Changes in auditory brainstem evoked response latencies and amplitudes, and distortion product otoacoustic emission amplitudes in Fisher rats following electrical lesions made to the inferior colliculi

Bilateral lesions in the inferior colliculi (IC) of six experimental rats were created by passage of electrical current through a surgically inserted needle electrode. Nine control animals received sham surgery. Short latency auditory evoked responses (ABR) and distortion product otoacoustic emissions (DPOAE) with and without noise to the contralateral ear were measured pre and post surgery on both groups. Changes in the parameters on ABR and DPOAE were related to changes in the efferent responses mediated at the level of the IC.; The reduction of DPOAE amplitude with contralateral noise was confirmed in this study. There were no statistically significant changes in the ABR responses of cochlear microphonic amplitude, wave I latency and amplitude, wave V latency and amplitude and I–V interpeak latency, with contralateral noise. There were some overall changes in both ABR and DPOAE parameters during the course of this experiment that may be due to developmental changes. There were no statistically significant changes in ABR measurements between the treated and untreated rats, but there were significant reductions in DPOAE amplitude in the experimental rats. Under these experimental conditions, lesions to the IC resulted in increased inhibition of the cochlear responses in anesthetized rats. Therefore the IC may have an inhibitory effect on the action of the crossed olivocochlear bundle (COCB) which has the effect of increased inhibition of cochlear responses.