Temporal Coding of Single Auditory Nerve Fibers Is Not Degraded in Aging Gerbils

Abstract

People suffering from age-related hearing loss typically present with deficits in temporal processing tasks. Temporal processing deficits have also been shown in single-unit studies at the level of the auditory brainstem, midbrain, and cortex of aged animals. In this study, we explored whether temporal coding is already affected at the level of the input to the central auditory system. Single-unit auditory nerve fiber recordings were obtained from 41 Mongolian gerbils of either sex, divided between young, middle-aged, and old gerbils. Temporal coding quality was evaluated as vector strength in response to tones at best frequency, and by constructing shuffled and cross-stimulus autocorrelograms, and reverse correlations, from responses to 1 s noise bursts at 10-30 dB sensation level (dB above threshold). At comparable sensation levels, all measures showed that temporal coding was not altered in auditory nerve fibers of aging gerbils. Furthermore, both temporal fine structure and envelope coding remained unaffected. However, spontaneous rates were decreased in aging gerbils. Importantly, despite elevated pure tone thresholds, the frequency tuning of auditory nerve fibers was not affected. These results suggest that age-related temporal coding deficits arise more centrally, possibly due to a loss of auditory nerve fibers (or their peripheral synapses) but not due to qualitative changes in the responses of remaining auditory nerve fibers. The reduced spontaneous rate and elevated thresholds, but normal frequency tuning, of aged auditory nerve fibers can be explained by the well known reduction of endocochlear potential due to strial dysfunction in aged gerbils.

Sound source localization patterns and bilateral cochlear implants: Age at onset of deafness effects

“…Decreases in amplitude for all waves and increases in latency for early waves of the auditory brainstem response occur with increasing age in humans [ 102 ], suggesting fewer or less consistent inputs. Interestingly, the temporal resolution of individual auditory nerve fibers seems to remain intact during aging in the absence of hearing loss [ 103 ]. Thus, changes in the auditory brainstem response may be related to the loss of auditory nerve fibers associated with aging.…”

Olivocochlear projections contribute to superior intensity coding in cochlear nucleus small cells

“…Human studies suggest that such perceptual deficits reflect difficulties in coding temporal envelopes (Bharadwaj et al 2014), expected to impair speech recognition (Shannon et al 1995). Temporal-coding deficits after synaptopathy are not apparent in ANFs (Heeringa et al 2020), suggesting their origin may be central. The enhanced intensity coding and precise envelope coding shown here for small cells primes the SCC as an important subcortical area for speech processing, and is expected to be preferentially targeted by cochlear synaptopathy.…”

Cochlear nucleus small cells use olivocochlear collaterals to encode sounds in noise

“…Human studies suggest that such perceptual deficits reflect difficulties in coding temporal envelopes (Bharadwaj et al, 2014), expected to impair speech recognition (Shannon et al, 1995). Temporal-coding deficits after synaptopathy are not apparent in ANFs (Heeringa et al, 2020), suggesting their origin may be central. The enhanced intensity coding and precise envelope coding shown here for small cells primes the SCC as an important subcortical area for speech processing is expected to be preferentially targeted by cochlear synaptopathy.…”