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Abstract
This study is part of ongoing efforts to characterize and determine the neural bases
of presbycusis. These efforts utilize humans and animals in sets of overlapping hypotheses
and experiments. Here, 50 young adult and elderly subjects, with normal audiometric
thresholds or high-frequency hearing loss, were presented three types of linguistic
materials at suprathreshold levels to determine speech recognition performance in
noise. The study sought to determine how peripheral and central auditory system dysfunctions
might be implicated in the speech recognition problems of elderly humans. There were
four main findings. (1) Peripheral auditory nervous system pathologies, manifested
as reduced sensitivity for speech-frequency pure tones and speech materials, contribute
to elevated speech reception thresholds in quiet, and to reduced speech recognition
in noise. (2) Good cognitive ability was demonstrated in the old subjects who took
advantage of supportive context as well or better than young subjects, strongly indicating
that the cortical portions of the speech/language nervous system did not account for
the speech understanding dysfunctions of the old subjects. (3) When audibility and
cognitive functioning were not affected, the demonstrated speech-recognition in-noise
dysfunction remained in old subjects. This implicates auditory brainstem or auditory
cortex temporal-resolution dysfunctions in accounting for the observed differences
in speech processing. (4) Performance differences between young and elderly subjects
with elevated thresholds illustrate the effects of age plus hearing loss and thereby
implicate both peripheral and central dysfunctions in presbycusics. This is because
the differences in performance between young and elderly subjects with normal peripheral
sensitivity identified a central auditory dysfunction.