Stimulus Rate and Subcortical Auditory Processing of Speech: Comparison between Younger and Older Adults

Abstract

Perception of acoustic signal depends on accurate encoding of temporal events of auditory signals. The auditory brainstem reflects processing of temporal events those are diagnostically significant in the assessment of hearing loss and neurological function (Hall, 1992). Degenerative changes occur with advancing age in the central auditory pathway, including both sub cortical and cortical structures. 17 young aged individuals (30 ears 18 to30 years) and 15 older adults (30 ears- 40 to 55 years) with normal hearing sensitivity participated in the study). The brainstem responses to click stimulus was recorded presented at 80 dB SPL across three repetition rates) and was analyzed for wave V latency. The brainstem response to speech evoked ABR was also recorded by using syllable /da/ presented at 80 dB SPL across the three repetition rates (6.9, 10.9 & 15.4). The speech ABR waveforms were analyzed for both the onset (latency of wave V and A) and the sustained (latencies of wave - D, E and F) responses. FFT was done to find the raw amplitude of FO, Fl and higher harmonics (F2) frequency components. The increase in latency of speech evoked ABR (transient and sustained response) and click evoked ABR for the older adults suggest that the brain stem timing might be affected for the older adults. A reduction in amplitude for the coding of FO, F 1 and F2 for the older adult group was seen. Reduction in coding of FO, F 1 and F2 might be leading to the speech perception problems in older individuals.