Electrocochleographic frequency following responses as a potential marker of age-related peripheral degeneration

Wave-I of the transient-evoked auditory brainstem response has been proposed as a marker of peripheral neural degeneration, since it represents auditory nerve (AN) activity and can be separated in time from presynaptic hair-cell potentials and subsequent brainstem potentials. However, evidence suggests that steady-state potentials, like frequency following responses (FFRs), might be more representative measures of neural degeneration. In steady-state potentials, activity from different generators overlap in time and may be difficult to disentangle. Here, we measure peripheral responses with electrocochleography to quantify age-related AN degeneration. We disentangle AN responses from 'central' FFRs associated with brainstem generators and corroborate the absence of such brainstem activity with transient stimuli. We estimate the magnitude of the presynaptic cochlear microphonic that can obscure the AN contribution to the overall peripheral response with a high-frequency tone where the AN is minimally responsive. Recordings were obtained from 15 young and 14 older human participants with near-normal audiograms in response to high-level 10-ms tones at 516, 1032 and 3096 Hz and 100-µs clicks. Our results show a reduced peripheral response to the tones and clicks in the older participants, consistent with the presence of age-related degeneration across AN fibers in older listeners with near-normal hearing thresholds.