| This dissertation examines the relationship between amplitude/latency changes of the auditory brainstem (ABR) and middle latency (MLR) responses with intensity and behavioral loudness growth measures to 1000-Hz tones. Studies of both normal-hearing and sensorineurally-impaired listeners examined (1) the accuracy of the ABR and MLR in predicting behavioral hearing threshold, and (2) the relation between the rate of ABR/MLR amplitude growth/latency decrease and behavioral loudness growth with increasing threshold shift. Results indicate that the ABR and MLR give equally accurate estimates of hearing threshold for 1000-Hz tones for both groups of subjects. In addition, ipsilateral broadband masking effectively shifted the thresholds of the normal-hearing listeners to 40 and 60 dB nHL levels for both the electrophysiologic and behavioral measures. Relative to the normal response, electrophysiologic measures to supra threshold stimuli reveal (1) differences between the ABR and MLR in subjects with decreased hearing sensitivity, and (2) the effects of noise masking on the ABR/MLR are different than the effects of cochlear impairment. ABR and MLR intensity-amplitude/latency functions show a slower rate of change with intensity compared to behavioral loudness growth rates for both normal and impaired listeners. However, ipsilateral broadband masking studies of the normal listeners reveal that the relative increase in slope with increasing masker is similar for both the electrophysiologic and behavioral measures, indicating that the ABR/MLR reflect, at least in part, the same neural processes that underlie loudness perception. Hearing loss (masking and cochlear impairment) causes an increase in the rate of ABR/MLR amplitude growth, ABR latency decrease, and loudness growth with intensity increase, and these electrophysiologic and behavioral functions steepen with increasing threshold shift. These supra threshold results indicate that the ABR/MLR may serve as potential predictors/indicators of loudness growth in persons who are unable to report their judgement of the loudness of sound. |
