Abstract
The auditory spatial tuning of bimodal (auditory-visual) units in the optic tectum of the barn owl was altered by raising animals with one ear occluded. Changes in spatial tuning were assessed by comparing the location of a unit's auditory best area with that of its visual receptive field. As shown previously, auditory best areas are aligned with visual receptive fields in the tecta of normal birds (Knudsen, E. I. (1982) J. Neurosci. 2: 1177–1194). It was demonstrated in this study that, when birds were raised with one ear occluded, best areas and visual receptive fields were aligned only as long as the earplug was in place. When the earplug was removed, best areas and visual receptive fields became misaligned, indicating that a change in auditory spatial tuning had taken place during the period of occlusion. However, in a bird that received an earplug as an adult, no such alterations in auditory spatial tuning were observed; even after 1 year of monaural occlusion, auditory best areas and visual receptive fields were misaligned so long as the earplug was in place, and were aligned when the earplug was removed. These results suggest that exposure to abnormal localization cues modifies the auditory spatial tuning of tectal units only during a restricted, sensitive period early in development. After the earplug was removed from a juvenile bird that had been raised with an occluded ear, the initial misalignment between auditory best areas and visual receptive fields decreased gradually over a period of weeks. In contrast, when earplugs were removed from two adult birds that had been raised with monaural occlusions, auditory- visual misalignments persisted for as long as measurements were made, which was up to 1 year after earplug removal. These data indicate that auditory cues become permanently associated with locations in visual space during a critical period which draws to a close at about the age when the animal reaches adulthood. Horseradish peroxidase was injected into two optic tecta (in a single animal) that contained units with permanently altered auditory spatial tuning. The positions of retrogradely labeled cells in the external nucleus of the inferior colliculus (ICX) were the same as those observed in control birds (Knudsen, E. I., and P. F. Knudsen (1983) J. Comp. Neurol. 218: 187–196). Thus, the changes in spatial tuning were not due to a shift in the topographic projection from the ICX to the optic tectum.(ABSTRACT TRUNCATED AT 400 WORDS)