Aging Affects Hemispheric Asymmetry in the Neural Representation of Speech Sounds

Abstract

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.

Right-Hemisphere Auditory Cortex Is Dominant for Coding Syllable Patterns in Speech

“…To promote subject stillness during long recording sessions as well as diminish attention to the auditory stimuli, subjects watched a videotape movie of his or her choice and listened to the soundtrack to the movie in the nontest ear with the sound level set Ͻ40 dB SPL. This paradigm for measuring cortical-evoked potentials has been used in previous studies investigating cortical asymmetry for speech sounds (Bellis et al, 2000;Abrams et al, 2006) as well as other forms of cortical speech processing (Kraus et al, 1996;Banai et al, 2005;Wible et al, 2005). Although it is acknowledged that cortical activity in response to a single stimulus presentation includes contributions from both the experimental speech stimulus and the movie soundtrack, auditory information in the movie soundtrack is highly variable throughout the recording session.…”

Speech-Evoked Neurophysiologic Responses in Children with Learning Problems: Development and Behavioral Correlates of Perception

“…The normal group (WNL [within normal limits]) included 1) children between the ages of 5 and 15 yr (N ϭ 130, 70 female, 60 male) with no history of learning or attention problems based on a detailed parent questionnaire; moreover, children displayed scores within normal limits (including no discrepancy between ability and achievement) on all standardized tests of learning and academic achievement described below; 2) young adults between the ages of 19 and 27 yr (N ϭ 10, all female); and 3) senior adults between the ages of 55 and 78 yr (N ϭ 10, all female). Young and senior adult data were obtained from Bellis, Nicol, and Kraus (2000). The group with learning problems (LP) included children between the ages of 8 and 15 yr (N ϭ 86, 21 female and 65 male) previously diagnosed as having a learning disability (LD; N ϭ 35, 12 female and 23 male), attention deficit disorder (ADD; N ϭ 30, four female and 26 male) or both (ADLD; N ϭ 21, five female and 16 male).…”

Auditory Brainstem Timing Predicts Cerebral Asymmetry for Speech

“…Like ABRs, cortical potentials rely on stimulus-locked, synchronous firing from neuronal ensembles; however, they provide an abstract representation of features in acoustic stimuli. The early components of cortical responses (Ͻ150 ms) reflect obligatory acoustic processing of speech stimuli (Sharma et al, 2000) and left-asymmetric responses in this time range is thought to describe its preference for processing of rapid acoustic signals, including speech (Liégeois-Chauvel et al, 1999;Bellis et al, 2000).…”