Effects of ageing on neural onset and offset responses in core and belt auditory cortex of alert macaque monkeys

Age-related hearing loss is a ubiquitous malady in the geriatric population, yet the underlying neural mechanisms remain incompletely understood. We compared onset and offset responses of single neurons in the primary (A1), rostral (R) and caudolateral (CL) areas of the auditory cortex using spectral, spatial and temporal stimuli in a younger macaque monkey with normal-hearing and an aged macaque monkey with high frequency hearing loss. In the younger monkey, core areas exhibited a significantly higher proportion of neurons responsive to both the onset and the offset of stimuli compared to the belt area CL, indicating functional differentiation. However, this distinction diminished with age, with the proportion of neurons with both onset and offset responses becoming more uniform across cortical areas. Onset firing rates were generally higher in the aged monkey, but with a lower signal-to-noise ratio, suggesting increased neural excitability but reduced response fidelity. Notably, CL neurons in the aged monkey exhibited a significantly greater disparity between the best frequencies for onset and offset responses, suggesting reduced spectral precision. Additionally, spectral tuning bandwidths (BW) were broader in the CL neurons in the aged monkey, whereas, in the younger monkey, A1 neurons exhibited significantly narrower onset BWs compared to offset BWs, a distinction that was lost in the aged monkey. These findings highlight a fundamental asymmetry in auditory cortical processing and suggest that belt area CL is particularly susceptible to age-related changes. Understanding these neural mechanisms provides insights into auditory ageing and potential strategies for mitigating hearing deficits. KEY POINTS: Age-related hearing loss affects how the brain processes auditory information in older adults, with neural changes observed in both core and belt areas of the auditory cortex. In the younger monkey, different auditory areas show distinct patterns of onset and offset responses, but this functional separation was reduced in aged monkeys. The aged monkey showed higher onset firing rates but reduced signal fidelity, indicating increased neural excitability and reduced response fidelity. The caudolateral auditory area shows the greatest age-related changes, suggesting that it is especially vulnerable to auditory decline. These findings highlight how age-related changes impact the auditory processing in different areas of the auditory cortex.