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.

Fig. 1.

Effect of age on the degree of temporal lobe asymmetry in the P1–N1 response as calculated by the equation [(TL − TR)/(TL + TR)]. Results indicate that children and young adults demonstrated a significantly greater degree of temporal lobe asymmetry favoring the left hemisphere than did older adults. For older adults, responses were essentially symmetrical.

Fig. 2.

Grand averaged P1–N1 responses from the right and left temporal lobe electrode sites for children (n= 15), young adults (n = 11), and older adult subjects (n = 10). Responses were elicited by 1800–2500 repetitions of synthetic /da/ and /ga/ stimuli presented to the right ear at an intensity of 75 dB SPL and a rate of 1.9/sec. Grand averages indicate larger responses over the left temporal lobe than over the right for children and young adults and essentially symmetrical responses for older adults.

Fig. 3.

Individual subject data from children (n = 15), young adults (n = 11), and older adults (n = 10) for the degree of hemispheric asymmetry in the P1–N1 response. The degree of asymmetry was calculated using the equation [(TL − TR)/(TL + TR)].Positivevalues indicate larger responses over the left temporal lobe; negativevalues indicate larger responses over the right temporal lobe. The majority of children and all of the young adult subjects exhibited larger responses over the left temporal lobe. Overall, older adults exhibited symmetrical responses.

Fig. 4.

Effect of age group on the P1–N1 peak-to-peak amplitude obtained over right and left temporal electrode sites. Error bars represent one SD. Responses were significantly larger over the left temporal lobe than over the right for children and young adults but were symmetrical for older adults. Response amplitudes were significantly larger overall for children compared with young adult and older adult subjects.

Fig. 5.

MMN response amplitude, duration, and area obtained over the right and left temporal lobes from children, young adults, and older adults. Error bars represent one SD. Results indicate symmetrical responses over the temporal lobes for all MMN parameters.

Fig. 6.

JNDs obtained for /da/-to-/ga/ and /ba/-to-/wa/ stimulus continua obtained from children (n = 17), young adults (n = 12), and older adults (n = 12). Bottom, For the /da/-to-/ga/ continuum, each JND point represents a difference of 10 Hz in frequency of the third formant. Top, For the /ba/-to-/wa/ continuum, each JND point represents a difference of 1 msec in the duration of the first and second formant transition. Results indicate that elderly adults exhibit a significantly poorer ability to discriminate the /da–ga/ contrast compared with children and young adults as evidenced by the larger JNDs for the older subject group. There was no effect of subject group on the ability to discriminate the /ba–wa/ contrast.