ENHANCED EVOKED-RESPONSE AMPLITUDES IN THE INFERIOR COLLICULUS OF THE CHINCHILLA FOLLOWING ACOUSTIC TRAUMA

Evoked response amplitude-level functions were measured from electrodes in the inferior colliculus of the chinchilla before and after exposure to a 2 kHz pure tone of 105 dB SPL. The exposure produced approximately 20-30 dB of permanent threshold shift from 2 to 8 kHz, but little or no hearing loss at higher or lower frequencies. Generally less than 60% of the outer hair cells were missing in the region of hearing loss. The amplitude-level functions measured at 4 and 8 kHz generally showed a loss in sensitivity at low sound levels, a reduction in the maximum amplitude and sometimes steeper than normal slopes. The amplitude-level functions measured at 2 kHz also showed a loss in sensitivity; however, the maximum amplitude was often greater than normal. Even though there was no loss in sensitivity at 0.5 kHz, the amplitude-level function was steeper than normal and the maximum amplitude of the evoked response was almost always substantially larger than normal. The enhancement of the evoked response amplitude from the inferior colliculus does not appear to originate in the cochlea, but may reflect a reorganization of neural activity in the central auditory pathway.