Role of the caudate nucleus in performance of the conditioned reactions connected with sound localization.

The instrumental differentiation of left-right positions of sound-conditioned stimuli (series of clicks under dichotic stimulation) in 8 dogs was examined before and after the bilateral electrolytic lesions of various parts of the caudate nucleus (NCd). It was demonstrated that after damage of the NCd the correct differentiation of the side of monaural sound stimulation was preserved in all dogs. However, the differentiation of lateralization of sound produced by interaural time differences was selectively impaired. Lesions of the dorsal part of the NCd heads caused the absolute absence of the differentiation of the binaural sound image lateralization for 21-50 days after the operation. This function partially recovered but the percentage of correct responses remained significantly reduced in comparison with preoperative performance. Lesions of the ventral segment of the NCd heads of different parts of the NCd bodies had less influence on the performance of this behavioral task. In these cases there was no period of absolute absence of the differentiation, but the level of correct responses to the binaural signals was significantly reduced. It is suggested that the NCd is a part of the system of brain structures in which the parameters of the conditioned sound signals are compared with a reference system for recognition of sound location. The result of this comparison is used by the NCd for choosing a corresponding program of the motor response.