Investigations in MASTER (Multiple Auditory STEady-state Evoked Response): Pure tone and speech audiometry applications

This thesis considered the objective evaluation of hearing in both normal-hearing and hearing-impaired adults using multiple auditory steady-state evoked responses (MASTER). To this end, three studies were performed. The first evaluated air-conducted and bone-conducted pure tone thresholds using MASTER. Behavioral thresholds were compared to the thresholds for detecting scalp-recorded auditory steady-state evoked potentials (ASSR) in normal-hearing and hearing-impaired adults. Differences between ASSR and behavioral thresholds were on average 8 dB and there was a significant correlation (r = 0.92) between behavioral thresholds and ASSR. The second study dealt with ASSRs elicited by independent amplitude and frequency modulation (IAFM) in young normal-hearing subjects. The results of this experiment confirmed previous results that ASSRs evoked by amplitude modulation (AM) and frequency modulation (FM) are mostly independent (John et al 2001b). A combined assessment of AM- and FM- ASSRs did not allow for better detectability at near threshold intensities. This procedure however, did not yield better detection at low intensities compared to the simple AM response. However, the detection IAFM was related to the ability to perceive speech. There was a significant correlation (r = 0.74) between the number of significant IAFM responses and word recognition scores (WRS) when the stimuli was presented at different intensities. This last observation led to the third study, which used IAFM stimuli designed to be more representative of speech with respect to FM and AM components. Behavioral word recognition and ASSRs evoked by IAFM were recorded in normal-hearing and impaired subjects in masking and with hearing aids. A significant relationship was observed between the number of significant IAFM responses detected and the word recognition score with an overall correlation of 0.78. The results of these experiments suggest that MASTER can be used to objectively estimate hearing thresholds and that it can potentially be used to evaluate suprathreshold measures such as word recognition and benefit of hearing aid amplification.