Inner hair cell loss leads to enhanced response amplitudes in auditory cortex of unanesthetized chinchillas: evidence for increased system gain

Carboplatin preferentially destroys inner hair cells (IHCs) in the chinchil la inner ear, while retaining a near-normal outer hair cell (OHC) populatio n. The present study investigated the functional consequences of IHC loss o n the compound action potential (CAP), inferior colliculus potential (ICP) and auditory cortex potential (ACP) recorded from chronically implanted ele ctrodes. IHC lass led to a reduction in CAP amplitude that was roughly prop ortional to IHC loss. The ICP amplitude was typically reduced by IHC loss, but the magnitude of this reduction was generally less than that observed f or the CAP. In contrast to the CAP and ICP, ACP amplitudes were generally n ot reduced following IHC loss. In some animals, the ACP amplitude remained at pre-carboplatin values despite substantial IHC loss. However, in other a nimals, IHC loss led to an increase ('enhancement') of ACP amplitude. ACP e nhancement was greatest at 1-2 weeks post-carboplatin, returning towards ba seline amplitudes at 5 weeks post-carboplatin. In other animals, the ACP re mained enhanced up to 5 weeks post-carboplatin. We interpret the transient and sustained enhancement of ACP amplitude following partial IHC loss as ev idence of functional reorganization occurring at or below the level of the auditory cortex. These results suggest that the gain of the central auditor y pathway increases following IHC loss to compensate for the reduced input from the cochlea. (C) 2000 Elsevier Science B.V. All rights reserved.