik, KatzPutter, Rubinstein, & Hildesheimer, 1993), they do improve with increasing age during infancy (John, Brown, Muir, & Picton, 2004; Lins et al., 1996; Savio, Cardenas, Perez-Abalo, Gonzalez, & Valdes, 2001; Suzuki & Kobayashi, 1984).  It has been found that the 40-Hz response cannot be reliably recorded in young infants (Levi, Folsom, & Dobie, 1993, 1995; Maurizi et al., 1990; Stapells, Galambos, Costello, & Makeig, 1988). Although an enhancement in the ASSR is evident in adults at 40 Hz, Stapells and colleagues (Stapells et al., 1988) found no corresponding 40-Hz augmentation in children aged 3 weeks to 29 months. It has therefore been suggested that maturational changes are responsible for the amplitude peak found near 40 Hz in adults. In contrast, ASSRs at rates near 80 Hz are readily recorded in newborns (Rickards et al., 1994; Lins et al., 1996).  Arousal State Although ASSR amplitudes to 40-Hz stimuli are two to three times larger than those at 80 Hz in adults, they decrease in amplitude considerably during sleep and anesthesia (Galambos et al., 1981; Jerger, Chmeil, Frost, & Coker, 1986; Linden, Campbell, Hamel, & Picton, 1985). ASSRs to 80 Hz, however, are much less affected by state of arousal (Cohen et al., 1991; Levi et al., 1993; Lins et al., 1995), and for this reason much of the current clinical interest has been in  7  the 80-Hz range. The present study also chose to investigate this modulation range, and the remainder of this literature review will focus primarily on 80-Hz ASSRs.  Measurement and Analysis of the ASSR As mentioned above, an important feature of ASSRs is the ability to use statistical methods to objectively determine response presence/absence (Picton et al., 2003). Although several clinical ASSR systems are currently available, the following discussion pertains specifically to the multiMASTER research system (John & Picton, 2000b), the system utilized in the current study.  Steady-state responses have stable amplitudes and phases, and therefore may be recorded in either the time or frequency domain. However, because ASSRs are composed of discrete frequency components and have a repeating fundamental, they are more amenable to measurement in the frequency domain. In order to convert ASSRs to the frequency domain for response measurement, either a Fourier analyzer (Regan, 1966, 1989; Stapells, Linden, Suffield, Hamel, & Picton, 1984) or the Fast Fourier Transform (FFT; Rickards & Clark, 1984) may be employed. Most current systems (including multiMASTER) employ the FFT and measure the amplitude and phase at each MF. The spectrum generated displays vertical lines representing the ASSRs at each of the frequencies at which the CFs were modulated (John & Purcell, 2002).  An electroencephalogram (EEG) recording of brain electrical activity is recorded through scalp electrodes. The ASSR EEG includes not only the activity of interest (i.e., the signal), but  8  also normal physiological and electrical activity (i.e., background noise). The EEG is filtered both to remove low-frequency energy and to deter aliasing, and then amplified for conversion from analog to digital (AD) form without loss of required information or addition of artifact. Because ASSR sources vary as a function of both the MF being used and the information one wishes to acquire, optimal electrode placement may also vary (Herdman et al., 2002a; John & Purcell, 2002; Van der Reijden, Mens, & Snik, 2005). If results from both ears are to be obtained, a midline channel is often necessary. To be consistent with previous research (e.g., Herdman & Stapells, 2001; John et al., 2002a; Picton et al., 1998), and to reduce the possibility of contamination from post-auricular muscle responses (Small & Stapells, 2008), this study positioned the inverting electrode at the nape, the non-inverting electrode high on the forehead at midline, and the ground electrode on the left mastoid.  ASSR detection algorithms are based primarily on the signal-to-noise ratio (SNR); that is, the ASSR signal must be significantly larger than the noise in order for the ASSR to be detected. To determine response presence/absence, amplitudes at the MFs are compared against amplitudes at adjacent frequencies (Picton et al., 2003). In the multiMASTER system, the F-test determines whether the amplitude at the MF exceeds the amplitude of noise in 120 adjacent frequency bins (i.e., 60 bins on either side of the MF), and declares significant responses with a probability of p < .05. The minimum SNR that indicates a response is statistically different from noise is 1.75 (John et al., 2002a).  9  MultiMASTER records noise in terms of circle radius, which represents the 95% confidence interval of the noise for response detection. In other words, if a response amplitude is larger than the circle radius value at that MF, there is a 95% certainty that the response is present (i.e., that the signal is significantly larger than the surrounding background noise).  As with most other evoked responses, ASSRs become more easily detected through the process of averaging. Assuming random background noise, averaging reduces the noise by the square root of the number of samples in the average (Picton, Linden, Hamel & Maru, 1983). The multiMASTER system averages recording “sweeps” together, with sweeps being composed of small segments call “epochs”. Linking epochs into sweeps lengthens the data segments submitted to the FFT, thus increasing the frequency resolution of the amplitude spectra used to evaluate the ASSR (John & Purcell, 2002).  The reduction in noise due to averaging assumes constant noise. However, muscle activity due to subject movement can be very erratic. Artifact rejection discards any samples in which the voltage of an EEG exceeds a predetermined value (e.g., ± 50 µV). As artifact rejection discards the response along with the noise, longer test times may result (Cone-Wesson & Dimitrijevic, 2009); however, the fact that sweeps are divided into epochs allows for the rejection of individual epochs instead of entire sweeps.  Along with normal averaging, weighted averaging is another method used to increase the signal-to-noise ratio (Lütkenhöner, Hoke, & Pantev, 1985; John, Dimitrijevic, & Picton, 2001a).  10  Weighted averaging assigns more emphasis to epochs with lower noise levels compared to those with higher noise levels (John, Dimitrijevic, & Picton, 2003). Because the effects of weighted averaging are not always predictable (John et al., 2001a), it may be prudent to initially use nonweighted averaging and then subsequently re-analyze the data using weighted averaging if this was deemed desirable.  Stopping Criteria Stopping criteria differ depending on the nature of the study. For threshold studies, recordings continue until a response is detected (p <.05). Usually a minimum of at least two consecutive sweeps at significance are required. However, Luts, Van Dun, Alaerts, and Wouters (Luts et al., 2008) recently proposed a minimum of eight sweeps be presented in order to decrease the error rate due to variable recording lengths.  In the absence of a response (p $.05), recordings continue until a predetermined EEG noise criterion is met. More accurate threshold estimations may be obtained with a stricter noise criterion, however testing times will also increase (John, Purcell, Dimitrijevic, & Picton, 2002b; Picton et al., 1983). The noise criterion for 80-Hz threshold studies is commonly set at #20 nV (CR; Dimitrijevic et al., 2002; Herdman & Stapells, 2003; Picton, Dimitrijevic, Perez-Abalo, & van Roon, 2005; Small & Stapells, 2005; Van Maanen & Stapells, 2005), and at #60 nV for 40Hz studies (Fontaine, 2006; Van Maanen & Stapells, 2005).  11  In contrast to threshold studies, accuracy of measures in suprathreshold studies can record ASSRs until a noise criterion is reached, regardless of whether response significance has been met. The noise criterion may not be as strict as that for threshold studies (e.g., #30 nV for 80-Hz recordings), because of the larger response amplitudes obtained at suprathreshold.  For all studies, a minimum and maximum number of sweeps are typically decided upon to ensure both reliability and efficiency of results. Recording times of three minutes per stimulus are typical, however, this may increase to between 10-17 minutes at near-threshold levels (Dimitrijevic et al., 2002; Herdman & Stapells, 2001).  Stimulus Factors Larger amplitude ASSRs are detected more rapidly and at lower intensity levels. As such, determining optimal stimulus settings which will produce increased response amplitudes is essential for efficient threshold estimations. Factors such as modulation rate, intensity, carrier frequency, and rise time all play an important role in ASSR response amplitudes.  Modulation Rate Modulation rate refers to the frequency at which stimuli vary in amplitude, frequency, or both (Purcell & Dajani, 2008). Depending on the stimulus type, rate may refer to the MF (e.g., for AM or FM stimuli), the fluctuation in amplitude envelope (e.g., for tone pairs), or the frequency of stimulus repetition (e.g., for clicks or tone bursts). Because the majority of stimuli  12  utilize amplitude modulation, rate is commonly referred to as MF. It is this frequency at which the response is evaluated in the EEG spectrum (Purcell & Dajani, 2008).  If all other stimulus parameters are held constant, rate has a definite effect on response amplitude in adults. ASSRs are largest at either very low rates (e.g., <10 Hz) or in the 40-Hz range (Stapells et al., 1984). Another, albeit smaller, peak is evident in the 80-100 Hz range (Lins et al., 1995). Above 100 Hz, ASSR amplitude tends to decrease towards zero and ultimately cannot be distinguished from background noise (Purcell & Dajani, 2008). Infants do not appear to have the same amplitude enhancement in the 40-Hz range, again, likely due to maturational effects (Stapells et al., 1988).  Intensity Intensity refers to the root-mean-square level at which a stimulus is presented (Purcell & Dajani, 2008). Although great individual variability exists, increasing stimulus intensity generally increases ASSR amplitude (Campbell et al., 1977; Galambos et al., 1981; Lins, Picton, Picton, Champagne, & Durieux-Smith, 1995b; Stapells et al., 1984). This amplitude growth tends to be steeper above 60 dB SPL (Picton, van Roon, & John, 2007), and then saturates above 90 dB HL (Picton et al., 2003).  Amplitude growth may be explained by cochlear physiology. Higher stimulus intensities cause a greater spread of energy along the basilar membrane. The subsequent involvement of more hair cells leads to additional activation of afferent nerve fibres, which in turn translates into  13  more input to ASSR generators (Purcell & Dajani, 2008). The end result is an increase in ASSR amplitudes.  Carrier Frequency ASSR amplitudes vary depending upon the CF utilized in each of the main modulation rate ranges. In the 40-Hz modulation range, an increase in CF results in a decrease in response amplitude to AM stimuli (Galambos et al., 1981; Picton et al., 1987; Rodriguez, Picton, Linden, Hamel, & Laframboise, 1986; Ross, Draganova, Picton, & Pantev, 2003; Stapells et al., 1984), resulting in the largest ASSR responses at 0.5 kHz. This is in contrast to the pattern in the 80-Hz modulation range, where ASSR responses to AM stimuli are smallest for 0.5 kHz, largest for 1 and 2 kHz, and decrease again at 4 kHz (John, Dimitrijevic, van Roon, & Picton, 2001b; John et al., 2001b).  Rise Time As previously mentioned, a wide array of stimuli may be used to elicit ASSRs, such as periodically repeating brief tones (John et al., 2003; Mo & Stapells, 2008; Stapells et al., 1984; Stapells, Makeig, & Galambos, 1987), clicks (Galambos et al., 1981; John et al., 2003), beats, noise bursts, and sinusoidally amplitude-modulated tones (Campbell et al., 1977; Herdman & Stapells, 2001; Lins & Picton, 1995; Lins et al., 1995; Lins et al., 1996). A key difference between these stimuli is their rise time, a feature closely linked to response amplitudes. Physiological responses vary significantly with various characteristics of the rise function (Stapells & Picton, 1981; Suzuki & Horiuchi, 1981). In general, steeper slopes or greater changes  14  in slope over time (i.e., acceleration) produce larger and earlier responses, possibly due to greater synchronicity in neural firing (John et al., 2002a). As it is thought that AM envelopes with steeper slopes or greater accelerations will evoke larger steady-state responses, variations on the standard sinusoidal AM stimulus have been developed, such as AM2 tones (i.e., AM tones with a more rapid or exponential envelope; John et al., 2002a, 2004).  Multiple Simultaneous Stimuli As previously mentioned, one of the key advantages of ASSRs over ABRs is the ability of ASSRs to be recorded to multiple stimuli simultaneously presented to one or both ears, thus potentially speeding up testing times (Herdman & Stapells, 2001; John et al., 1998b; Lins & Picton, 1995; Lins et al., 1996). The responses to up to four separate tones per ear can be separated and independently assessed according to their corresponding MFs (Dimitrijevic et al., 2002; Herdman & Stapells, 2001, 2003; John et al., 1998b, 2002b; Lins & Picton, 1995; PerezAbalo et al., 2001; Stapells et al., 2005). Monotic multiple (MM) refers to the simultaneous presentation of four stimuli to one ear, and dichotic multiple (DM) refers to the simultaneous presentation of four stimuli to both ears (i.e., 8 stimuli total).  The multiple-ASSR technique potentially results in faster threshold estimation times compared to the single-stimulus ASSR technique because the amount of available information for a given recording time is increased regardless of whether one or multiple stimuli are being presented (John et al., 2002b; Picton et al., 1983). However, the multiple technique may not  15  prove significantly faster and/or more efficient than the single technique if “interactions” resulting in reduced response amplitudes occur (see below).  Relative Efficiency Recording multiple responses simultaneously in one session may result in a significant reduction in recording time, as many responses can be recorded in the same time that it takes to record one. However, due to “interactions”, or decreases in response amplitudes when stimuli are presented simultaneously as compared to when they are presented alone, testing times may not be reduced as significantly as first thought. If amplitudes do not decrease, the multiple method is faster by a factor of the number of stimuli presented. But even if there is a decrease in amplitude when presenting stimuli simultaneously, testing multiple stimuli may still be more efficient than testing each individually. The “relative efficiency” measure estimates whether faster testing times arise from using multiple versus single stimulus presentation (Herdman & Stapells, 2001; John et al., 1998a).  As previously mentioned, background noise in an EEG recording decreases at the rate of the root number of sweeps averaged (John et al., 2002b), and the decrease in noise is consistent regardless of the number of stimuli being presented. Therefore, for multiple stimuli recordings to be more efficient than testing single stimuli independently, any decrease in ASSR amplitude when simultaneously presenting multiple frequencies must not be more than 1/%N, where N is the number of stimuli (John et al., 1998a). For example, in order for four simultaneous stimuli to be more efficient than when each is presented alone, amplitude reductions of the multiple  16  responses cannot exceed 50% of the amplitudes in the single condition. Likewise, the amplitude cannot decrease more than 65% of the single condition amplitude to declare an eight-stimulus multiple condition more efficient than single-stimulus presentation.  The separation between modulation rates of adjacent carrier frequencies may have an effect on the responses to multiply presented tones. It has been ascertained that the presentation of MFs separated by at least 1.3 Hz in adjacent carrier frequencies will not attenuate responses to multiple stimuli in the 80-Hz range (John et al., 1998a). Modulation rate separations of 7 to 9 Hz have commonly been used in 80-Hz threshold studies with no adverse amplitude effects (Armstong, 2006; Herdman & Stapells, 2001; Lins & Picton, 1995; Van Maanen and Stapells, 2005)  ASSRs to 80-Hz multiple simultaneous stimuli were first investigated by Lins and Picton in 1995. No significant decrease in response amplitude was found at 60 dB SPL when four AM tones (0.5, 1, 2, 4 kHz) were presented simultaneously in one or both ears versus when they were presented alone, as long as the CFs were separated by at least one octave (Lins & Picton, 1995). John et al. (1998a) also found no significant change in amplitude when four 80-Hz AM stimuli in one ear were separated by at least one octave at 60 dB SPL.  Herdman and Stapells (2001) studied the effects of multiple ASSR stimuli when presented at different intensities. Their study confirmed that, for 30 and 60 dB SPL, the amplitude of responses to multiple AM stimuli presented simultaneously are not significantly  17  different from those when the stimuli are presented alone, provided all carrier frequencies are at least one octave apart (Herdman & Stapells, 2001). Attenuated responses were found when stimuli were presented simultaneously at higher intensities (e.g., 75 dB SPL) by John et al. (1998b). This was thought to be due to destructive interference of responses resulting in attenuation of the response amplitude, especially to lower carrier frequencies (John et al., 1998b).  The effect of intensity on RE is shown by Armstrong’s 2006 study. At high intensities (i.e., 80 dB SPL), ASSRs to multiple AM tones were not more efficient than ASSRs to single AM tones for ASSRs to 14-, 40-, and 80-Hz modulation rate ranges (Armstrong, 2006). John et al. (2002b) used previous data (Dimitrijevic et al., 2002; Herdman & Stapells, 2001; John et al., 2001, 2002a) of multiply-presented AM, AM/FM, and AM2 tones (#60 dB SPL) in order to estimate the multiple-stimulus technique (DM) to be two to three times faster than testing each frequency and ear separately (John et al., 2002b). Most recently, Jenny Hatton's MSc thesis (Hatton, 2008) found that infants show significant interactions at even 60 dB SPL. Despite these interactions in infants, the multiple stimulus ASSR remained the most efficient technique at this intensity.  It should be noted that all of the previously mentioned studies investigated response interactions using only AM stimuli. Hence, we have no idea if the broader spectra of AM/FM and AM2 stimuli result in greater interactions and thus less efficiency.  18  Physiology Underlying Interactions Between ASSRs to Multiple Simultaneous Stimuli As alluded to above, interactions refer to changes in the amplitude of one or more response(s) when multiple stimuli are presented simultaneously versus singly (John et al., 1998a). While most interactions between responses to different stimuli are inhibitory, it is also possible for one stimulus to enhance or sensitize the response to another (Dolphin & Mountain, 1993).  The most common interactions result in an attenuation of the responses to lowerfrequency stimuli when presented with stimuli of higher frequency, and an enhancement of the responses to higher-frequency stimuli when presented with lower-frequency stimuli. These changes in amplitude indicate physiologic interactions occurring in the cochlea and/or auditory nervous system (John et al., 2002b). Below is an explanation behind interactions at each of these locations.  Cochlear Interactions Components of an input signal composed of multiple stimuli often interact in the cochlea, and almost always in a suppressive manner (Geisler, 1998). Although there is no limit to the number of stimulus components that can interact within the cochlea, this phenomenon has become known as “two-tone” suppression because the majority of studies investigating it have used two tones. Two-tone suppression has been proven to be a cochlear phenomenon because it persists even after sectioning the auditory nerve (Kiang, Watanabe, Thomas, & Clark, 1965).  19  The basic characteristics of two-tone suppression can be accounted for by the compressive nature of the outer hair cells (OHCs). The active process of the cochlea, generated by the OHCs, is called the cochlear amplifier. The cochlear amplifier serves to refine the sensitivity and frequency selectivity of the mechanical vibrations of the cochlea. Because of the non-linear processing in the cochlea, responses to simultaneously-presented stimuli can physically interact on the basilar membrane, resulting in constructive and/or destructive interference (John et al., 1998a). Destructive interference occurs when a suppressor tone “jams” the cochlear amplifier. The frequency separation of the stimulus tones is crucial: only if a suppressor’s response peak lies within the amplification zone of the probe tone along the basilar membrane is the suppressor able to affect the probe tone’s amplification process (Geisler, 1998).  Two-tone suppression may be classified as either “low-side” or “high-side” (Ruggero, Robles, & Rich, 1992; Sachs & Kiang, 1968). If the suppressor tone has a frequency below that of the probe tone CF, low-side suppression is said to occur, and, accordingly, if the suppressor has a frequency greater than the probe tone CF, high-side suppression is said to occur. It takes less intensity to cause high-side suppression (Sachs & Kiang, 1968). Two-tone suppression is not the same phenomenon as masking, whereby low-frequency tones attenuate high-frequency tones, and not vice versa (Moore, 1985).  The fact that no interactions were present when simultaneous multiple stimuli were presented to adults at #60 dB SPL (Herdman & Stapells, 2001; Lins & Picton, 1995) suggests no overlap of excitation patterns occurs at or below this intensity, at least not to the extent that OHC  20  suppression occurs. In contrast, the findings of interactions between responses to simultaneously presented stimuli at intensities >60 dB SPL and/or at half-octave separations (John et al., 1998a; Lins & Picton, 1995) suggests an overlap of excitation patterns along the basilar membrane resulting in destructive interference. This is not surprising, as the bandwidth of the cochlear filter increases with increasing sound pressure levels (Moore, 1993). Therefore, more intense stimuli should interact even when their carrier frequencies are largely separated due to the wider cochlear filters.  It is speculated that the presentation of stimuli with broader spectra (e.g., AM/FM and AM2) may also increase cochlear interactions. Stimuli with an inherently broader spread of acoustic energy may cause greater interference between responses to simultaneous stimuli such that the multiple-stimulus technique becomes less efficient than the single-stimulus technique.  Neural Interactions Responses to multiple ASSR stimuli may also interact subsequent to the cochlea in more neural locations such as the brainstem and/or cortex. John et al. (1998a) assessed neural interactions by to comparing ASSRs at different modulation rates. At 40 Hz, a decrease was seen in response amplitudes when going from single to multiple stimuli at 60 dB SPL; in contrast, no amplitude reduction was evidenced for 80-Hz multiple ASSRs. As this pattern of decreases cannot be accounted for solely by cochlear processes, it was concluded that a neural component exists when presenting multiple stimuli simultaneously at 40 Hz (John et al., 1998a).  21  Neural interactions may also be assessed through binaural stimulation. John et al. (1998b) presented tones in the MM (four tones to one ear) and the DM (four tones to both ears) conditions to adult subjects and found greater reductions in amplitude in the DM condition (John et al., 1998b). Such a result can only be due to binaural interactions at the level of the brainstem or higher in the auditory pathway.  Magnetoencephalography has also been employed to assess neural mechanisms at 40 Hz (Ross et al., 2003). Significant interactions were found between two simultaneously-presented tones at 80 dB SPL. The greatest decrease in ASSR amplitude occurred when the carrier frequency of the interfering tone was higher than that of the test tone.  The effects of either cochlear or neural interactions were illustrated in the Mo and Stapells (2008) study investigating the effects of interfering brief-tone stimuli on response amplitudes. The largest amplitude decrease for 0.5-kHz stimuli resulted from 1-kHz interfering stimuli. The 4-kHz interfering stimuli had the greatest negative effect on the 2-kHz stimulus, but there was also a small inhibitory effect from the 1-kHz interfering stimuli (Mo & Stapells, 2008).  In Picton et al.’s 2009 study, responses at 53 dB SPL were largest for 1 and 2 kHz, but responses at 73 dB SPL were largest for 0.5 and 4 kHz (Picton, van Roon, & John, 2009). The relative sparing of the 4 kHz response at 73 dB SPL may have been due to a form of high-side suppression of either cochlear or central origin (Picton et al., 2009).  22  In contrast to adults, infants display significantly reduced amplitudes in the multiple versus the single stimulus condition at 60 dB SPL (Hatton, 2008). Although the exact mechanisms are not fully understood, infant reductions in amplitudes in the multiple condition may be explained by cochlear and/or neural factors such as broader tuning filters and decreased brainstem processing time (Hatton, 2008).  Frequency Specificity of Stimuli A compromise must be reached between stimuli that have sufficiently rapid onsets to evoke easily recognizable responses and stimuli that are sufficiently frequency-specific to estimate pure-tone thresholds. More rapid onsets lead to larger responses but also decrease the frequency specificity of stimuli (Cobb, Skinner, & Burns, 1978; Hecox, Squires, & Galambos, 1976; Kodera, Yamane, Yamada, & Suzuki, 1977). The frequency specificity of a stimulus may be looked at in terms of its acoustic specificity, cochlear place specificity, or the frequency specificity of the central auditory neurons.  Acoustic Specificity The acoustic specificity of a stimulus depends on the amount of spectral splatter around the nominal frequency (Durrant, 1983). A large amount of spectral splatter causes activation of cochlear regions other than the frequency of interest, possibly resulting in an underestimation of the threshold for the target frequency (Herdman, Picton, & Stapells, 2002b). An increase in stimulus intensity also causes greater spread of activation as acoustic side lobes become more intense and exceed the thresholds at their respective frequencies. Besides rise/fall time, other  23  factors determining frequency specificity include stimulus duration, gating, the transfer function of the transducer, and the resonant properties of the acoustic coupler (Burkard, 1984; Durrant, 1983; Harris, 1978; Nuttal, 1981).  Clicks are broad-band stimuli which elicit relatively large ASSRs and ABRs (John et al., 2003; Stürzebecher, Cebulla, & Neumann, 2003). However, because they contain energy across a wide-frequency spectrum, they are not very frequency-specific, and usually show more energy at the harmonics of the stimulus rate (Mo & Stapells, 2008; Picton et al., 2005). Beats, although quite frequency-specific (i.e., they contain acoustic energy at only two points in the spectrum), produce response amplitudes of only 70% of those to sinusoidal AM (Picton et al., 2005).  Cochlear Place Specificity Cochlear place specificity refers to the ability of a stimulus to activate only discrete regions of the cochlea along the basilar membrane. This is both facilitated and countered by the upward and downward spread of activation present in the cochlear basilar membrane. Upward spread of excitation results in displacement of basal cochlear regions which have characteristic frequencies above the stimulus’ spectral components (Dallos, 1996), causing the possible underestimation of hearing thresholds; conversely, stimulation of high frequency sidelobes may result in overestimation of hearing thresholds. It is important to note that few, if any, studies have investigated place specificity of AM/FM and AM2 stimuli for moderate-to-high (i.e., >60 dB SPL) stimuli (Herdman et al., 2002b).  24  AM tones demonstrate both high acoustic frequency specificity and reasonable place specificity (Hartmann, 1977; Herdman et al, 2002b; Herdman & Stapells, 2003; John et al., 2002a; Stapells et al., 2005). Derived-response analysis has shown that ASSRs to sinusoidally amplitude-modulated tones have place specificity similar to that of brief tones (Herdman et al., 2002b). Using AM/FM or AM2 rather than AM decreases the frequency specificity of the response because the spectra for the AM/FM and AM2 stimuli spread more widely than the spectrum of the AM tone. A spectral comparison of these three stimuli can be seen in Figure 1.1, where the -20 dB bandwidth for 2-kHz stimuli are 174, 653, and 370 for AM, AM/FM, and AM2, respectively.  Specificity of Central Auditory Neurons The frequency specificity of central auditory neurons is tied to the activation patterns of the basilar membrane through the tuning of cochlear filters (Pickles, 1988). Central neurons primarily activated by fibres emanating from the same primary cochlear filter have good frequency specificity; however, central neurons activated by converging fibres emanating from a range of cochlear filters have broader tuning curves (Rhode & Greenberg, 1992). Herdman et al., (2002b) found that ASSRs to stimuli with MFs near 80 Hz have broader frequency specificity than primary auditory neurons.  25  20 dB  SINE AM  AM/FM  SINE AM2  0.1  1  10  FREQUENCY (kHz) Figure 1.1. Acoustic spectra of stimuli used in this study (AM, AM/FM, and AM2) for carrier frequencies of 0.5, 1, 2, and 4 kHz. Y axis ticks represent 20 dB intervals.  26  Stimuli Used in Current Study As previously mentioned, threshold estimation can be improved if the amplitude of a response is increased without significantly affecting its frequency specificity. Currently, the two most common approaches to achieving this goal are the use of “mixed modulation” and “exponential” envelopes. As such, this study compared amplitudes obtained from AM, AM/FM and AM2 stimuli.  Sinusoidal AM The most common stimulus used to date has been continuous sinusoidally amplitudemodulated (AM) tones. Sinusoidally AM stimuli are created by multiplying or adding two sine waves together; the sine wave with the higher frequency becomes the carrier (fc), and the sine wave with the lower frequency becomes the modulating envelope (fm; John et al., 1998b). The high acoustic frequency specificity of this stimulus is due to the presence of spectral energy only at fc ± fm. The modulation depth for AM tones is usually set to 100%, which results in the amplitude envelope decreasing to zero for every cycle, and the amplitude of the sidebands to be 50% of the carrier (John & Purcell, 2002).  Several studies have investigated ASSRs to AM tones in the 80-Hz modulation range in addition to those previously outlined (see Relative Efficiency section). John et al. (2002b) investigated ASSRs to AM stimuli modulated at 80 Hz in the MM condition at intensities less than 50 dB SPL. They found responses to low-frequency stimuli to be attenuated by the presence of higher frequency stimuli (John et al., 2002b). This corresponds to an earlier finding stating that  27  responses to CFs between 1 and 3 kHz were generally larger than those outside this range for AM stimuli (John et al., 2001b).  Picton et al. (2009) found that amplitudes for 1 and 2 kHz decreased in the DM condition as compared to the MS condition, but there were no differences in amplitudes between the DM and MM conditions. At 53 dB SPL, the largest responses were present at 1 and 2 kHz; at 73 dB SPL, the largest responses were at 0.5 kHz, and MM and DM responses were significantly smaller than MS responses, especially at 1 and 2 kHz (Picton et al., 2009).  Mixed Modulation (AM/FM) Stimuli created through mixed modulation have both amplitude and frequency modulation occurring at the same modulation rate. Sidebands for the AM component are at fc ± fm, and for the FM component are at fc ± integer multiples of the fm (John et al., 2001b). Modulation depths are commonly set to 100% for amplitude and 20-25% for frequency (Cohen et al., 1991; John et al., 2003). John et al. (2001b) used 25% FM to determine the optimal FM phase setting to obtain the largest percent of responses detected (John et al., 2001b). A setting of 25% FM indicates stimulus fluctuation of ±12.5% from the CF (John & Picton, 2000b).  It has been proposed that an AM/FM stimulus may produce larger amplitudes due to two possible mechanisms: (1) the increased spread of spectra energy present with AM/FM stimuli (see Figure 1.1) may cause more neurons to be activated (Cohen et al., 1991); and/or (2) the AM and FM components may evoke independent responses, the addition of which would result in a larger  28  combined response (John et al., 2001b, 2004). Psychophysical and neuromagnetic studies have indicated that certain systems in the auditory neural pathways are selectively sensitive to frequency change, rather than amplitude modulation (Kay & Matthews, 1972; Tansley & Regan, 1979; Tansley, Regan, & Suffield, 1982; Tansley & Suffield, 1983; Rees & Kay, 1985; Mäkelä, Hari, & Linnankivi, 1987).  Because the maximum amplitude of the FM response occurs slightly earlier than that of the AM response, the phase of the FM modulation envelope is adjusted to ensure the maximum combined response is obtained (Cohen et al., 1991; John et al., 2001b). However, it must be noted that the optimal relative phase between AM and FM may vary with each subject and carrier frequency, and will have to be adjusted accordingly. In addition to this phase adjustment, a frequency adjustment is also necessary because peaks do not occur exactly at desired CF values (see asymmetrical AM/FM spectra in Figure 1). In order for the maximum energy of the spectra to occur at 0.5, 1, 2, and 4 kHz, Dimitrijevic et al. (2002) shifted CF values to 0.5, 0.92, 1.85, and 3.81 kHz (Dimitrijevic et al., 2002). Because stimulus parameters for the current study differed slightly from Dimitrijevic et al.’s, our CF values were adjusted to 0.5, 0.96, 1.92, and 3.9 kHz for 0.5, 1, 2 and 4 kHz, respectively.  Cohen, Rickards, and Clark (1991) originally showed that the use of simultaneous amplitude and frequency modulation produced larger responses in adults than when presenting AM stimuli alone. Their 80-Hz data indicated that responses to AM/FM stimuli were significantly larger than responses to AM stimuli at 2 and 4 kHz for intensities less than 55 dB HL (Cohen et al., 1991).  29  John and Picton (2000a) confirmed Cohen et al.’s findings, reporting that AM/FM stimuli with 25% FM at 50 dB SPL evoked responses that reached significance almost twice as fast as AM stimuli (John & Picton, 2000). When John et al. (2001b) compared responses to AM and AM/FM tones in the MM condition at 50, 40, and 30 dB SPL, they found an enhancement in AM/FM at the middle CFs over the lower and higher frequencies. The average AM/FM responses were 27, 40, and 24% larger than the AM responses at 50, 40, and 30 dB SPL, respectively (John et al., 2001b). John and colleagues later demonstrated 20% larger responses for both adults (at CFs of 0.5, 1, and 2 kHz) and infants (at CFs of 1 and 2 kHz) when using AM/FM versus AM stimuli in the DM condition (John et al., 2004).  Dimitrijevic et al. (2002) estimated the audiogram using multiple (DM) ASSRs to AM/FM stimuli modulated in the 80 Hz range. Similar to other studies comparing results to behavioural thresholds, Dimitrijevic et al. found the largest discrepancy for the 0.5 kHz stimulus (Aoyagi et al., 1994; Rance, Rickards, Cohen, DeVidi, & Clark, 1995; Lins et al., 1996; Herdman & Stapells, 2001; Perez-Abalo et al., 2001). The decrease at 0.5 kHz may be related to issues of neural synchrony. The broader region of activation on the basilar membrane at this frequency versus at higher frequencies (due to the tonotopic distribution of the cochlea) may result in more latency jitter in the responding neurons, which would decrease the time-locked summation of responses (Dimitrijevic et al., 2002). This effect is compounded by the greater spectral spread resulting from using AM/FM stimuli.  30  Exponential Envelope Modulation (AM2) An exponential envelope modulated stimulus consists of a sine wave envelope raised to a power of N (i.e., sinN; John et al., 2002a). Increasing the value of N results in a decrease in tone duration, and a corresponding decrease in rise time. Because response amplitudes do not increase appreciably beyond N=2, AM2 has been chosen as the optimal exponential envelope (John et al., 2002a). AM2 stimuli are easier to setup than AM/FM stimuli because there does not need to be any adjustment of the relative phase or frequency and there is no concern that in certain subjects the optimum relative phases of the AM and FM components may differ from normal values (John et al., 2004).  Altering the envelope of a stimulus has the following effects: (i) it increases the maximum slope and the maximum acceleration of the stimulus, (ii) it decreases the durations at which the slope and the acceleration are near their maximum value, (iii) it changes the timing of these maxima so that they occur later within the cycle, and (iv) it increases the duration when the stimulus is below half its maximum amplitude (John et al., 2002). The increased periods of low level sound between successive peaks of the modulation envelope (see Figure 1.1) increase the likelihood that refractory periods (i.e., periods of neuronal response to each stimulus presentation) will end before the start of the next stimulus presentation (John et al., 2002a).  John, Dimitrijevic, and Picton (2003) used noise and tones with exponential envelopes to obtain ASSRs. In all cases, the ASSRs for the exponential modulations were larger than those for either AM or AM/FM tones. John et al. (2004) compared infant ASSRs to 80-Hz modulated AM,  31  AM/FM and AM2 stimuli in the DM condition at 50 dB SPL. They found that AM/FM and AM2 stimuli produced responses 15% larger than AM stimuli in newborns, and 17% larger than AM stimuli in older infants.  The amplitude enhancement by this stimulus is likely caused by the steeper slopes of AM2 envelopes, which increase the synchrony of the neural responses (John et al., 2002a, 2004). John and colleagues (John et al., 2002a) demonstrated that the AM2 stimulus increased response amplitudes over those from the AM stimulus by 39% at 35 dB pSPL and by 18% at 55 dB pSPL in the DM condition (CFs: 0.5-6 kHz). Contrary to what is found with AM/FM stimuli, AM2 stimuli tend to produce larger responses at lower (e.g., 0.5 kHz) and higher (e.g., 4 kHz) CFs in both adults and infants (John et al., 2002a, 2004). This increase at lower CFs (i.e., <1 kHz) might be due to individual waves of the stimulus activating responses more quickly to exponential envelopes. The response enhancement at higher CFs (e.g., >2 kHz) may be the result of responses following the overall shape of the stimulus envelope (John et al., 2002a).  A decrease in stimulus duration may affect response amplitude in the following ways: (1) it causes a wider acoustic frequency spread which stimulates a greater number of neural elements; (2) it causes an increase in the rise time, resulting in greater neural synchrony; and (3) it results in longer silent periods between stimuli, allowing for more response recovery and thus larger amplitudes (John et al., 2002a).Mo and Stapells (2008) used brief tones to investigate the effect of stimulus duration on ASSR amplitudes. When brief tone durations were set under three or four cycles, a significant amplitude increase was evident for 0.5 and 2 kHz tones presented alone. When presented  32  simultaneously with other stimuli, the 2 kHz response still increased with decreasing duration; however, the 0.5 kHz stimulus displayed no change as duration decreased (Mo & Stapells, 2008).  No study has directly compared ASSRs to AM/FM and AM2 stimuli in adults. It is also important to note that previous AM/FM and AM2 studies have not made direct comparisons of single versus multiple responses for all CFs (i.e., 0.5, 1, 2, and 4 kHz). Nor have AM/FM or AM2 ASSRs been fully investigated at higher intensities (e.g., 60-80 dB SPL). The current study addressed all of these previous shortcomings.  Summary and Rationale for the Study The clinical application of ASSRs, especially for threshold estimation in infants and young children, has increased tremendously in recent years. This is evidenced by the existence of several commercially available systems for recording ASSRs (as reviewed in Cone-Wesson & Dimitrijevic, 2009). An emerging concern is the lack of standardization among the different systems. Some of these systems are fairly closely based on the equipment and techniques used in much of the foundational ASSR research; however, many are not (Stapells et al., 2005; D’haenens et al., 2007). As researchers, clinicians, and equipment companies begin to use various multiple stimuli to elicit ASSRs, it is important to better understand the effects of stimulus factors on subsequent responses. Most current ASSR systems use AM or AM/FM stimuli. Even though new stimuli (e.g., AM2) are being implemented, there are no data to indicate their effects on response interactions and/or testing efficiency.  33  The effects of stimulus rise time, intensity, and single versus multiple presentation all need to be investigated. Increases in intensity result in greater activation of the basilar membrane, possibly increasing response interactions. Rise time affects frequency specificity, and may contribute to response interactions even at lower intensities. The presentation of simultaneous stimuli may produce such interactions that the multiple-stimulus technique might become less efficient than the single-stimulus technique with new stimuli. Very few studies have obtained single- versus multiple-stimuli data, especially for new stimuli.  34  Chapter 2: Multiple brainstem auditory steady-state response interactions for different stimuli  35  Introduction Auditory steady-state responses (ASSRs) are an objective, efficient, and frequencyspecific means by which to test hearing thresholds (Picton, John, Dimitrijevic, & Purcell, 2003). As such, ASSRs may be useful in the assessment of individuals who are unable to respond behaviourally (e.g., infants) or those who choose not to cooperate (e.g., medical-legal cases). ASSRs employ statistical techniques to objectively determine if a response is present, as opposed to subjective analyses of waveforms by clinicians (John & Picton, 2000b; Stapells, Herdman, Small, Dimitrijevic, & Hatton, 2005).  ASSRs provide an accurate means of objectively estimating hearing thresholds across the audiometric range (for a review, see: Picton et al., 2003). The differences between ASSR thresholds and behavioural pure-tone thresholds in adults are generally between 5 and 15 dB (e.g., Dimitrijevic et al., 2002; Herdman & Stapells, 2001, 2003; Perez-Abalo et al., 2001; Rance & Rickards, 2002).  In order to evoke ASSRs, repeating stimuli are presented at such a rate that the response to any one stimulus overlaps the responses to preceding stimuli, creating a periodic waveform (John, Lins, Boucher, & Picton, 1998b; Lins & Picton, 1995). The discrete frequency components of this periodic waveform remain constant in amplitude and phase during stimulus presentation (Regan, 1989, p.35).  36  A wide array of stimuli may be used to evoke ASSR responses, such as clicks, beats, noise bursts, tone pips, sinusoidally amplitude-modulated tones (AM), frequency-modulated tones (FM), mixed-modulation tones (AM/FM), and exponentially modulated tones (AM2). ASSR stimuli may be presented to subjects in one of three conditions: individual stimuli in one ear (monotic single; MS), multiple stimuli simultaneously in one ear (monotic multiple; MM), or multiple stimuli simultaneously in both ears (dichotic multiple; DM). Carrier frequencies (CFs) of 0.5, 1, 2, and 4 kHz are commonly utilized.  In order to evoke ASSRs, CFs are modulated in either the amplitude or frequency domain, or both (Cone-Wesson & Dimitrijevic, 2009). A wide range of modulation frequencies (MFs) may be used (e.g., 10-200 Hz), but the two most studied regions are 40 and 80 Hz. Research into intracranial sources has shown that ASSRs to MFs in the 40-Hz range correspond primarily to activity in the auditory cortical areas of the brain, whereas ASSRs to MFs in the 70110 Hz range correspond primarily to brainstem activity (Herdman et al., 2002a; Mauer & Döring, 1999; Wong & Stapells, 2004). Although the 40-Hz ASSR cannot be reliably recorded in young infants (Levi, Folsom, & Dobie, 1993, 1995; Maurizi et al., 1990; Stapells, Galambos, Costello, & Makeig, 1988), ASSRs to rates near 80 Hz are readily recorded in newborns (Rickards et al., 1994; Lins et al., 1996).  An important feature of ASSRs is the ability to use statistical methods to objectively determine response presence/absence (Picton et al., 2003). Because ASSRs are composed of discrete frequency components and have a repeating fundamental, they are amenable to  37  measurement in the frequency domain. Either a Fourier analyzer (Regan, 1966, 1989; Stapells, Linden, Suffield, Hamel, & Picton, 1984) or the Fast Fourier Transform (FFT; Rickards & Clark, 1984) may be employed to convert ASSRs to the frequency domain. The multiMASTER research system (John & Picton, 2000b) employs the FFT to measure the amplitude and phase at each MF. The spectrum generated displays vertical lines representing the ASSRs at each of the frequencies at which the CFs were modulated (John & Purcell, 2002).  ASSR detection algorithms are based primarily on the signal-to-noise ratio; that is, the ASSR signal must be significantly larger than the noise in order for the ASSR to be detected. To determine response presence/absence, amplitudes at the MFs are compared against amplitudes at adjacent frequencies (Picton et al., 2003). In the multiMASTER system, the F-test determines whether the amplitude at the MF exceeds the amplitude of noise in 120 adjacent frequency bins (i.e., 60 bins on either side of the MF), and declares significant responses with a probability of p < .05.  As with most other evoked responses, ASSRs become more easily detected through the process of averaging. Assuming random background noise, averaging reduces the noise by the square root of the number of samples in the average (Picton, Linden, Hamel & Maru, 1983). The multiMASTER system averages recording “sweeps” together, sweeps being composed of smaller segments called “epochs”. Linking epochs into sweeps lengthens the data segments submitted to the FFT, thus increasing the frequency resolution of the amplitude spectra used to evaluate the ASSR (John & Purcell, 2002).  38  One of the key advantages of ASSRs is the ability of responses to be recorded to simultaneously presented (i.e., multiple) stimuli. Indeed, ASSRs may be evoked by simultaneously presenting at least four separate tones per ear (Herdman & Stapells, 2001; John, Lins, Boucher, & Picton, 1998b; Lins, Picton, Picton, Champagne, & Durieux-Smith, 1995; Lins et al., 1996), and the responses to these multiple stimuli can be separated and independently assessed according to their corresponding MFs (Dimitrijevic et al., 2002; Herdman & Stapells, 2001, 2003; John et al., 1998b, John, Purcell, Dimitrijevic, & Picton, 2002b; Lins & Picton, 1995; Perez-Abalo et al., 2001; Stapells et al., 2005).  Recording multiple responses simultaneously in one session may result in a significant reduction in recording time as many responses can be recorded in the same time that it takes to record one. However, due to interactions between responses, recording ASSRs to simultaneous stimuli often results in decreased response amplitudes compared to amplitudes when stimuli are presented alone. The “Relative Efficiency” (RE) measure takes both of these factors into consideration when determining whether faster testing times arise from using single versus multiple stimulus presentation (Herdman & Stapells, 2001; John, Lins, Boucher, & Picton, 1998a).  The most common interactions result in attenuation of the responses to lower-frequency stimuli when presented with stimuli of higher frequency, and an enhancement of the responses to higher-frequency stimuli when presented with lower-frequency stimuli. These changes in amplitude indicate physiologic interactions in the cochlea and/or auditory nervous system (John  39  et al., 2002b). Because the bandwidth of the cochlear filter enlarges with increasing sound pressure levels (Moore, 1993), very intense stimuli may interact even when their carrier frequencies are well separated. It is possible that the presentation of stimuli with broader spectra (e.g., AM/FM or AM2) may also increase cochlear interactions.  As previously mentioned, a wide array of stimuli may be used to elicit ASSR responses. A key difference between these stimuli is their rise time, a feature closely linked to response amplitudes. In general, steeper slopes or greater changes in slope over time (i.e., acceleration) produce larger and earlier responses, possibly due to greater synchronicity in neural firing (John, Dimitrijevic, & Picton, 2002a). Larger responses are desired as they are recognized as significant more rapidly and at lower intensities than smaller responses (John, Brown, Muir, & Picton, 2004). Current research and clinical interest has focused on the use of AM/FM and AM2 stimuli.  Using AM/FM stimuli produces significantly larger responses in adults compared to presenting AM stimuli alone (Cohen, Rickards, & Clark, 1991; John, Dimitrijevic, van Roon, & Picton, 2001b). John et al. (2001b) compared responses to AM and AM/FM tones in the monotic multiple (MM) condition at 50, 40, and 30 dB SPL and found an enhancement in AM/FM at the middle CFs over the lower and higher frequencies. The larger amplitudes produced by AM/FM stimuli may be due to two possible mechanisms: (1) the wider frequency spectra may cause more neurons to be activated (Cohen et al., 1991); and/or (2) the AM and FM components may evoke independent responses, the addition of which would result in a larger combined response (John et al., 2001b, 2004).  40  The amplitude enhancement of AM2 stimuli over AM stimuli is likely caused by the steeper slopes of AM2 envelopes, which increase the synchrony of the neural responses (John et al., 2002a, 2004). John et al. (2002a) demonstrated that the AM2 stimulus increased response amplitudes over those from the AM stimulus by 39% at 35 dB pSPL and by 18% at 55 dB pSPL in the dichotic multiple (DM) condition. Contrary to what is found with AM/FM stimuli, AM2 stimuli produce larger responses at lower (e.g., 0.5 kHz) and higher (e.g., 4 kHz) CFs in both adults and infants (John et al., 2002a, 2004).  A tradeoff must often be made between stimuli with faster rise times, producing larger and earlier responses, and those with better frequency specificity (Cobb, Skinner, & Burns, 1978; Hecox, Squires, & Galambos, 1976; Kodera, Yamane, Yamada, & Suzuki, 1977; John et al., 2002a; Mo & Stapells, 2008; Stapells & Picton, 1981). Sinusoidally amplitude-modulated (AM) tones demonstrate high acoustic frequency specificity (Herdman, Picton, & Stapells, 2002b; Herdman & Stapells, 2003) because their spectral power occurs only at the carrier frequency (fc) and at two side bands (fc ± fm; John et al., 2004). In contrast, the spectra for AM/FM and AM2 stimuli spread more widely than sinusoidal AM, causing them to have reduced frequency specificity. Spectral bandwidths for 2-kHz stimuli at -20 dB, as calculated from Figure 2.1, are 174, 653, and 370 Hz for AM, AM/FM, and AM2, respectively.  Stimuli with increased rise times may pose some issues when presented simultaneously and/or at higher intensities. In a recent study conducted by Mo and Stapells (2008), shorter duration stimuli (i.e., brief tones of <12 ms) displayed increased interactions when presented  41  simultaneously at 75 dB SPL. No study has directly compared ASSRs to AM/FM and AM2 stimuli in adults. It is also important to note that previous AM/FM and AM2 studies have not made direct comparisons of single versus multiple responses for all CFs (i.e., 0.5, 1, 2, and 4 kHz). Nor have AM/FM or AM2 ASSRs been fully investigated at higher intensities (e.g., 60-80 dB SPL).  It is predicted that stimuli with broader spectra, while providing increased response amplitudes, will also result in greater response interactions. These interactions may be such that testing individual stimuli will be more efficient than multiple presentation, even at reduced intensities. The purpose of my thesis research was twofold: (1) to compare amplitudes of ASSRs to three stimuli (AM, AM/FM, AM2) in the single and multiple conditions at moderate and high intensities to determine which stimuli elicits the largest amplitudes and whether interactions are greater for stimuli with broader frequency spectra (e.g., AM/FM) and/or faster rise times (e.g., AM2); and (2) to assess whether any reductions in amplitude(s) result in the multiple technique becoming less efficient to the extent that it is actually faster to use single stimuli for the different stimuli investigated.  42  20 dB  SINE AM  AM/FM  SINE AM2  0.1  1  10  FREQUENCY (kHz) Figure 2.1. Acoustic spectra of stimuli used in this study (AM, AM/FM, and AM2) for carrier frequencies of 0.5, 1, 2, and 4 kHz. Y axis ticks represent 20 dB intervals.  43  Methods Subjects Data for 15 subjects (10 females) between the ages of 19 and 40 years (mean = 25 years) were included in this study. Twelve additional participants were excluded due to excessive EEG “noise” (i.e., circle radius noise values exceeded 30 nV after 60 sweeps for three consecutive stimulus presentations). All subjects had pure-tone behavioural thresholds equal to or better than 15 dB HL (re: ANSI S3.6-1996) at 0.5, 1, 2, and 4 kHz for both ears.  Auditory Stimuli For all stimuli, CFs of 0.5, 1, 2, and 4 kHz were used, and modulation rates in the 70-110 Hz range were used, with rates of 77.15, 84.96, 92.77, 100.59 (left ear) and 81.06, 88.87, 96.68, 105.47 (right ear) for 0.5, 1, 2, and 4 kHz, respectively. The modulation rates were chosen to ensure each EEG recording contained an integer number of MF cycles and each recording sweep contained an integer number of CFs (John & Picton, 2000b). Both monotic single (MS) and monotic multiple (MM) stimuli were presented to the left (test) ear only, and dichotic multiple (DM) stimuli were presented to both ears (but results were only considered for the left ear).  Stimuli consisted of: (i) 100% sinusoidally amplitude-modulated tones (AM); (ii)100% sinusoidally amplitude-modulated and 25% frequency-modulated (AM/FM) tones; and (iii) 100% amplitude-modulated tones with exponential (sine2) envelopes (AM2). All stimuli were generated by the Rotman MultiMASTER research system (John & Picton, 2000b), attenuated  44  through Tucker-Davis Technologies (TDT) PA5 attenuators, then routed to a TDT HB7 module and ER-3A insert earphones.  AM/FM stimuli were created with AM of 100% and FM of 25% (John et al., 2001b; John & Picton, 2000b). The FM modulation depth of 25% (i.e., stimulus fluctuation of ±12.5% from CF; John & Picton, 2000b) was chosen because John et al. (2001b) determined it to be optimal for obtaining the largest percent of responses detected (John et al., 2001b). Because of the spectral asymmetry in AM/FM stimuli, CF values were adjusted from 0.5, 1, 2, and 4 kHz to 0.5, 0.96, 1.92, and 3.9 kHz so that the spectral peaks occurred at or close to the octave CF values (see Figure 1), similar to those used by Dimitrijevic et al. (2002). The same values were used for both ears.  The exponential component of the AM2 stimulus was formed by taking only the square of the modulation function prior to its multiplication with the CF (John et al., 2002a). The modulation envelope was based on a function using sinN where N was 2. Figure 1 shows the spectra of AM, AM/FM and AM2 stimuli used in this study. Notice the AM spectra only contains energy at the carrier frequency plus two side-bands, whereas both the AM/FM and AM2 stimuli have broader spectral widths. The asymmetry of the AM/FM stimuli is also evident.  Stimuli were measured in dB peak SPL, converted to dB SPL by subtracting 3 dB, and subsequently calibrated in dB HL (re: ANSI S3.6-1996). A Larson Davis System 824 sound level  45  meter and a G.R.A.S. Sound & Vibration RA0113 2-cc coupler were used in the calibration procedure. All tones were calibrated individually for each carrier frequency.  Stimulus intensities were 60 and 80 dB HL. Although several prior research studies presented stimuli at “equal SPL”, this study used equivalent dB HL levels in order to be consistent with current clinical practices. The differences between dB HL and dB SPL are fairly small: 5.5, 0, 3, and 5.5 dB at 0.5, 1, 2, and 4 kHz, respectively.  Recordings ASSRs were recorded using a single EEG channel. Three gold-cup electrodes were placed on the scalp: the non-inverting electrode was placed high on the forehead at midline, the inverting electrode was placed at the midline of the nape (just below the hairline), and the ground electrode was placed on the left mastoid. All electrode impedances were kept below 3 kΩ at 10 Hz. The EEG signal was band-pass filtered from 30-250 Hz (12 dB/octave) and amplified with a gain of 80,000 (Herdman & Stapells, 2001). Analog-to-digital (A/D) conversion of the EEG recording was performed at 1250 Hz (Small & Stapells, 2004). Sixteen consecutive data epochs of 0.8192 seconds each were linked together to form sweeps of 13.072 seconds. An artifact rejection level of ±60 µV was set to minimize the effect of muscle artifacts due to movement.  Responses were averaged in the time domain and converted on-line to the frequency domain by the MultiMASTER system using a Fast Fourier Transform. Non-weighted averaging in the multiMASTER system was used. Recording continued until EEG noise (circle radius; CR)  46  was reduced to 30 nV or less. In addition to this noise level criterion, a minimum of 12 and a maximum of 60 sweeps were recorded. If the CR did not reach 30 nV after 60 sweeps, the condition was repeated and/or excluded from subsequent data analyses. Although not a stopping criterion, response significance (i.e., p <.05) was also noted.  Procedure After informed written consent was obtained, a standard behavioural hearing threshold test was administered to confirm normal hearing status. EEG electrodes were applied to the scalp. Each subject was asked to relax and/or sleep in a double-walled sound-attenuated booth throughout the recordings. Subjects were given ample opportunity to take breaks between test blocks. A total of 36 test blocks were presented to each subject.  The test blocks were broken up as follows: (1) 24 blocks in MS condition (all four carrier frequencies presented individually to the test ear for all three stimuli at both intensities); (2) six blocks in MM condition (all four carrier frequencies presented simultaneously to the test ear for all three stimuli at both intensities); and (3) six blocks in the DM condition (eight stimuli presented simultaneously -- four carrier frequencies per ear -- for all three stimuli at both intensities). The order of all 36 blocks was randomized across subjects (see below for details). The total testing time was spread across two test sessions, and did not exceed 2.5 hours per session.  47  Stimuli were presented via air conduction through EAR-3A insert earphones. The left ear was the “test ear” for all conditions; right ear results were not considered in the DM condition. All test parameters were randomized across subjects; first the session intensity was chosen (i.e., 60 versus 80 dB HL), then the order of stimulus type (i.e., AM, AM/FM, AM2), and finally the order of conditions within each stimulus (i.e., DM, MM, MS). The second session followed the same randomization procedure for the remaining intensity.  Data Analyses Amplitude Response amplitudes were averaged across subjects for each test block, and analyzed within and between the three test stimuli. Analyses were performed using two- (i.e., stimulus x frequency) and three-way (i.e., stimulus x condition x frequency) repeated-measures ANOVAs (intensities were kept separate to facilitate interpretation of results). Huynh-Feldt kf correction factors for degrees of freedom were used as appropriate for all repeated-measures ANOVAs. Differences in amplitudes were considered significant at the p < .05 level and a trend at the p < .1 level. Newman-Keuls post-hoc analyses (p < .05) were performed for significant main effects and interactions. Because initial analyses revealed few differences between response amplitudes for MM and DM conditions at both 60 and 80 dB HL, we also combined the MM and DM data (i.e., “MULT”) at each intensity.  48  Relative Efficiency Recording multiple responses simultaneously in one session may result in a significant reduction in recording time as many responses can be recorded in the same time that it takes to record one. However, due to interactions, recording ASSRs to simultaneous stimuli often results in decreased response amplitudes compared to amplitudes when stimuli are presented alone. The “Relative Efficiency” (RE) measurement takes both of these factors into consideration when determining whether faster testing times arise from using single versus multiple stimulus presentation (Herdman & Stapells, 2001; John et al., 1998a). RE was calculated for both the MM and DM conditions (note: the RE of the MS condition is always “1"). If the RE is greater than 1 in the MM or DM condition, the multiple-stimulus technique is more efficient than the singlestimulus technique. The RE formula was: (multiple condition amplitude / single condition amplitude) * %N, where N = the number of simultaneously presented frequencies (i.e., 4 or 8; Herdman & Stapells, 2001; John et al., 2002b; John et al., 1998a). T-tests were performed to determine the significance of each of the 48 MM and DM blocks (i.e., 2 intensities x 3 stimuli x 2 conditions x 4 frequencies) relative to the MS value of 1. The Bonferroni correction for p < .05 was used to determine whether MM and DM were significantly greater than 1 (the value of MS); thus p <.001 (i.e., .05/48) for significance and p < .002 (i.e., .1/48) for a trend.  Results ASSR Amplitudes Figure 2.2 shows the average amplitudes for each stimulus (AM, AM/FM, and AM2) grouped by carrier frequency at intensities of 60 and 80 dB HL for all conditions (MS, MM, and  49  DM). In order to simplify the presentation of results, the effect of stimulus type in the MS condition will first be presented.  Monotic Single Condition MS condition amplitudes in response to 60 dB HL stimuli were analyzed using a two-way repeated measures ANOVA (3 stimuli x 4 frequencies). A significant main effect for stimulus was present (F=29.0; df =2, 28; 0=.92; p < .001), with Newman Keuls post-hoc analysis revealing response amplitudes were greatest to the AM/FM stimulus compared to the AM (p = .001) and AM2 (p = .001) stimuli. However, a stimulus x frequency interaction (F=6.3; df=6, 84; 0=.58; p < .001) was also present, with post-hoc analyses showing AM/FM amplitudes significantly larger than AM and AM2 amplitudes at 1, 2, & 4 kHz, but no significant difference in amplitudes between stimuli at 0.5 kHz. Individual stimuli also showed amplitude differences according to frequency: AM responses at 0.5 kHz were significantly larger than 2 (p = .03) and 4 kHz (p = .05); AM/FM responses at 1 and 2 kHz were significantly (p = .020-.023) larger than 0.5 and 4 kHz; and AM2 responses at 4 kHz were significantly smaller than 2 kHz (p = .02). There was no significant main effect for frequency (F=1.97; df=3, 42; 0=.70; p = .155). A twoway repeated measures ANOVA (3 stimuli x 4 frequencies) was performed on the 80 dB HL results for the MS condition, revealing a main effect for stimulus (F=58.0; df=2, 28; 0=1.0; p < .001). The post-hoc analysis of this effect revealed that responses to AM/FM stimuli were larger than those to AM (p = .0001) and AM2 (p = .0001) stimuli. No stimulus x frequency interaction was found (F=2.3; df=6, 84; 0=.67; p = .068), but there was a non-significant trend suggesting  50  MONOTIC SINGLE  MONOTIC MULTIPLE DICHOTIC MULTIPLE  200 AM AM/FM AM2  150  AMPLITUDE (nV)  100  60 dB HL  50 0 150 100  80 dB HL  50 0 0.5  1.0  2.0  4.0  0.5  1.0  2.0  4.0  0.5  1.0  2.0  4.0  FREQUENCY (kHz) Figure 2.2. Mean amplitudes (±1 SD) for all stimuli (AM, AM/FM, and AM2) in all conditions (MS, MM, and DM) at both intensities (60 and 80 dB HL). 51  that the responses to the AM/FM stimuli were greater to the responses to the other stimuli at all frequencies except 0.5 kHz.  Monotic Multiple Condition A two-way repeated-measures (3 stimuli x 4 frequencies) ANOVA performed for the MM condition at 60 dB HL revealed significant main effects for stimuli (F =6.2; df=2, 28; 0=1.0; p = .006) and frequency (F=3.2; df=3, 42; 0=.91; p = .039), and a significant stimulus x frequency interaction (F=4.72; df=6, 84; 0=1.00; p < .001). The post-hoc analysis of the stimulus main effect showed that response amplitudes to AM stimuli were significantly smaller than those to both AM/FM (p = .005) and AM2 (p = .037), with no significant difference between amplitudes to AM/FM and AM2. The post-hoc analysis for the main effect of frequency revealed that response amplitudes at 4 kHz were significantly smaller than those at 2 kHz (p = .044). Posthoc analyses of the stimulus x frequency interaction revealed that response amplitudes to AM stimuli were significantly smaller than those to AM/FM stimuli at 2 kHz (p = .029), and significantly smaller than both AM/FM (p = .0002) and AM2 (p = .05) stimuli at 4 kHz. There were no significant differences between amplitudes for stimuli at 0.5 (p = .09-.67) or 1 kHz (p = .11-.54).  A two-way repeated-measures ANOVA (3 stimuli x 4 frequencies) for the MM condition at 80 dB HL revealed a significant main effect for stimuli (F=5.6; df=2, 28; 0=.99; p = .009). The post-hoc analysis for this effect showed that AM2 response amplitudes were significantly larger than both AM (p = .009) and AM/FM (p = .015) response amplitudes. A significant main effect  52  for frequency (F=39.3; df=3, 42; 0=.64; p < .05) was also present, with a post-hoc analysis revealing significantly larger amplitudes to 4 kHz (p = .0001). Post-hoc analyses of a significant stimulus x frequency interaction (F=14.5; df=6, 84; 0=1.0; p < .001) revealed significant differences between all responses at 0.5 kHz (i.e., AM2 > AM > AM/FM) and significantly larger amplitudes to AM/FM stimuli compared to AM (p = .0009) and AM2 (p = .014) at 4 kHz. The post-hoc analysis of this interaction also revealed that: (i) the responses to AM stimuli were significantly (p = .00012-.00013) larger at 4 kHz than at all other frequencies; (ii) responses to AM/FM stimuli were significantly (p = .00012-.00015) smaller at 0.5 kHz, and significantly (p = .00012-.00013) larger at 4 kHz than at all other frequencies; and (iii) responses to AM2 stimuli were significantly (p = .0012-.0001) larger at 4 kHz than at all other frequencies, and those at 0.5 kHz were significantly (p = .049) larger than those at 2 kHz.  Dichotic Multiple Condition A two-way (3 stimuli x 4 frequencies) repeated measures ANOVA performed for the DM condition at 60 dB HL indicated no main effects for stimulus (F=.69; df=2, 28; 0=.98; p = .51) or frequency (F=1.3; df=3, 42; 0=.93; p = .29). A significant stimulus x frequency interaction (F=4.5, df=6, 84; 0=1.0; p < .001) was present, with post-hoc analyses revealing AM2 amplitudes to be larger than AM/FM amplitudes at 0.5 kHz (p = .004), but not different at other frequencies (p = .44-.67). As well, response amplitudes to AM/FM stimuli at 2 kHz were significantly larger than those at 0.5 kHz (p = .004).  53  At 80 dB HL, main effects were found for both stimulus (F=4.6; df=2, 28; 0=1.0; p = .019) and frequency (F=18.0; df=3, 42; 0=.54; p < .001). Post-hoc analyses revealed AM2 amplitudes were slightly but significantly larger than both AM (p = .021) and AM/FM (p = .027) amplitudes, and 4 kHz amplitudes were significantly (p = .00013-.00031) larger than those at all other frequencies. A significant stimulus x frequency interaction was also present (F=3.9; df=6, 84; 0=.85; p = .003), with post-hoc analyses revealing that AM/FM response amplitudes were significantly smaller than both AM (p = .002) and AM2 (p = .0001) response amplitudes at 0.5 kHz. Individual stimuli also showed differences according to frequency: (i) AM response amplitudes were significantly (p = .00012-.00013) larger at 4 kHz than at all other frequencies; (ii) AM/FM response amplitudes at 0.5 kHz were significantly (p = .00012-.00017) smaller than at other frequencies, and those at 4 kHz were significantly (p = .00012-.00013) larger than at other frequencies; and (iii) AM2 response amplitudes at 4 kHz were significantly (p = .00012.00027) larger than amplitudes at all other frequencies.  Monotic Multiple versus Dichotic Multiple Conditions At 60 dB HL, a three-way repeated measures ANOVA (3 stimuli x 2 conditions x 4 frequencies) was performed comparing the MM and DM conditions. No main condition effect was found (p =.07), but there was a non-significant trend suggesting amplitudes were larger in the MM condition. A stimulus x condition significant interaction (F=3.34; df=2, 28; 0=1.0; p = .049) was present, with post-hoc analyses indicating responses amplitudes to AM/FM stimuli were significantly larger in the MM condition (p = .02), and response amplitudes to AM stimuli  54  were significantly smaller than those to AM/FM (p = .0008) and AM2 (p = .04) in the MM condition.  At 80 dB HL, a three-way repeated measures ANOVA (3 stimuli x 2 conditions x 4 frequencies) revealed no main effect for condition (p = .19) and no significant interactions involving condition (p = .19-.99). Because of the lack of significant condition effects at 80 dB HL and the presence of only one significant interaction at 60 dB HL, the MM and DM data were combined into a “MULT” condition for each intensity. To determine significant differences between amplitudes in the MS condition versus those in the multiple conditions, a three-way repeated measures ANOVA (3 stimuli x 2 conditions x 4 frequencies) was performed at each intensity (see below).  Single vs Multiple Condition At 60 dB HL, a three-way repeated measures ANOVA revealed main effects for stimulus (F=21.9; df=2, 28; 0=1.0; p < .001) and condition (F=13.1; df=1, 14; 0=1.0; p = .003). The posthoc analysis of the stimulus main effect revealed AM/FM response amplitudes to be significantly larger than both AM (p = .0001) and AM2 (p = .0002) response amplitudes. Post-hoc analysis of the condition main effect revealed amplitudes in the MS condition to be significantly larger than those in the MULT condition (p = .003) for all stimuli. A significant stimulus x condition interaction was also present (F=21.3; df=2, 28; 0=.91; p < .001), with post-hoc analyses revealing that the single-condition amplitudes to AM/FM stimuli were significantly (p = .00013.00014) larger than those to other stimuli, and for the multiple condition, amplitudes to AM/FM  55  stimuli were significantly (p = .016) larger than amplitudes to AM stimuli, but not significantly (p = .321) different from amplitudes to AM2 stimuli. Post-hoc analyses of the significant stimulus x frequency interaction (F=9.7; df=6, 84; 0=.62; p < .001) revealed the following: (i) AM/FM response amplitudes were significantly (p = .00012-.00021) larger than those for other stimuli at 1, 2, and 4 kHz; (ii) responses to AM2 stimuli were significantly (p = .038) larger than responses to AM stimuli at 0.5 kHz; and (iii) responses to AM2 stimuli were almost significantly (p = .051) larger than those to AM/FM stimuli at 0.5 kHz.  A significant stimulus x frequency x condition interaction was also present at 60 dB HL (F=2.6; df=6, 84; 0=.80; p = .034). Post-hoc analyses for this complicated interaction revealed that, in the MS condition, there were no significant (p = .13-.67) differences between responses to different stimuli at 0.5 kHz, but in the MULT condition, responses to AM2 stimuli were significantly (p = .008) larger than those to AM/FM stimuli at 0.5 kHz. As well, responses to AM/FM stimuli were significantly (p = .00012-.00020) larger than those to AM and AM2 stimuli at 1, 2 and 4 kHz in the MS condition, but responses to AM/FM stimuli were only significantly (p = .002) larger than those to AM stimuli at 4 kHz in the MULT condition.  At 80 dB HL, a three-way repeated measures ANOVA indicated main effects for both stimulus (F=19.8; df=2, 28; 0=1.0; p < .001) and condition (F=90.2; df=1, 14; 0=1.0; p < .001). Post-hoc analyses revealed significantly (p = .0001-.001) larger response amplitudes to AM/FM stimuli than to either AM or AM2 stimuli (stimulus main effect), and significantly (p = .0002) larger response amplitudes in the MS condition compared to the MULT condition (condition  56  main effect). There was no main effect for frequency (F=3.3; df=3, 42; 0=.68; p = .052), but a trend was evident such that response amplitudes were larger at 4 kHz compared to other frequencies. Post-hoc analysis of a significant stimulus x condition interaction (F=66.0; df=2, 28; 0=1.0; p < .001) revealed AM/FM response amplitudes to be significantly (p = .0001) larger than AM and AM2 response amplitudes in the single condition, and AM2 response amplitudes to be significantly (p = .002) larger than AM and AM/FM response amplitudes in the multiple condition. A significant stimulus x frequency interaction was present (F=6.5; df=6, 84; 0=.77; p < .001), with post-hoc analyses indicating responses to AM/FM stimuli were significantly (p = .0001-.039) larger than those to AM and AM2 stimuli at 1, 2 and 4 kHz, and responses to AM/FM stimuli were significantly (p = .00012-.00037) smaller at 0.5 kHz than at all other frequencies.  Post-hoc analyses of a significant condition x frequency interaction (F=15.5; df=3, 42; 0=.63; p < .001) at 80 dB HL showed that in the MS condition, amplitudes decreased significantly (p = .026) as frequency increased from 1 to 4 kHz, but in the MULT condition, amplitudes increased significantly (p = .00013) as frequency increased from 1 to 4 kHz. Although the stimulus x condition x frequency interaction did not quite reach significance, there was a trend evident (F=2.6; df=6, 84; 0=.60; p = 0.055) such that there was no difference between stimuli at 0.5 kHz in the MS condition, but a near significant difference (i.e., AM2 > AM > AM/FM) between stimuli at this frequency in the MULT condition.  57  Relative Efficiency Significant reductions in amplitude do not necessarily mean the multiple condition is less efficient than the single condition. Recall that as long as the amplitudes are at least 50% that of the single amplitudes in the monotic multiple condition and at least 35% for the dichotic multiple condition, multiple stimuli are more efficient than single.  Figure 2.3 displays the mean RE results for each stimulus at 60 and 80 dB HL. As previously mentioned, the RE value of all stimuli in the MS condition is, by default, “1”; t-tests were used to determine whether RE values in the MM and DM conditions were significantly different from 1.  All RE values obtained for the multiple stimulus conditions and frequencies at 60 dB HL were greater than “1”, indicating that, for 60 dB HL stimuli, the multiple conditions (MM, DM) were never less efficient than the single condition. In fact, 18 out of 24 comparisons (i.e., 75%) showed mean RE values that were significantly (or approaching significance) larger than the MS value of 1 at 60 dB HL. Only the MM RE values for the AM and AM/FM stimuli at 0.5 kHz did not reach or approach significance at this intensity.  At 80 dB HL, most (83%) multiple conditions were larger than 1, however fewer comparisons were significantly so (33%). At 4 kHz, all stimuli showed significantly better efficiency in the multiple conditions than in the single condition. As well, the AM2 stimulus in the DM condition was significantly more efficient than the MS condition at 1 and 2 kHz. It is  58  MONOTIC MULTIPLE  DICHOTIC MULTIPLE ** **  4 ** **  RELATIVE EFFICIENCY  3 * 2  *  **  * ** ** ** **  **  *  ** * ** *  **  ** **  60 dB HL  ** **  1 0 AM AM/FM AM2  4 3  **  **  ** * ** *  2 1  ** **  **  80 dB HL  **  0 0.5  1  2  4  0.5  1  2  4  FREQUENCY (kHz) Figure 2.3. Mean (±1 SD) relative efficiency (RE) values for all stimuli in MM and DM conditions at both intensities. Double asterisks over individual bars indicate values are significantly higher or lower than 1; single asterisks indicate nearly significant trends. Dashed line represents RE of MS condition (i.e., 1).  59  worth noting that four of the RE values for the AM/FM stimuli at 80 dB HL were less than 1 (i.e., worse than the RE in the MS condition) when presented simultaneously; significantly so for 0.5 kHz in the MM condition.  Discussion Amplitudes This study revealed a number of interesting and new findings. In terms of response amplitudes, when stimuli were presented in the single condition, responses were largest to AM/FM stimuli at all frequencies except 0.5 kHz. However, when stimuli were presented in either of the multiple conditions at both intensities, responses to AM/FM were no longer the largest. Responses to AM and AM2 stimuli also showed this interaction, but only at the higher intensity.  Other less obvious yet significant findings included the significantly smaller responses to AM/FM stimuli at 0.5 kHz in both multiple conditions, and the larger responses at 4 kHz versus all other frequencies at 80 dB HL in the multiple conditions but not in the single condition. Overall, it appears that when moving from the single condition to either multiple condition, differences between responses for the different stimuli (when they exist) are much smaller.  To date, no other study has shown responses to be larger for AM/FM stimuli compared to other stimuli at 1, 2, and 4 kHz in the single condition for both 60 and 80 dB HL. However, in the multiple condition (MM and DM), John et al. (2001) revealed that AM/FM stimuli evoke  60  responses one-third larger than AM stimuli at intensities of 30-50 dB SPL for CFs between 0.5 to 5 kHz.  The 4 kHz rise in amplitude with increasing stimuli evidenced in the multiple conditions for all stimuli at 80 dB HL and not at 60 dB HL was similar to what Picton and colleagues (Picton, van Roon, & John, 2009) found with AM stimuli at 73 dB SPL. They postulated it originated from processes which become more apparent at higher intensities. The attenuation of the responses to lower-frequency stimuli when presented with stimuli of higher frequency explain the relative drops in amplitude for 0.5, 1 and 2 kHz. The lack of higher frequency suppression in turn explains the relative sparing of the amplitude to 4 kHz stimuli (John et al., 1998a; Ross, Draganova, Picton,& Pantev, 2003).  In addition to 73 dB SPL, Picton et al. (2009) tested AM stimuli at a lower intensity (53 dB SPL) and found the largest responses in the multiple condition to be at 1 and 2 kHz. Although Picton et al. (2009) showed responses at 0.5 kHz to be larger than those at 1 and 2 kHz in the multiple condition at 73 dB SPL, the present study’s results did not replicate this. We did, however, show an increase at 0.5 kHz for AM in the single condition at 60 and 80 dB HL. This discrepancy between studies in the multiple condition may be due to differences in methodology.  Previous studies in adults have only investigated AM2 in the multiple (DM) condition. Picton et al. (2005) used AM2 stimuli to estimate audiometric thresholds. They showed response amplitudes to AM2 stimuli presented in the multiple condition at 70 dB SPL to be highest at 0.5  61  kHz and lowest at 2 kHz. In contrast, the present study’s results showed lowest response amplitudes at 0.5 kHz and largest amplitudes at 4 kHz.  John et al. (2002a) compared responses to AM2 stimuli with, among others, responses to AM stimuli at 35 and 55 SPL. They showed that using AM2 stimuli increased response amplitudes by 39% at 35 dB SPL and 18% at 55 dB SPL compared to AM stimuli. Their pattern of responses for the DM condition at 55 dB SPL was similar to that found in the present study at 60 dB HL.  It was hypothesized that stimuli with faster rise times would result in greater amplitudes due to their increased synchronicity of neural responses. This was demonstrated by AM2 stimuli in the multiple conditions, which produced slightly but significantly higher response amplitudes than AM and AM/FM stimuli, in particular at 0.5 kHz.  Similarly, it was predicted that stimuli with broader spectra would result in greater amplitudes due to their increased spread of spectral energy activating more neurons. This was demonstrated in by the AM/FM stimulus in the MS condition, which evoked responses significantly larger than those to both AM and AM2 stimuli at all frequencies except 0.5 kHz. However, it was also predicted that more interactions would be found when using stimuli with broader spectra, even at moderate intensities. This was illustrated by the drop in amplitude by the AM/FM stimulus in the multiple conditions versus in the single conditions, even at 60 dB HL.  62  Relative Efficiency Relative efficiency is an important factor to consider when selecting optimal clinical stimuli. The ability to present multiple ASSR stimuli simultaneously allows for a potential decrease in testing times. However, due to response interactions occurring at cochlear and/or neural levels, response amplitudes may be reduced to the extent that single stimuli presentation proves more efficient than multiple. Clinically, a stimulus that provides a large response and does not decrease significantly when presented simultaneously with other stimuli is desirable.  The RE values calculated from this study’s amplitude data showed the multiple conditions were never less efficient than the single condition at 60 dB HL, and at 80 dB HL, the multiple conditions were primarily (83%) just as efficient as the single condition. Four of the AM/FM RE values were worse in the multiple conditions than in the single condition at 80 dB HL; significantly so for 0.5 kHz in the MM condition. The problematic frequency when looking at RE seems to be 0.5 kHz. As such, it is predicted 0.5 kHz will be the limiting factor ultimately determining testing time.  Only a handful of previous ASSR studies have calculated relative efficiency; the majority of which investigated AM stimuli (Armstrong, 2006; Fontaine, 2006; Hatton, 2008). John et al. (1998b) recorded responses to AM stimuli at 35, 60, and 75 dB SPL in MS and MM conditions at 1 and 2 kHz. For 75 dB SPL stimuli, their responses in the MM condition decreased from those in the MS condition by a similar amount to what was found in the current study. For example, at 75 dB SPL, their MM values were 56 and 49% of MS values at 1 and 2 kHz,  63  respectively, and this study’s 80 dB HL MM values were 50 and 59% of MS values at 1 and 2 kHz, respectively.  Unfortunately, previous studies examining AM/FM and AM2 stimuli did not test both single and multiple conditions, thus preventing the subsequent calculation of their RE values. Amstrong (2006) revealed the MM condition was not significantly more efficient than the MS condition for AM stimuli at 80 dB SPL. Fontaine (2006) investigated ASSRs to 40 Hz at 80 dB SPL and found that MM and DM conditions were significantly more efficient than the MS condition, but not significantly different from each other.  Examination of raw amplitude data from Herdman and Stapells (2001) reveals their relative efficiency values for AM stimuli at 60 dB SPL to be quite similar to what was found in the current study at 60 dB HL. John et al (1998b) recorded ASSRs to AM stimuli at 60 and 75 dB SPL in MS and MM conditions at 1 and 2 kHz only. Their 60 dB SPL results are similar to ours at 60 dB HL, but at 75 dB SPL their RE value for 2 kHz is below that in the MS condition, a finding not duplicated in the current study.  Clinical Implications In choosing optimal clinical testing parameters, both stimulus choice and presentation condition must be considered. The results of this study indicate that multiple-stimulus conditions are more efficient than the single condition for AM and AM2 stimuli at 60 dB HL, and at least as efficient at 80 dB HL. If one were to choose the single-stimulus condition, AM/FM stimuli are  64  best; however, use of AM/FM stimuli in multiple test presentations is not recommended due to the significant decrease in response amplitudes at 0.5 kHz (compared to other frequencies and stimuli).  Perhaps response interactions would be lessened if stimuli with broader spectral widths were only combined with one or two other frequencies per ear. For example, 1, 2, and 4 kHz could be combined into a multiple condition and 0.5 kHz could be tested separately in a single condition for AM/FM and AM2 stimuli. Likewise, stimuli could be grouped to maximize frequency separation (i.e., 0.5 and 2 kHz versus 1 and 4 kHz). By exploring similar combinations, the most efficient methodology by which to attain response amplitudes may be determined.  Future Directions Future research is required to investigate interactions in individuals with sensorineural hearing loss. Hearing-impaired individuals may have more interactions due to their broader cochlear filters (Moore, 1993) causing a given stimulus to activate a broader region along the basilar membrane. This may cause even a stimulus with a relatively narrow acoustic spectra (e.g., AM) to evoke more response interactions than what is seen in normal-hearing individuals. However, the effects of stimulus intensity on normal cochlear physiology must also be remembered, as cochlear filters may be wider just due to increased intensity. As well, individuals with SNHL often have unequal thresholds across the frequency range, both intra and inter-  65  aurally. Such differences may significantly decrease the efficiency of testing multiple stimuli simultaneously, resulting in monotic testing being more efficient than dichotic testing.  Infants comprise another important population to investigate. Possible cochlear and/or neural auditory pathway immaturities may increase response interactions, negatively affecting response amplitudes (Hatton, 2008) and thus testing efficiency. Research is needed to investigate response amplitudes to (and efficiency of) various stimuli in infants. Such research should be extended to hearing-impaired infants, as this is ultimately the population of most interest.  66  References Aoyagi, M., Furuse, H., Yokota, M., Kiren, T., Suzuki, Y. & Koike, Y. (1994). Detectability of amplitude-modulation following response at different carrier frequencies. Acta Otolaryngologica Supplement, 511, 23-27. Armstrong, M.T. (2006). 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Ear and Hearing, 25, 57-67.  79  Appendix A: Corrected amplitude (nV) data  80  Appendix A1: Corrected amplitude (nV) data for AM stimuli at 80 dB HL by condition and subject MS MM Subject 0.5 kHz 1 kHz 2 kHz 4 kHz 0.5 kHz 1 kHz 2 kHz 1 187 144 164 102 40 51 80 2 70 107 118 78 60 40 62 3 44 51 77 91 60 49 63 4 169 134 111 104 88 62 78 5 138 108 107 90 48 48 42 6 27 68 73 83 61 77 37 7 106 93 100 90 70 48 65 8 189 158 92 75 75 68 107 9 171 141 126 119 51 74 56 10 44 66 81 68 32 41 37 11 153 128 121 111 70 66 74 12 139 98 83 58 60 51 54 13 88 86 63 66 22 43 37 14 53 87 72 106 75 40 54 15 122 102 88 68 36 44 25 Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  4 kHz 76 66 61 118 83 76 90 122 87 57 108 58 58 117 49  DM 0.5 kHz 62 62 31 40 33 52 67 41 31 49 56 78 19 17 38  1 kHz 66 50 51 52 79 41 50 40 39 46 77 70 34 37 39  Amplitudes have been corrected for residual noise (Picton et al., 2003). Bold values indicate amplitudes that met noise criterion but did not differ significantly from the background EEG noise.  81  2 kHz 89 41 47 74 45 54 52 127 78 71 52 40 43 27 29  4 kHz 66 68 62 133 96 61 67 150 139 78 123 54 57 93 32  Appendix A2: Corrected amplitude (nV) data for AM/FM stimuli at 80 dB HL by condition and subject MS MM Subject 0.5 kHz 1 kHz 2 kHz 4 kHz 0.5 kHz 1 kHz 2 kHz 1 183 225 194 126 34 60 80 2 123 139 144 86 38 34 57 3 81 94 118 129 18 65 57 4 169 151 87 167 58 100 71 5 159 152 154 135 18 72 74 6 56 97 123 138 32 42 58 7 47 129 146 129 6 59 56 8 91 232 163 90 42 59 51 9 168 204 118 145 39 47 35 10 85 84 134 75 35 45 71 11 173 204 163 155 59 63 86 12 159 153 113 101 31 57 65 13 118 121 98 98 19 47 46 14 101 162 103 136 26 47 63 15 146 85 104 102 15 51 48 Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  4 kHz 101 86 93 123 124 75 91 81 126 72 141 83 79 119 78  DM 0.5 kHz 36 31 31 32 24 20 9 21 9 45 56 42 13 26 20  1 kHz 88 61 29 68 91 51 41 48 27 65 41 67 30 41 55  Amplitudes have been corrected for residual noise (Picton et al., 2003). Bold values indicate amplitudes that met noise criterion but did not differ significantly from the background EEG noise.  82  2 kHz 74 72 54 73 91 69 34 53 40 86 92 66 53 39 49  4 kHz 75 79 80 154 121 75 83 111 106 91 113 53 82 101 51  Appendix A3: Corrected amplitude (nV) data for AM2 stimuli at 80 dB HL by condition and subject MS MM Subject 0.5 kHz 1 kHz 2 kHz 4 kHz 0.5 kHz 1 kHz 2 kHz 1 167 159 159 111 84 71 77 2 84 89 107 77 66 57 54 3 31 43 59 85 46 55 47 4 127 174 125 113 90 91 77 5 153 130 138 112 89 64 67 6 66 86 91 103 56 67 70 7 98 95 87 84 95 40 61 8 160 131 159 97 73 65 84 9 115 140 113 137 49 47 45 10 57 56 83 75 73 64 42 11 146 139 135 126 90 86 71 12 132 145 85 76 93 87 85 13 102 99 72 73 40 44 47 14 79 64 86 142 64 29 56 15 76 77 96 69 62 47 58 Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  4 kHz 93 72 74 102 104 75 97 123 107 40 114 58 69 109 72  DM 0.5 kHz 69 56 18 48 47 57 100 59 37 65 69 103 22 49 50  1 kHz 81 48 54 36 83 54 63 60 57 50 97 84 46 36 51  Amplitudes have been corrected for residual noise (Picton et al., 2003). Bold values indicate amplitudes that met noise criterion but did not differ significantly from the background EEG noise.  83  2 kHz 80 39 68 124 68 77 78 65 63 92 54 51 48 51 34  4 kHz 102 70 66 191 93 56 85 97 104 94 92 53 63 96 67  Appendix A4: Corrected amplitude (nV) data for AM stimuli at 60 dB HL by condition and subject MS MM Subject 0.5 kHz 1 kHz 2 kHz 4 kHz 0.5 kHz 1 kHz 2 kHz 1 37 41 65 55 15 41 38 2 83 35 54 27 53 63 42 3 23 23 27 71 21 31 47 4 83 83 63 80 42 46 37 5 63 63 57 68 47 75 65 6 53 27 53 52 37 26 61 7 66 59 44 48 66 59 46 8 111 56 42 35 81 66 36 9 118 102 64 61 64 57 52 10 30 60 47 37 42 38 27 11 74 55 58 67 50 83 67 12 51 70 29 40 53 69 60 13 44 35 49 26 17 15 37 14 53 47 46 58 65 59 39 15 48 30 39 32 32 43 55 Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  4 kHz 27 27 51 30 64 39 21 26 34 32 60 29 24 43 30  DM 0.5 kHz 19 42 40 33 36 34 90 77 42 38 59 78 13 48 31  1 kHz 51 33 57 56 44 33 44 32 63 52 74 92 33 52 46  2 kHz 49 32 41 69 60 46 38 65 52 50 77 35 37 37 57  Amplitudes have been corrected for residual noise (Picton et al., 2003). Bold values indicate amplitudes that met noise criterion but did not differ significantly from the background EEG noise.  84  4 kHz 37 40 34 68 74 29 30 32 37 32 79 36 23 33 29  Appendix A5: Corrected amplitude (nV) data for AM/FM stimuli at 60 dB HL by condition and subject MS MM Subject 0.5 kHz 1 kHz 2 kHz 4 kHz 0.5 kHz 1 kHz 2 kHz 1 93 97 94 70 36 55 46 2 52 79 83 45 38 37 59 3 27 50 77 79 33 63 56 4 110 124 71 76 40 84 42 5 54 68 93 72 56 77 90 6 60 51 102 80 32 49 95 7 45 84 77 54 56 55 33 8 95 92 64 36 41 49 50 9 98 104 96 74 39 47 44 10 31 51 55 54 70 52 74 11 68 65 99 97 60 83 76 12 51 89 93 63 53 72 71 13 61 34 67 57 18 29 46 14 121 148 70 92 65 59 58 15 59 64 70 64 24 70 55 Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  4 kHz 42 35 57 65 69 64 41 36 75 48 71 45 53 74 45  DM 0.5 kHz 32 41 28 32 39 44 32 52 35 38 46 55 20 53 29  1 kHz 40 40 55 67 73 52 52 57 52 29 51 72 30 62 40  Amplitudes have been corrected for residual noise (Picton et al., 2003). Bold values indicate amplitudes that met noise criterion but did not differ significantly from the background EEG noise.  85  2 kHz 39 48 29 43 65 70 32 51 42 87 78 67 53 55 74  4 kHz 43 47 62 53 49 51 35 36 69 41 70 42 43 68 42  Appendix A6: Corrected amplitude (nV) data for AM2 stimuli at 60 dB HL by condition and subject MS MM Subject 0.5 kHz 1 kHz 2 kHz 4 kHz 0.5 kHz 1 kHz 2 kHz 1 85 67 76 48 44 54 54 2 53 36 58 26 66 30 39 3 38 26 36 60 23 46 45 4 127 105 96 64 63 62 76 5 53 48 66 37 59 56 51 6 51 26 54 47 42 28 64 7 70 56 56 45 78 56 53 8 107 57 51 34 33 40 53 9 105 116 62 65 81 45 44 10 38 31 59 30 51 68 73 11 79 30 69 64 45 36 62 12 51 71 48 36 77 85 68 13 43 27 42 27 27 27 41 14 108 83 51 34 80 54 67 15 38 36 44 41 41 49 66 Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  4 kHz 42 37 44 37 52 36 24 47 55 44 56 43 41 57 34  DM 0.5 kHz 53 64 24 60 45 51 86 89 53 58 62 72 27 34 50  1 kHz 25 27 42 37 42 54 40 34 50 31 65 76 25 67 53  Amplitudes have been corrected for residual noise (Picton et al., 2003). Bold values indicate amplitudes that met noise criterion but did not differ significantly from the background EEG noise.  86  2 kHz 63 39 43 66 53 50 51 43 53 48 58 55 36 43 62  4 kHz 53 40 48 34 49 48 26 33 73 37 67 39 27 49 24  Appendix B: Circle radius noise (CR)  87  Appendix B1: EEG noise (nV) for AM stimuli at 80 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 30 28 24 29 30 22 18 24 30 19 21 17 29 30 16  1 kHz 11 14 18 27 24 20 20 21 30 29 29 16 29 23 16  2 kHz 22 11 15 28 18 16 20 23 30 22 21 14 29 30 14  4 kHz 16 25 16 27 30 16 13 20 29 19 17 27 28 20 16  MM 0.5 kHz 29 25 26 30 22 29 30 28 29 29 25 25 14 22 19  1 kHz 29 23 24 28 19 27 27 27 28 25 23 25 14 20 17  2 kHz 27 22 22 23 18 27 28 28 27 25 20 24 12 17 15  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  88  4 kHz 26 20 19 23 18 24 25 28 24 24 21 23 11 16 13  DM 0.5 kHz 28 29 25 30 29 28 24 29 30 29 29 29 15 30 21  1 kHz 29 28 26 28 26 29 23 28 27 25 25 29 12 27 19  2 kHz 27 26 23 26 25 23 20 28 27 27 23 27 12 25 17  4 kHz 30 25 22 25 24 23 19 29 26 27 22 26 12 23 16  Appendix B2: EEG noise (nV) for AM/FM stimuli at 80 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14  MS 0.5 kHz 16 20 21 27 29 22 29 30 30 18 30 19 15 26  1 kHz 24 21 21 20 29 19 30 30 30 29 29 21 20 17  2 kHz 11 17 18 29 29 16 29 29 29 20 28 14 13 18  4 kHz 21 18 30 29 28 30 30 28 27 24 18 24 12 14  MM 0.5 kHz 21 28 20 30 19 28 30 24 29 25 30 23 14 30  1 kHz 22 29 18 25 18 29 28 22 27 25 29 21 13 26  2 kHz 18 28 19 24 17 26 27 23 27 25 27 19 12 23  15 19 30 14 13 19 17 17 Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  89  4 kHz 18 27 15 23 17 24 24 23 25 24 24 17 12 23  DM 0.5 kHz 22 26 27 30 30 28 29 30 15 29 29 26 17 27  1 kHz 22 24 24 26 28 29 30 26 15 28 28 23 15 26  2 kHz 21 25 23 25 27 29 29 28 13 26 25 21 13 23  4 kHz 21 22 22 24 25 28 26 29 14 27 24 19 13 22  15  19  16  16  15  Appendix B3: EEG noise (nV) for AM2 stimuli at 80 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 12 21 23 29 29 18 18 25 24 17 19 29 14 30 30  1 kHz 13 30 19 30 27 25 29 30 19 22 18 18 12 26 21  2 kHz 10 14 29 26 26 16 30 30 18 29 18 15 12 20 14  4 kHz 9 15 15 19 24 16 30 19 26 20 22 30 11 19 14  MM 0.5 kHz 21 13 22 30 29 18 30 30 28 28 29 19 13 23 24  1 kHz 17 11 20 29 29 17 26 30 25 27 27 18 11 20 23  2 kHz 18 12 19 27 24 16 28 28 22 26 28 16 12 18 24  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  90  4 kHz 19 12 19 28 24 14 26 26 21 24 27 14 12 16 22  DM 0.5 kHz 16 28 22 21 30 24 30 30 17 26 27 29 17 23 23  1 kHz 13 25 23 22 30 21 30 28 15 27 29 27 17 20 20  2 kHz 12 24 20 19 27 23 29 26 14 29 27 24 17 20 20  4 kHz 11 25 19 18 22 20 28 29 12 27 25 27 16 20 17  Appendix B4: EEG noise (nV) for AM stimuli at 60 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 20 28 22 18 29 19 30 19 25 27 22 18 29 16 29  1 kHz 12 19 20 18 30 17 29 16 27 26 29 30 26 29 16  2 kHz 13 19 26 29 28 17 17 20 30 16 30 23 14 21 13  4 kHz 15 27 16 29 27 12 19 19 25 21 23 13 17 15 29  MM 0.5 kHz 14 29 24 21 29 21 18 22 29 16 30 17 15 18 17  1 kHz 14 25 24 19 27 21 17 22 25 16 29 17 15 17 14  2 kHz 13 25 19 18 24 19 17 20 22 16 27 15 14 15 14  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  91  4 kHz 12 24 18 17 25 19 17 19 21 16 25 12 13 14 12  DM 0.5 kHz 29 23 28 28 30 22 30 22 29 22 30 29 15 27 24  1 kHz 27 19 25 27 26 22 28 24 27 20 29 30 14 23 17  2 kHz 26 19 24 29 25 19 27 22 26 21 28 29 13 21 17  4 kHz 25 17 24 26 22 20 28 23 26 19 27 28 12 19 15  Appendix B5: EEG noise (nV) for AM/FM stimuli at 60 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 24 15 16 24 30 26 29 16 29 28 30 30 30 30 22  1 kHz 21 15 16 21 20 20 30 16 29 30 21 18 13 30 17  2 kHz 19 23 18 26 29 25 30 15 30 29 30 26 14 29 14  4 kHz 14 22 27 20 29 16 23 18 15 29 24 12 29 22 12  MM 0.5 kHz 29 24 30 30 28 18 30 19 30 30 30 20 15 25 29  1 kHz 28 24 25 25 28 18 29 16 27 28 28 17 14 23 24  2 kHz 28 23 25 22 30 16 26 15 23 27 28 15 13 20 22  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  92  4 kHz 27 22 24 21 28 13 25 13 23 23 27 14 12 19 23  DM 0.5 kHz 29 22 28 29 30 26 30 21 30 28 29 20 17 20 28  1 kHz 29 22 26 29 28 25 26 20 27 27 30 19 15 20 25  2 kHz 28 20 24 25 25 20 27 17 26 25 28 15 15 16 26  4 kHz 26 20 22 26 27 22 23 18 26 27 28 14 15 16 24  Appendix B6: EEG noise (nV) for AM2 stimuli at 60 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 30 25 17 30 18 17 30 21 30 27 24 21 16 29 30  1 kHz 20 28 16 28 29 21 29 16 24 26 28 16 16 17 12  2 kHz 11 18 29 25 22 14 30 29 22 30 29 14 12 16 11  4 kHz 19 26 20 29 30 29 30 14 30 23 20 13 15 30 29  MM 0.5 kHz 18 19 28 22 30 19 29 30 30 29 25 19 24 27 27  1 kHz 17 19 25 20 24 18 26 27 26 27 23 17 24 29 25  2 kHz 16 19 27 20 21 17 25 25 27 23 21 14 20 23 22  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  93  4 kHz 17 19 21 19 19 16 23 24 25 22 23 12 20 23 21  DM 0.5 kHz 24 22 21 27 30 26 29 26 29 27 30 30 22 22 19  1 kHz 23 21 19 29 29 23 26 23 27 25 26 28 23 19 17  2 kHz 21 21 17 26 25 22 26 22 26 26 25 29 22 17 16  4 kHz 20 20 15 25 23 20 24 22 23 24 23 24 22 15 15  Appendix C: Phase (degrees)  94  Appendix C1: Phase (degrees) of AM stimuli at 80 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 265.039 199.664 209.805 264.867 231.234 161.620 245.214 232.609 240.058 191.986 212.613 228.541 247.735 267.502 226.708  1 kHz 230.432 164.771 183.793 223.155 183.621 70.176 200.065 193.877 232.896 186.257 172.907 173.423 197.945 212.670 189.351  2 kHz 215.764 138.931 188.434 210.092 185.856 121.685 188.434 183.220 197.029 184.366 172.506 155.718 183.621 231.406 171.818  4 kHz 185.913 110.798 164.714 176.574 173.709 145.348 160.073 146.837 174.225 160.989 140.879 163.281 163.682 191.471 166.547  MM 0.5 kHz 179.095 125.294 99.798 210.493 147.296 113.090 218.572 197.601 164.485 139.790 175.256 143.285 156.463 198.805 134.920  1 kHz 218.801 105.986 126.440 176.287 123.461 128.274 175.142 165.802 209.404 158.067 166.031 148.957 160.989 163.797 138.873  2 kHz 208.029 125.122 184.825 178.293 154.286 143.629 180.126 157.781 192.846 162.995 153.598 137.212 179.042 187.804 143.285  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  95  4 kHz 188.262 103.808 169.756 175.256 192.216 157.552 161.906 110.856 178.694 160.015 136.925 154.171 161.677 186.658 156.864  DM 0.5 kHz 225.046 136.066 159.557 238.167 172.678 125.008 234.672 235.646 190.783 128.216 182.361 178.923 182.762 -35.019 131.826  1 kHz 217.770 127.873 127.758 183.449 148.614 114.236 175.485 198.690 200.008 137.269 140.478 142.483 156.578 175.944 120.711  2 kHz 219.546 149.473 190.955 261.372 187.346 135.149 215.993 162.250 202.071 173.709 171.818 162.365 171.360 194.622 176.689  4 kHz 202.185 104.324 154.802 253.923 238.396 148.900 153.656 121.627 202.185 181.387 137.498 179.954 152.281 191.012 169.584  Appendix C2: Phase (degrees) of AM/FM stimuli at 80 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz -80.226 231.692 242.407 -79.366 237.823 211.238 -20.695 -75.699 264.752 233.812 -88.534 256.903 244.641 -85.382 244.412  1 kHz 218.400 145.462 155.432 195.883 163.682 84.099 190.726 201.097 200.237 145.233 179.152 155.432 175.371 187.804 169.183  2 kHz 208.545 136.696 159.729 244.240 169.928 122.372 199.836 174.339 194.794 169.297 153.598 145.405 160.703 189.236 170.157  4 kHz 203.560 145.233 194.221 180.871 173.537 155.203 169.584 172.105 193.533 169.240 150.332 161.448 171.818 196.513 169.469  MM 0.5 kHz 234.385 186.372 90.573 210.837 158.526 132.399 97.563 184.653 225.562 128.560 188.320 136.581 205.451 150.447 217.254  1 kHz 214.275 125.753 115.955 165.057 109.595 99.855 164.198 173.595 165.860 129.534 147.353 122.315 150.848 149.874 115.096  2 kHz 192.330 148.671 175.485 176.517 147.869 137.212 196.570 178.006 203.789 165.573 167.578 144.145 149.072 185.283 158.526  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  96  4 kHz 203.102 143.056 164.026 200.638 173.079 170.157 158.354 147.869 170.042 159.156 151.650 177.605 175.199 199.148 174.225  DM 0.5 kHz 224.702 198.633 96.245 254.267 165.172 177.548 168.954 244.011 186.658 116.700 180.585 153.656 159.328 -69.397 161.906  1 kHz 198.003 115.726 103.178 190.268 113.549 123.461 168.323 177.720 178.980 123.633 110.569 130.279 133.602 202.013 120.309  2 kHz 205.394 147.640 172.678 -79.939 150.963 128.617 188.835 159.213 183.908 176.975 161.276 149.645 142.196 203.904 184.080  4 kHz 209.290 125.982 184.710 -85.669 176.058 182.017 159.786 157.151 178.866 165.000 134.977 171.303 183.736 212.899 178.923  Appendix C3: Phase (degrees) of AM2 stimuli at 80 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 245.100 185.340 143.743 243.324 213.358 97.563 208.087 215.363 226.765 139.675 196.226 205.336 216.394 240.745 232.265  1 kHz 220.061 148.556 160.359 197.945 156.005 69.603 202.644 217.369 201.154 163.568 165.688 149.645 174.912 189.408 191.471  2 kHz 199.378 119.794 191.127 183.220 175.371 103.751 191.929 212.384 177.949 172.621 159.729 153.369 158.698 193.419 152.452  4 kHz 187.632 109.366 159.442 176.918 153.140 142.827 172.334 153.255 157.437 175.772 140.363 165.630 152.395 171.704 156.177  MM 0.5 kHz 193.132 109.423 82.265 186.944 143.686 79.629 186.085 171.990 160.989 113.147 146.093 156.291 145.577 177.376 109.882  1 kHz 206.826 99.912 119.565 164.427 130.909 102.834 173.251 150.390 188.721 132.112 149.301 147.525 146.952 188.778 106.100  2 kHz 199.836 124.091 170.787 192.903 151.478 117.330 170.902 167.177 138.816 177.777 142.139 146.494 154.744 170.959 129.419  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  97  4 kHz 179.897 113.262 150.734 213.530 162.021 139.618 140.306 131.253 159.099 150.161 132.342 147.238 147.067 176.058 154.171  DM 0.5 kHz 194.393 149.473 119.393 190.955 178.236 127.586 210.780 195.482 161.505 107.131 169.698 146.952 151.135 227.510 134.576  1 kHz 192.903 100.657 114.465 168.266 120.882 106.788 167.980 193.877 160.359 147.754 138.873 152.452 148.499 173.423 127.987  2 kHz 201.097 115.153 170.042 -77.705 136.696 120.768 184.194 177.090 174.912 162.135 145.119 162.135 172.048 175.543 171.016  4 kHz 190.955 112.231 146.150 268.018 157.437 148.098 154.916 121.742 168.667 167.006 136.009 150.161 173.537 163.625 158.812  Appendix C4: Phase (degrees) of AM stimuli at 60 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 252.090 142.311 158.010 242.636 208.087 157.666 211.524 195.596 222.984 120.367 216.738 165.115 208.946 247.907 169.584  1 kHz 203.216 99.110 127.013 177.777 136.639 49.836 156.807 170.099 177.777 132.513 152.968 160.417 161.849 201.899 95.730  2 kHz 200.867 110.856 189.236 206.024 133.946 82.838 170.787 145.691 151.192 151.536 149.301 136.753 132.972 196.742 136.925  4 kHz 183.220 113.434 154.687 178.006 151.135 125.695 120.424 114.752 166.318 159.271 137.040 141.795 136.983 159.557 136.868  MM 0.5 kHz 188.721 137.384 142.027 192.731 166.203 138.014 179.840 164.771 167.407 125.351 192.388 157.724 197.315 199.435 149.072  1 kHz 194.794 122.601 101.370 169.469 133.258 93.667 154.171 165.860 187.460 133.430 140.363 152.796 153.770 195.825 102.605  2 kHz 193.018 182.246 155.360 190.440 128.847 98.537 179.381 133.774 166.031 158.067 160.932 149.015 145.290 197.601 143.514  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  98  4 kHz 176.116 128.675 162.977 201.841 128.388 145.749 132.399 118.304 164.599 149.989 152.911 125.237 141.108 168.209 146.666  DM 0.5 kHz 269.737 146.551 175.256 193.705 215.993 142.483 206.139 196.112 183.736 135.894 233.469 171.933 194.450 -84.007 152.682  1 kHz 196.857 115.669 136.238 168.667 136.811 103.178 168.209 194.851 175.428 124.377 152.395 149.301 148.785 209.118 105.470  2 kHz 207.456 166.891 172.391 257.590 151.822 89.141 228.770 166.834 200.237 167.865 172.964 142.368 138.014 220.119 148.270  4 kHz 197.029 103.694 153.426 225.676 142.483 132.743 169.698 111.486 176.631 148.327 150.332 150.103 138.472 177.892 145.462  Appendix C5: Phase (degrees) of AM/FM stimuli at 60 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 265.440 184.481 158.698 -84.236 235.302 159.958 246.360 201.555 252.720 159.213 221.379 181.043 219.947 -57.537 204.821  1 kHz 180.012 92.693 79.801 153.942 116.413 61.352 130.966 143.113 179.095 120.711 100.199 121.742 122.028 179.038 65.535  2 kHz 168.782 127.070 129.935 173.308 115.325 93.724 181.845 143.915 157.265 155.088 134.175 138.300 130.107 195.252 130.336  4 kHz 179.897 149.358 167.349 152.281 126.497 133.316 142.941 136.123 155.833 158.526 130.222 140.478 163.797 177.720 142.368  MM 0.5 kHz 223.098 139.504 149.072 198.060 151.536 149.645 196.971 163.453 190.611 148.098 155.890 163.740 172.621 238.110 133.029  1 kHz 148.213 109.824 88.396 136.066 82.953 83.755 125.867 145.004 119.908 90.229 80.374 107.590 130.394 184.309 85.531  2 kHz 169.125 127.013 210.436 182.533 119.565 102.089 143.056 146.150 159.442 145.749 112.632 145.863 126.154 175.944 144.145  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  99  4 kHz 169.240 114.293 206.654 183.564 165.630 138.243 175.428 111.085 152.911 153.656 121.112 144.488 161.391 169.355 140.821  DM 0.5 kHz 244.298 173.251 150.275 204.248 148.270 134.232 190.268 170.386 213.530 133.373 125.180 178.465 174.970 -67.048 142.483  1 kHz 164.141 112.173 91.146 147.983 84.671 64.389 144.889 147.468 156.578 72.639 71.837 104.152 79.057 185.627 77.968  2 kHz 171.417 136.696 135.264 161.161 116.127 85.073 154.802 161.906 178.293 156.349 137.785 138.071 114.179 184.080 151.765  4 kHz 186.658 120.481 170.271 166.948 189.179 135.149 176.689 162.651 179.782 144.259 125.638 136.410 164.943 175.886 132.170  Appendix C6: Phase (degrees) of AM2 stimuli at 60 dB HL by condition and subject  Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  MS 0.5 kHz 269.336 128.216 115.038 243.553 171.646 119.737 193.304 180.699 205.279 132.170 177.376 142.196 203.675 256.215 134.633  1 kHz 179.954 116.528 128.331 148.213 129.591 59.977 147.353 139.618 168.667 116.012 94.469 127.586 170.386 177.376 67.139  2 kHz 167.808 94.584 162.938 162.880 131.138 56.253 167.177 167.865 158.297 125.237 107.819 129.419 127.242 169.698 123.690  4 kHz 160.760 83.927 140.363 134.060 148.957 121.570 133.029 118.934 156.463 140.535 131.425 126.612 106.444 188.148 140.191  MM 0.5 kHz 168.209 97.563 138.169 166.433 126.326 117.559 173.079 191.986 154.802 122.143 163.740 145.119 194.106 199.951 125.523  1 kHz 162.193 88.281 97.118 157.895 112.002 74.874 178.579 154.286 195.539 128.732 107.819 114.351 118.304 179.210 96.245  2 kHz 174.053 122.429 140.275 169.813 121.799 72.124 168.724 191.242 168.896 134.633 146.837 120.080 125.982 182.017 129.821  Conditions: monotic single (MS), monotic multiple (MM), dichotic multiple (DM).  100  4 kHz 190.210 87.479 164.587 167.808 119.278 125.982 150.218 125.810 138.701 141.108 108.105 134.232 125.122 165.516 139.618  DM 0.5 kHz 213.186 122.028 132.988 178.694 105.355 115.382 193.075 166.490 172.735 99.855 181.158 140.649 214.446 246.188 129.706  1 kHz 183.048 112.575 84.895 196.971 80.088 71.493 153.598 159.901 171.188 102.089 106.329 120.481 130.107 196.627 102.204  2 kHz 163.682 103.694 157.730 181.731 153.083 72.697 154.802 149.473 176.631 133.545 130.107 125.294 116.184 182.361 120.596  4 kHz 143.342 100.256 151.227 165.229 140.535 135.837 167.063 107.131 156.177 135.321 117.445 150.390 119.106 156.234 108.507  Appendix D: Relative efficiency data  101  Appendix D1: Relative efficiency data of AM stimuli at 80 dB HL by condition and subject MM DM Subject 0.5 1 2 4 0.5 0.43 0.71 0.98 1.49 0.94 1 1.71 0.75 1.05 1.69 2.51 2 2.73 1.92 1.64 1.34 1.99 3 1.04 0.93 1.41 2.27 0.67 4 0.70 0.89 0.79 1.84 0.68 5 4.52 2.26 1.01 1.83 5.45 6 1.32 1.03 1.30 2.00 1.79 7 0.79 0.86 2.33 3.25 0.61 8 0.60 1.05 0.89 1.46 0.51 9 1.45 1.24 0.91 1.68 3.15 10 0.92 1.03 1.22 1.95 1.04 11 0.86 1.04 1.30 2.00 1.59 12 0.50 1.00 1.17 1.76 0.61 13 2.83 0.92 1.50 2.21 0.91 14 0.59 0.86 0.57 1.44 0.88 15 Conditions: monotic multiple (MM) and dichotic multiple (DM).  1 1.30 1.32 2.83 1.10 2.07 1.71 1.52 0.72 0.78 1.97 1.70 2.02 1.12 1.20 1.08  2 1.53 0.98 1.73 1.89 1.19 2.09 1.47 3.90 1.75 2.48 1.22 1.36 1.93 1.06 0.93  Note: Relative efficiency values for monotic single (MS) condition all equal "1".  102  4 1.83 2.47 1.93 3.62 3.02 2.08 2.11 5.66 3.30 3.24 3.13 2.63 2.44 2.48 1.33  Appendix D2: Relative efficiency data of AM/FM stimuli at 80 dB HL by condition and subject MM DM Subject 0.5 1 2 4 0.5 0.37 0.53 0.82 1.60 0.56 1 0.62 0.49 0.79 2.00 0.71 2 0.44 1.38 0.97 1.44 1.08 3 0.69 1.32 1.63 1.47 0.54 4 0.23 0.95 0.96 1.84 0.43 5 1.14 0.87 0.94 1.09 1.01 6 0.26 0.91 0.77 1.41 0.54 7 0.92 0.51 0.63 1.80 0.65 8 0.46 0.46 0.59 1.74 0.15 9 0.82 1.07 1.06 1.92 1.50 10 0.68 0.62 1.06 1.82 0.92 11 0.39 0.75 1.15 1.64 0.75 12 0.32 0.78 0.94 1.61 0.31 13 0.51 0.58 1.22 1.75 0.73 14 0.21 1.20 0.92 1.53 0.39 15 Conditions: monotic multiple (MM) and dichotic multiple (DM).  1 1.11 1.24 0.87 1.27 1.69 1.49 0.90 0.59 0.37 2.19 0.57 1.24 0.70 0.72 1.83  2 1.08 1.41 1.29 2.37 1.67 1.59 0.66 0.92 0.96 1.82 1.60 1.65 1.53 1.07 1.33  Note: Relative efficiency values for monotic single (MS) condition all equal "1".  103  4 1.68 2.60 1.75 2.61 2.54 1.54 1.82 3.49 2.07 3.43 2.06 1.48 2.37 2.10 1.41  Appendix D3: Relative efficiency data of AM2 stimuli at 80 dB HL by condition and subject MM DM Subject 0.5 1 2 4 0.5 1.01 0.89 0.97 1.68 1.17 1 1.57 1.28 1.01 1.87 1.89 2 2.97 2.56 1.59 1.74 1.64 3 1.42 1.05 1.23 1.81 1.07 4 1.16 0.98 0.97 1.86 0.87 5 1.70 1.56 1.54 1.46 2.44 6 1.94 0.84 1.40 2.31 2.89 7 0.91 0.99 1.06 2.54 1.04 8 0.85 0.67 0.80 1.56 0.91 9 2.56 2.29 1.01 1.07 3.23 10 1.23 1.24 1.05 1.81 1.34 11 1.41 1.20 2.00 1.53 2.21 12 0.78 0.89 1.31 1.89 0.61 13 1.62 0.91 1.30 1.54 1.75 14 1.63 1.22 1.21 2.09 1.86 15 Conditions: monotic multiple (MM) and dichotic multiple (DM).  1 1.44 1.53 3.55 0.59 1.81 1.78 1.88 1.30 1.15 2.53 1.97 1.64 1.31 1.59 1.87  2 1.42 1.03 3.26 2.81 1.39 2.39 2.54 1.16 1.58 3.14 1.13 1.70 1.89 1.68 1.00  Note: Relative efficiency values for monotic single (MS) condition all equal "1".  104  4 2.60 2.57 2.20 4.78 2.35 1.54 2.86 2.83 2.15 3.54 2.07 1.97 2.44 1.91 2.75  Appendix D4: Relative efficiency data of AM stimuli at 60 dB HL by condition and subject MM DM Subject 0.5 1 2 4 0.5 0.81 2.00 1.17 0.98 1.45 1 1.28 3.60 1.56 2.00 1.43 2 1.83 2.70 3.48 1.44 4.92 3 1.01 1.11 1.17 0.75 1.12 4 1.49 2.38 2.28 1.88 1.62 5 1.40 1.93 2.30 1.50 1.81 6 2.00 2.00 2.09 0.88 3.86 7 1.46 2.36 1.71 1.49 1.96 8 1.08 1.12 1.63 1.11 1.01 9 2.80 1.27 1.15 1.73 3.58 10 1.35 3.02 2.31 1.79 2.26 11 2.08 1.97 4.14 1.45 4.33 12 0.77 0.86 1.51 1.85 0.84 13 2.45 2.51 1.70 1.48 2.56 14 1.33 2.87 2.82 1.88 1.83 15 Conditions: monotic multiple (MM) and dichotic multiple (DM).  1 3.52 2.67 7.01 1.91 1.98 3.46 2.11 1.62 1.75 2.45 3.81 3.72 2.67 3.13 4.34  2 2.13 1.68 4.30 3.10 2.98 2.45 2.44 4.38 2.30 3.01 3.75 3.41 2.14 2.28 4.13  Note: Relative efficiency values for monotic single (MS) condition all equal "1".  105  4 1.90 4.19 1.35 2.40 3.08 1.58 1.77 2.59 1.72 2.45 3.34 2.55 2.50 1.61 2.56  Appendix D5: Relative efficiency data of AM/FM stimuli at 60 dB HL by condition and subject MM DM Subject 0.5 1 2 4 0.5 0.77 1.13 0.98 1.20 0.97 1 1.46 0.94 1.42 1.56 2.23 2 2.44 2.52 1.45 1.44 2.93 3 0.73 1.35 1.18 1.71 0.82 4 2.07 2.26 1.94 1.92 2.04 5 1.07 1.92 1.86 1.60 2.07 6 2.49 1.31 0.86 1.52 2.01 7 0.86 1.07 1.56 2.00 1.55 8 0.80 0.90 0.92 2.03 1.01 9 4.52 2.04 2.69 1.78 3.47 10 1.76 2.55 1.54 1.46 1.91 11 2.08 1.62 1.53 1.43 3.05 12 0.59 1.71 1.37 1.86 0.93 13 1.07 0.80 1.66 1.61 1.24 14 0.81 2.19 1.57 1.41 1.39 15 Conditions: monotic multiple (MM) and dichotic multiple (DM).  1 1.17 1.43 3.11 1.53 3.04 2.88 1.75 1.75 1.41 1.61 2.22 2.29 2.50 1.18 1.77  2 1.17 1.64 1.07 1.71 1.98 1.94 1.18 2.25 1.24 4.47 2.23 2.04 2.24 2.22 2.99  Note: Relative efficiency values for monotic single (MS) condition all equal "1".  106  4 1.74 2.95 2.22 1.97 1.92 1.80 1.83 2.83 2.64 2.15 2.04 1.89 2.13 2.09 1.86  Appendix D6: Relative efficiency data of AM2 stimuli at 60 dB HL by condition and subject MM DM Subject 0.5 1 2 4 0.5 1.04 1.61 1.42 1.75 1.76 1 2.49 1.67 1.34 2.85 3.42 2 1.21 3.54 2.50 1.47 1.79 3 0.99 1.18 1.58 1.16 1.34 4 2.23 2.33 1.55 2.81 2.40 5 1.65 2.15 2.37 1.53 2.83 6 2.23 2.00 1.89 1.07 3.47 7 0.62 1.40 2.08 2.76 2.35 8 1.54 0.78 1.42 1.69 1.43 9 2.68 4.39 2.47 2.93 4.32 10 1.14 2.40 1.80 1.75 2.22 11 3.02 2.39 2.83 2.39 3.99 12 1.26 2.00 1.95 3.04 1.78 13 1.48 1.30 2.63 3.35 0.89 14 2.16 2.72 3.00 1.66 3.72 15 Conditions: monotic multiple (MM) and dichotic multiple (DM).  1 1.06 2.12 4.57 1.00 2.47 5.87 2.02 1.69 1.22 2.83 6.13 3.03 2.62 2.28 4.16  2 2.34 1.90 3.38 1.94 2.27 2.62 2.58 2.38 2.42 2.30 2.38 3.24 2.42 2.38 3.99  Note: Relative efficiency values for monotic single (MS) condition all equal "1".  107  4 3.12 4.35 2.26 1.50 3.75 2.89 1.63 2.75 3.18 3.49 2.96 3.06 2.83 4.08 1.66  Appendix E: Behavioural research ethics board (BREB) approval  108  The University of British Columbia Office of Research Services Behavioural Research Ethics Board Suite 102, 6190 Agronomy Road, Vancouver, B.C. V6T 1Z3  CERTIFICATE OF APPROVAL - MINIMAL RISK PRINCIPAL INVESTIGATOR:  INSTITUTION / DEPARTMENT: UBC/Medicine, Faculty of/Audiology & Speech Sciences  David R. Stapells  UBC BREB NUMBER: H08-00874  INSTITUTION(S) WHERE RESEARCH WILL BE CARRIED OUT: Institution  Site  UBC  Vancouver (excludes UBC Hospital)  Other locations where the research will be conducted:  WorkSafe BC, Richmond (Audiology division) Western Institute for the Deaf and Hard of Hearing, Vancouver  CO-INVESTIGATOR(S): Grace S Shyng Lori Wood  SPONSORING AGENCIES: Natural Sciences and Engineering Research Council of Canada (NSERC) PROJECT TITLE: Efficiency of multiple ASSRs in adults with hearing loss CERTIFICATE EXPIRY DATE: June 23, 2009 DOCUMENTS INCLUDED IN THIS APPROVAL:  DATE APPROVED: June 23, 2008  Document Name  Consent Forms: Consent form Advertisements: Recruitment flyer Letter of Initial Contact: Invitation_ASSRefficiency_May27_2008  Version  Date  N/A  May 28, 2008  N/A  May 28, 2008  N/A  May 27, 2008  The application for ethical review and the document(s) listed above have been reviewed and the procedures were found to be acceptable on ethical grounds for research involving human subjects.  Approval is issued on behalf of the Behavioural Research Ethics Board and signed electronically by one of the following: Dr. M. Judith Lynam, Chair Dr. Ken Craig, Chair Dr. Jim Rupert, Associate Chair Dr. Laurie Ford, Associate Chair Dr. Daniel Salhani, Associate Chair Dr. Anita Ho, Associate Chair  109  

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