Functional changes in peripheral and central auditory pathways following selective inner hair cell loss

When the inner hair cells of the cochlea are destroyed, the neural activity conveyed to the central auditory system is reduced. How does the central auditory system respond when it is deprived of information? To address this question, inner hair cells in the chinchilla cochlea were selectively destroyed with carboplatin. Inner hair cell lesions had no effect on distortion product otoacoustic emissions or the cochlear microphonic potential, showing that the outer hair cells were functionally intact. Recordings from single auditory nerve fibers showed that the surviving inner hair cells and type I neurons had normal thresholds and sharp tuning. The only notable deficit was a reduction of the compound action potential from the auditory nerve in proportion to the amount of inner hair cell loss. Despite reduced input from the cochlea and auditory nerve, the local field potentials in the inferior colliculus and the auditory cortex were not consistently reduced. In fact, potentials in the auditory cortex were frequently larger than normal. Enhancement of the cortical potential may be due to loss of γ-aminobutyric acid (GABA)-mediated inhibition. The data show that when the inner hair cells of the cochlea are damaged, the central auditory system increases its gain to compensate for a reduced sensory input.