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.

The ability of the two hemispheres of the brain to communicate with one another via the corpus callosum is important for a wide variety of sensory, motor, and cognitive functions, many of them communication related. Anatomical evidence suggests that aging results in structural changes in the corpus callosum and that the course over time of age-related changes in corpus callosum structure may depend on the gender of the individual. Further, it has been hypothesized that age- and gender-related changes in corpus callosum structure may result in concomitant decreased performance on tasks that are reliant on interhemispheric integrity. The purpose of this study was to investigate the effects of age and gender on auditory behavioral and visuomotor temporal indices of interhemispheric function across the life span of the normal adult. Results from 120 consistently right-handed adults from age 20 to 75 years revealed that interhemispheric integrity, as measured by dichotic listening, auditory temporal patterning, and visuomotor interhemispheric transfer time tasks, decreases relatively early in the adult life span (i.e., between the ages of 40 and 55 years) and shows no further decrease thereafter. In addition, the course over time of interhemispheric decline is different for men compared to women for some tasks. These findings suggest that decreased interhemispheric function may be a possible factor contributing to auditory and communication difficulties experienced by aging adults. In addition, results of this study hold implications for the clinical assessment of interhemispheric function in aging adults and for future research into the functional ramifications of decreased multimodality interhemispheric transfer.

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