Investigating Cortical Descending Control of the Peripheral Auditory System

Abstract

Corticofugal projections may modulate auditory signal processing at all levels of the ascending auditory system. The final link in this pathway, extending descending control to the cochlea, is the olivocochlear bundle, which originates in the olivary complex and consists of lateral and medial (MOC) systems. Classically, activation of the MOC system leads to an increase in cochlear microphonic and a decrease in cochlear action potential amplitudes.

Here, we investigated the effect of reversible cortical inactivation on the guinea pig cochlea. During cortical inactivation, CM and CAP amplitudes were decreased to varying extents. Reactivation of the cortex led to a recovery to control levels. These results indicate that even in the anaesthetized animal, the cortex is exercising control over the function of the cochlea.

Appendices

Comment by Stefan Strahl

Did you observe any changes to the compound action potential waveform? In the context that the second negative potential (N2) can disappear in lession studies (e.g., Meurice et al. 1991) it might be interesting if any effects (e.g., to the latencies) of N1, P1 and N2 are observable in your experiments.

Reply Alan R. Palmer

We have normalized and compared the CAP waveforms measured before and during cooling. There is a change in the shape of the CAP waveform as the N1 component is reduced by a larger proportion than the N2 component and there is an increase in latency of about 0.5 ms. There was no gross change in the shape such as the loss of the N2 peak that occurs when the auditory nerve is sectioned as it leaves the internal auditory canal (Meurice et al. 1991).