Site of auditory plasticity in the brain stem (VLVp) of the owl revealed by early monaural occlusion.

Abstract

1. The optic tectum of the barn owl contains a physiological map of interaural level difference (ILD) that underlies, in part, its map of auditory space. Monaural occlusion shifts the range of ILDs experienced by an animal and alters the correspondence of ILDs with source locations. Chronic monaural occlusion during development induces an adaptive shift in the tectal ILD map that compensates for the effects of the earplug. The data presented in this study indicate that one site of plasticity underlying this adaptive adjustment is in the posterior division of the ventral nucleus of the lateral lemniscus (VLVp), the first site of ILD comparison in the auditory pathway. 2. Single and multiple unit sites were recorded in the optic tecta and VLVps of ketamine-anesthetized owls. The owls were raised from 4 wk of age with one ear occluded with an earplug. Auditory testing, using digitally synthesized dichotic stimuli, was carried out 8-16 wk later with the earplug removed. The adaptive adjustment in ILD coding in each bird was quantified as the shift from normal ILD tuning measured in the optic tectum. Evidence of adaptive adjustment in the VLVp was based on statistical differences between the VLVp's ipsilateral and contralateral to the occluded ear in the sensitivity of units to excitatory-ear and inhibitory-ear stimulation. 3. The balance of excitatory to inhibitory influences on VLVp units was shifted in the adaptive direction in six out of eight owls. In three of these owls, adaptive differences in inhibition, but not in excitation, were found. For this group of owls, the patterns of response properties across the two VLVps can only be accounted for by plasticity in the VLVp. For the other three owls, the possibility that the difference between the two VLVps resulted from damage to one of the VLVps could not be eliminated, and for one of these, plasticity at a more peripheral site (in the cochlea or cochlear nucleus) could also explain the data. In the remaining two owls, there was no evidence of adaptive adjustment in the VLVp despite large adaptive adjustments in the optic tectum. 4. The adjustment of ILD coding in the VLVp was always substantially smaller than expected based on the adjustment of ILD tuning in the optic tectum measured in the same animals. This indicates the involvement of at least one additional site of adaptive plasticity in the auditory pathway above the level of the VLVp.(ABSTRACT TRUNCATED AT 400 WORDS)