Midline and lateral field sound localization in the ferret (Mustela putorius): contribution of the superior olivary complex.

1. The ability of ferrets to localize sound in space was determined before and after unilateral or bilateral lesions of the superior olivary complex (SOC). Lesions were made by pressure injection of kainic acid into the SOC through a stereotaxically positioned glass micropipette. The lesions destroyed the cell bodies in the superior olive without disrupting fibers of passage in the trapezoid body or other pathways in the auditory brain stem. The integrity of fibers was demonstrated by protargol staining of axonal processes and by the retrograde transport of horseradish peroxidase (HRP) from the inferior colliculus to other auditory brain stem nuclei. Behavioral tests were carried out separately for sound localization at midline and lateral field positions. Minimum audible angles were determined for single 45-ms noise bursts presented through paired loudspeakers positioned symmetrically around 0, -60, and +60 degrees azimuth. 2. Four ferrets received complete lesions of the left SOC, and two received complete lesions of the right SOC. In general, unilateral destruction of the superior olive resulted in impairments in sound localization in both left and right lateral fields. In some cases, deficits were also apparent on midline. Four additional animals received unilateral lesions that spared cells within the SOC. In most cases, deficits were apparent despite incomplete lesions of the SOC. The pattern of deficits was generally consistent with that found in animals with complete lesions. Most animals had difficulty localizing sounds in the lateral fields. 3. Four animals received bilateral lesions of the SOC. Three had complete or near-complete destruction of the superior olive on one side of the brain with relatively minor damage on the other side. Each of these animals exhibited behavioral deficits that were particularly severe ipsilateral to the more extensively damaged superior olive. One animal with complete bilateral destruction of the SOC was incapable of sound localization, even with 2-s noise bursts. This animal, however, suffered severe motor impairments after surgery that might have contributed to the apparent inability to localize sound. 4. Two animals with kainic acid lesions that caused little or no damage to the SOC were still capable of high levels of performance in tests of sound localization and had no elevation in minimum audible angles. These cases served as controls for the possible effects of nonspecific brain damage and demonstrated that kainic acid injections per se resulted in no obvious deficits in our test situation.(ABSTRACT TRUNCATED AT 400 WORDS)