The ability of ferrets to localize sounds in space was determined before and after large unilateral lesions of the inferior colliculus. Three animals sustained restricted lesions that destroyed the inferior colliculus caudally but spared some tissue rostrally. These restricted lesions did not extend substantially beyond the cytoarchitectonic boundaries of the inferior colliculus. Four animals received more extensive lesions that destroyed all of the inferior colliculus, including the rostral pole. These lesions also involved to various degrees other midbrain structures, including the dorsal nucleus of the lateral lemniscus and the lateral tegmentum. Psychophysical curves and minimum audible angles were obtained for midline and left and right lateral fields. Comparisons were made of deficits ipsilateral and contralateral to the lesion. 2. The animals were tested in a semicircular apparatus with loudspeakers located at various positions around the periphery. Separate sound localization tests were conducted for midline, left, and right field positions. In each test the animals were trained first to discriminate between speakers separated by 60 degrees. They were then tested with progressively smaller angles of speaker separation to obtain their psychophysical threshold (minimum audible angle). All critical tests of sound localization were based on the ability of animals to identify the location of a 45-ms noise burst presented at the beginning of each trial. Correct responses were rewarded by delivery of a small quantity of water from spouts located around the perimeter of the apparatus. 3. Unilateral lesions restricted to the inferior colliculus had relatively little effect on sound localization. All three animals with restricted lesions were still capable of localizing a single noise burst and minimum audible angles were similar before and after surgery for midline, left, and right field tests. There was no difference in postoperative performance ipsilateral and contralateral to the lesion site. 4. More extensive unilateral lesions that included the dorsal nucleus of the lateral lemniscus and the lateral tegmentum as well as the inferior colliculus resulted in severe deficits in sound localization. Each of the four animals with large unilateral lesions had a substantial impairment in sound localization in the field contralateral to the lesion site. Postoperative performance in the contralateral field was too low to obtain a minimum audible angle for these animals. Midline sound localization was also affected in some cases but ipsilateral sound localization was only slightly affected.
