EARLY MONAURAL OCCLUSION ALTERS THE NEURAL MAP OF INTERAURAL LEVEL DIFFERENCES IN THE INFERIOR COLLICULUS OF THE BARN OWL

Monaural occlusion during early life causes adaptive changes in the tu ning of units in the owl's optic tectum to interaural level difference s (ILD) that tend to align the auditory with the visual map of space. We investigated whether these changes could be due to experience-depen dent plasticity occuring in the auditory pathway prior to the optic te ctum. Units were recorded in the external nucleus of the inferior coll iculus (ICx), which is a major source of auditory input to the optic t ectum. The tuning of ICx units to ILD was measured in normal barn owls and in barn owls raised with one ear occluded. ILD tuning at each rec ording site was measured with dichotic noise bursts, presented at a co nstant average binaural level, 20 dB above threshold. The best ILD at each site was defined as the midpoint of the range of ILD values which elicited more than 50% of the maximum response. A physiological map o f ILD was found in the ICx of normal owls: best ILDs changed systemati cally from right-ear-greater to left-ear-greater as the electrode prog ressed from dorsal to ventral. Best ILDs ranged from 13 dB right-ear-g reater to 15 dB left-ear-greater and progressed at an average rate of 12 dB/mm. The representations of ILD were similar on both sides of the brain. In the ICx of owls raised with one ear occluded, the map of IL D was shifted in the adaptive direction: ILD tuning was shifted toward s values favoring the non-occluded ear (the direction that would resto re a normal space map). The average magnitude of the shift was on the order of 8-10 dB in each of 4 owls. In one owl, the mean shift in ILD tuning was almost identical on both sides of the brain. In another owl , the mean shift was much larger on the side ipsilateral to the occlus ion than on the contralateral side. In both cases, the mean shifts mea sured in each ICx were comparable to the mean shifts measured in the o ptic tectum on the same sides of the brain. Thus, the adjustments in I LD tuning that have been observed in the optic tectum in response to m onaural occlusion are almost entirely due to adaptive mechanisms that operate at or before the level of the ICx.