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Comparisons of auditory steady-state response and behavioural air- and bone-conduction thresholds in infants and adults with normal hearing Casey, Kelly-Ann


To improve our understanding of normal responses in infants, the present study compares air-conduction (AC) and bone-conduction (BC) auditory thresholds using both the auditory steady-state response (ASSR) and behavioural testing methods in normal-hearing infants (6-18 months of age) and adults. There are no correction factors available for estimating BC behavioural thresholds; this is a limiting factor to clinical implementation of the ASSR. Additionally, previous studies have reported that ASSR and visual reinforcement audiometry (VRA) thresholds (in dB HL) to air- and bone-conducted stimuli have different frequency-dependent trends and suggest that infants present with an air-bone gap that is not attributable to a conductive pathology; however, this relationship has not been assessed directly. The objectives of the present study are: (i) to compare BC thresholds between methods and provide the initial step towards positing correction factors to predict BC behavioural thresholds and; (ii) to directly compare AC and BC thresholds to provide a more accurate estimation of the maturational ABG. Thresholds were estimated at 500–4000 Hz using AM² stimuli for ASSRs and warbled-tone stimuli for behavioural testing. The results indicated that BC thresholds were, on average, 7–16 dB poorer for ASSR compared to VRA, but varied largely across infants. As expected for the ASSR, frequency-dependent differences in BC sensitivity were found— the 500- and 1000-Hz thresholds were better than the 2000-Hz threshold. For AC ASSR, the 500-Hz thresholds were higher than the other frequencies. There was a tendency for infant and adult ASSR thresholds to differ for BC, but not for AC. Behavioural thresholds for AC and BC were similar between infants and adults and across frequency. The results support the presence of a clinically significant maturational ABG (14 and 17 dB) in the low frequencies for infant ASSRs. The infant behavioural ABG also appeared at 500 Hz, as was posited by Hulecki and Small (2011), but was too small to be practically significant. Clinical consideration of the maturational ABG seems warranted when using ASSRs, but not for VRA. The results also provided preliminary normal levels for AC and BC ASSRs to AM² stimuli.

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