Objective: This experiment was designed to evaluate whether neural encoding of speech features at the brain stem level is altered in the aging auditory system. In addition, the effect of minimal peripheral hearing loss on the auditory brain stem response (ABR) evoked by speech stimuli and interactions with aging were examined.
Design: Speech-evoked ABRs (S-ABRs) were recorded using a synthetic 40-msec /da/ stimulus from both ears of participants in two groups: normal-hearing younger adults (n = 19) and normal-hearing older adults (n = 18). Latencies and amplitude for S-ABR peaks representing neural responses to the onset and offset of the speech syllable as well as a sustained frequency following response to the vowel content were analyzed. The role of hearing threshold differences between groups and the reduced overall stimulus level on the S-ABR were also examined. In addition, click-evoked ABRs (C-ABRs) were obtained from all participants, and age-group differences in the neural response to both types of stimuli at the brain stem level were compared.
Results: S-ABR latencies, amplitudes, and sustained response mean data were obtained for younger adults and older adults. Older adults were found to have significantly smaller C-ABRs with longer latencies, despite all latencies falling within normal limits. Older adults also had significantly smaller onset and offset responses for the S-ABR, with significantly delayed offset latencies in response to this synthetic consonant-vowel syllable. Many of the C-ABR and S-ABR variables were found to significantly correlate with high-frequency audiometric thresholds, and few of the group differences remained significant when this was taken into account. The remaining significant S-ABR effects were decreased amplitude at the onset and significantly delayed offset responses in the older group. These effects were different from those of simply decreasing the overall stimulus level, which caused significant shifts in latency across the entire S-ABR.
Conclusions: Results of this study partially supported the hypothesis of age-related differences in neural processing of speech at the brain stem level. There were significant delays in the timing of the offset portion of the S-ABR in older listeners compared with their younger counterparts, even after accounting for the differences in peripheral hearing threshold between groups. There were also significant reductions in amplitude of the S-ABR at the onset. These results are consistent with a reduction in neural synchrony in older adults to transient components of both speech and nonspeech sounds. However, sustained components of the S-ABR, which follow the harmonic components of the syllable, showed group differences but were not significant after adjusting for peripheral hearing loss, suggesting that they may be more affected by hearing sensitivity and other peripheral changes. These results support further investigation into the ability of the aging auditory system to encode temporal cues at the brain stem level, particularly the response to speech stimulus offset and its relationship to speech perception and temporal processing abilities.