Temghare, N. | Search bioRxiv

bioRxiv2026

Noise-induced temporary threshold shift in macaques disrupts electrophysiological temporal processing despite recovery of cochlear sensitivity and preserved ribbon synapse counts

Noise exposure can produce lasting auditory dysfunction in the absence of permanent threshold shifts or hair cell loss, yet the functional consequences of temporary threshold shift (TTS) remain poorly defined in translational models. We assessed auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) in rhesus macaques (n = 13) at 2 and 9-10 months following a single moderate noise exposure that induced TTS. Previous histological analyses of these macaques showed no significant loss of hair cells or ribbon synapses but revealed persistent broadening of inner and outer hair cell ribbon-volume distributions. After exposure, DPOAE amplitudes and thresholds and ABR thresholds returned to pre-exposure values and showed low-frequency enhancement at later time points. Suprathreshold click- and tone-evoked ABR amplitudes were largely preserved or enhanced after exposure, consistent with compensatory gain. In contrast, macaque-specific chirp-evoked ABRs showed modest amplitude reductions and latency prolongation across waves, indicating altered neural synchrony at standard stimulus presentation rates, but with variable time courses. More temporally demanding paradigms revealed persistent impairments. ABRs to faster click rates and shorter paired-click intervals showed reduced adaptability in response amplitude and timing after normalization, with deficits persisting through 9-10 months. Increased inner hair cell ribbon-volume variability was more consistently associated with temporal response measures, including latency, paired-click recovery, and rate adaptation, than with amplitude-based ABR measures. Together, these findings reveal a lasting dissociation between response magnitude and fidelity after TTS: suprathreshold responses may be preserved or enhanced, while neural synchrony and temporal adaptability remain impaired. Increased presynaptic ribbon volume variability may serve as a structural marker of synaptic remodeling accompanying hidden auditory dysfunction, rather than as a direct determinant of suprathreshold response magnitude. Temporally demanding ABR paradigms may supplement threshold-based diagnostics for detecting persistent noise-induced auditory dysfunction.

neuroscience↗

bioRxiv2022

Psychometric and subcortical neurometric measures of temporal discrimination in rhesus macaques

Temporal envelope fluctuations are abundant in nature and are critical for perception of complex sounds. While psychophysical sinusoidal amplitude modulation (SAM) processing studies have characterized the perception of SAM, and neurophysiological studies report a subcortical transformation from temporal to rate-based code, no studies have characterized this transformation in unanesthetized animals or in nonhuman primates. To address this, we recorded single-unit responses and compared derived neurometric measures in the cochlear nucleus (CN) and inferior colliculus (IC) to psychometric measures of modulation frequency (MF) discrimination in macaques. IC and CN neurons often exhibited tuned responses to SAM in their rate and spike-timing. Neurometric thresholds spanned a large range (2-200 Hz {Delta} MF). The lowest 40% of IC thresholds were less than or equal to psychometric thresholds, regardless of which code was used, while CN thresholds were greater than psychometric thresholds. Discrimination at 10-20 Hz could be explained by indiscriminately pooling 30 units in either structure, while discrimination at higher MFs was best explained by more selective pooling. This suggests that pooled brainstem activity was sufficient for AM discrimination. Psychometric and neurometric thresholds decreased as a function of stimulus duration, but IC and CN thresholds were greater and more variable than behavior at durations less than 500 ms. This slower subcortical temporal integration compared to behavior was consistent with a drift diffusion model which reproduced individual differences in performance and can constrain future neurophysiological studies of temporal integration. These measures provide an account of AM perception at the neurophysiological, computational, and behavioral levels. Significance statementListening in everyday environments tasks the brain with extracting information from sound envelopes. This process involves both sensory encoding and decision-making. Different neural codes for envelope representation have been well characterized in the auditory midbrain and cortex, but studies of the brainstem have usually been conducted in anesthetized rodents or cats. Moreover, these candidate neural codes have been studied in isolation from the decision-making process. In this study, we found that population activity in the primate subcortical auditory system contains sufficient information for discriminating sound envelope and applied a biologically plausible model of decision-making to sound envelope discrimination performance from rhesus macaques, a species with great phylogenetic and perceptual similarity to humans.

neuroscience↗