SPEECH RECOGNITION IN NOISE AND PRESBYCUSIS - RELATIONS TO POSSIBLE NEURAL MECHANISMS

This study is part of ongoing efforts to characterize and determine th e neural bases of presbycusis. These efforts utilize humans and animal s in sets of overlapping hypotheses and experiments. Here, 50 young ad ult and elderly subjects, with normal audiometric thresholds or high-f requency hearing loss, were presented three types of linguistic materi als at suprathreshold levels to determine speech recognition performan ce in noise. The study sought to determine how peripheral and central auditory system dysfunctions might be implicated in the speech recogni tion problems of elderly humans. There were four main findings. (1) Pe ripheral auditory nervous system pathologies, manifested as reduced se nsitivity for speech-frequency pure tones and speech materials, contri bute to elevated speech reception thresholds in quiet, and to reduced speech recognition in noise, (2) Good cognitive ability was demonstrat ed in the old subjects who took advantage of supportive context as wel l or better than young subjects, strongly indicating that the cortical portions of the speech/language nervous system did not account for th e speech understanding dysfunctions of the old subjects. (3) When audi bility and cognitive functioning were not affected, the demonstrated s peech-recognition in-noise dysfunction remained in old subjects. This implicates auditory brainstem or auditory cortex temporal-resolution d ysfunctions in accounting for the observed differences in speech proce ssing. (4) Performance differences between young and elderly subjects with elevated thresholds illustrate the effects of age plus hearing lo ss and thereby implicate both peripheral and central dysfunctions in p resbycusics. This is because the differences in performance between yo ung and elderly subjects with normal peripheral sensitivity identified a central auditory dysfunction.