UBC Theses and Dissertations
Effective masking levels for bone-conduction auditory brainstem response stimuli in infants and adults with normal hearing Lau, Ricky
Background: Early hearing detection and intervention programs aim to limit delays in identifying ear-specific type/degree of hearing loss in infants using the auditory brainstem response (ABR). Ear-specific assessment poses challenges as sound delivered to one ear can travel across the skull and activate the contralateral cochlea. Wave V amplitude and latency measures ipsilateral and contralateral to the bone oscillator can be compared to isolate the test cochlea (AC testing also when large asymmetries are present). However, when comparison of ipsilateral/contralateral responses cannot isolate the responding cochlea, clinical masking is required. Effective masking levels (EMLs) for bone-conduction ABR testing in infants and adults have not been measured directly. This study aims to determine EMLs for 500- and 2000-Hz BC stimuli for normal-hearing infants (0-18 months) and adults. Method: Participants were 10-13 adults and 13-15 infants with normal hearing. BC 500- and 2000-Hz tone pip stimuli at intensities approximating normal levels (Infants: 20 and 30 dBnHL at 500 and 2000 Hz, respectively; Adults: 500 and 2000 Hz at 20 and 30 dBnHL) were presented via a B-71 oscillator. White-noise masking was presented binaurally via ER-3A earphones (22-82 dBSPL; 10-dB steps). The lowest level of masking to eliminate a BC response was deemed the EML. Results: For stimuli presented at 20 dBnHL, adult mean(1SD) EMLs for 500 and 2000 Hz were 65(9) and 53(6) dBSPL, respectively. Mean EMLs for infants were 80(6) dBSPL for 500 Hz at 20 dBnHL and 64(9) dBSPL for 2000 Hz at 30 dBnHL. Compared to adults, infants required approximately 13 dB more masking at 500 Hz but a similar amount of masking at 2000 Hz. Infants required 26 dB more masking at 500 versus 2000 Hz, whereas, adults required only 12 dB more masking at 500 versus 2000 Hz. Conclusion: Maximum binaural effective masking levels for infant BC responses are as follows: 82 dBSPL for 20 dBnHL at 500 Hz; 72 and 82 dBSPL for 30 and 40 dBnHL, respectively, at 2000 Hz. Unsafe levels of white noise would be needed to effectively mask at greater stimulus levels.
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