Effects of age upon auditory evoked potentials from the inferior colliculus and cortex in the guinea pig

Stability of neural representations in the auditory midbrain across the lifespan despite age-related brainstem delays

The extent to which aging of the central auditory pathway impairs auditory perception in the elderly independent of peripheral cochlear decline is debated. To cause auditory deficits in normal hearing elderly, central aging needs to degrade neural sound representations at some point along the auditory pathway. However, inaccessible to psychophysical methods, the level of the auditory pathway at which aging starts to effectively degrade neural sound representations remains poorly differentiated. Here we tested how potential age-related changes in the auditory brainstem affect the stability of spatiotemporal multiunit complex speech-like sound representations in the auditory midbrain of old normal hearing CBA/J mice. Although brainstem conduction speed slowed down in old mice, the change was limited to the sub-millisecond range and only minimally affected temporal processing in the midbrain (i.e. gaps-in-noise sensitivity). Importantly, besides the small delay, multiunit complex temporal sound representations in the auditory midbrain did not differ between young and old mice. This shows that although small age-related neural effects in simple sound parameters in the lower brainstem may be present in aging they do not effectively deteriorate complex neural population representations at the level of the auditory midbrain when peripheral hearing remains normal. This result challenges the widespread belief of 'pure' central auditory decline as an automatic consequence of aging, at least up to the inferior colliculus. However, the stability of midbrain processing in aging emphasizes the role of undetected 'hidden' peripheral damage and accumulating effects in higher cortical auditory-cognitive processing explaining perception deficits in 'normal hearing' elderly.

The effect of noise-induced sloping high-frequency hearing loss on the gap-response in the inferior colliculus and auditory cortex of guinea pigs

Gap detection has been used as an evaluation tool for temporal processing in subjects with sensorineural hearing loss (SNHL). However, the results from other reports are varied making it difficult to clearly define the impact of SNHL on the temporal processing ability of the auditory system. Specifically, we do not know if and how a high-frequency hearing loss impacts, presumably through off-channel interaction, the temporal processing in low-frequency channels where hearing sensitivity is virtually normal. In this experiment, gap-evoked responses in a low-frequency band (0.5-8 kHz) were recorded in the inferior colliculus (IC) and auditory cortex (AC) of guinea pigs through implanted electrodes, before and after a slopping high-frequency hearing loss, which was induced by over-stimulation using a 12-kHz-tone. The results showed that the gap thresholds in the low-frequency region increased gradually and became significantly higher 8 weeks after the induced high-frequency hearing loss. In addition, the response latency was slightly increased in the IC but this was not true for the AC. These results strongly indicate that a high-frequency hearing loss exerted an off-channel impact on temporal processing in the low-frequency region of the auditory system.