SITE OF AUDITORY PLASTICITY IN THE BRAIN-STEM (VLVP) OF THE OWL REVEALED BY EARLY MONAURAL OCCLUSION

1. The optic tectum of the barn owl contains a physiological map of in teraural level difference (ILD) that underlies, in part, its map of au ditory space. Monaural occlusion shifts the range of ILDs experienced by an animal and alters the correspondence of ILDs with source locatio ns. Chronic monaural occlusion during development induces an adaptive shift in the tectal ILD map that compensates for the effects of the ea rplug. The data presented in this study indicate that one site of plas ticity underlying this adaptive adjustment is in the posterior divisio n of the ventral nucleus of the lateral lemniscus (VLVp), the first si te of ILD comparison in the auditory pathway. 2. Single and multiple u nit sites were recorded in the optic tecta and VLVps of ketamine-anest hetized owls. The owls were raised from 4 wk of age with one ear occlu ded with an earplug. Auditory testing, using digitally synthesized dic hotic 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. Evidenc e of adaptive adjustment in the VLVp was based on statistical differen ces between the VLVp's ipsilateral and contralateral to the occluded e ar in the sensitivity of units to excitatory-ear and inhibitory-ear st imulation. 3. The balance of excitatory to inhibitory influences on VL Vp units was shifted in the adaptive direction in six out of eight owl s. In three of these owls, adaptive differences in inhibition, but not in excitation, were found. For this group of owls, the patterns of re sponse properties across the two VLVps can only be accounted for by pl asticity in the VLVp. For the other three owls, the possibility that t he 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 e xplain the data. In the remaining two owls, there was no evidence of a daptive adjustment in the VLVp despite large adaptive adjustments in t he optic tectum. 4. The adjustment of ILD coding in the VLVp was alway s substantially smaller than expected based on the adjustment of ILD t uning in the optic tectum measured in the same animals. This indicates the involvement of at least one additional site of adaptive plasticit y in the auditory pathway above the level of the VLVp. 5. The results indicate an activity-dependent mechanism operating in the VLVp that ca n adjust the range of ILD sensitivity toward the range of ILDs that ar e experienced by the animal.