Functional segregation of ITD sensitivity in the inferior colliculus of decerebrate cats.

Decerebration allows single-unit responses in the central nucleus of the inferior colliculus (ICC) to be studied in the absence of anesthesia and descending efferent influences. When this procedure is applied to cats, three neural response types (V, I, and O) can be identified by distinct patterns of excitation and inhibition in pure-tone frequency-response maps. Similarities of the definitive response map features with those of projection neurons in the auditory brain stem have led to the proposal that the ICC response types are derived from different sources of ascending input that remain functionally segregated within the midbrain. Additional evidence for the existence of these hypothesized parallel processing pathways has been obtained in our previous investigations of the effects of interaural level differences, brain stem lesions, and pharmacological manipulations on physiologically classified units. This study extends our characterization of the functional segregation of single-unit activity in the ICC by investigating how sensitivity to interaural time differences (ITDs) is related to the response types that are observed in decerebrate cats. The results of these experiments support our parallel-processing model of the ICC by linking the ITD sensitivity of type V and I units to putative inputs from the medial superior olive and lateral superior olive and by showing that most type O units lack a systematic sensitivity to binaural temporal information presumably because their dominant ascending inputs arise from weakly binaural neurons in the dorsal cochlear nucleus.