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The Journal of Neuroscience, January 15, 2000, 20(2):791-797
Aging Affects Hemispheric Asymmetry in the Neural Representation
of Speech Sounds
Teri James
Bellis1, 2,
Trent
Nicol1, and
Nina
Kraus1, 3
1 Auditory Neuroscience Laboratory, Department of
Communication Sciences and Disorders, and 3 Departments of
Neurobiology and Physiology and Otolaryngology, Northwestern
University, Evanston, Illinois, and 2 Department of
Communication Disorders, University of South Dakota, Vermillion, South
Dakota
Hemispheric asymmetries in the processing of elemental speech
sounds appear to be critical for normal speech perception. This study
investigated the effects of age on hemispheric asymmetry observed in
the neurophysiological responses to speech stimuli in three groups of
normal hearing, right-handed subjects: children (ages, 8-11 years),
young adults (ages, 20-25 years), and older adults (ages > 55 years). Peak-to-peak response amplitudes of the auditory cortical
P1-N1 complex obtained over right and left temporal lobes were
examined to determine the degree of left/right asymmetry in the
neurophysiological responses elicited by synthetic speech syllables in
each of the three subject groups. In addition, mismatch negativity
(MMN) responses, which are elicited by acoustic change, were obtained.
Whereas children and young adults demonstrated larger P1-N1-evoked
response amplitudes over the left temporal lobe than over the right,
responses from elderly subjects were symmetrical. In contrast, MMN
responses, which reflect an echoic memory process, were symmetrical in
all subject groups. The differences observed in the neurophysiological
responses were accompanied by a finding of significantly poorer ability
to discriminate speech syllables involving rapid spectrotemporal
changes in the older adult group. This study demonstrates a biological,
age-related change in the neural representation of basic speech sounds
and suggests one possible underlying mechanism for the speech
perception difficulties exhibited by aging adults. Furthermore, results
of this study support previous findings suggesting a dissociation between neural mechanisms underlying those processes that reflect the
basic representation of sound structure and those that represent auditory echoic memory and stimulus change.
Key words:
laterality; cerebral dominance; aging; speech perception; evoked potentials; mismatch negativity
Copyright © 2000 Society for Neuroscience 0270-6474/00/202791-07$05.00/0
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