Auditory brainstem responses in ferrets following unilateral cochlear removal.

To examine the effect of unilateral or asymmetric hearing loss on the developing brain, ferrets had the right cochlea removed at postnatal day (P)5, P25, P40 or P90. Auditory brainstem responses (ABR) to free-field click stimulation were obtained before and after cochlear removal in the P40 and P90 groups, and from P28 in the other groups. Acute cochlear removal did not lead to any change in the morphology of the ABR waveform or to any change in the ABR threshold in response to stimulation on the side of the intact ear. There was a small, progressive increase in the latency of the four main waves of the ABR. Long-term survival following cochlear removal did not lead to any change in ABR waveform or threshold, or to any further change in wave latency, regardless of age at the time of removal or survival time (up to 1-2 years). In contrast to initial expectations based on previous single-unit studies, these results suggest that cochlear removal in infancy does not produce any large compensatory change in the functional organization of the auditory brainstem.