A double-labeling investigation of the pretectal visuo-vestibular pathways.

The projections of the nucleus of the optic tract were studied in the cat by simultaneous use of two distinct retrograde tracers (Fast Blue and Diamidino Yellow) injected in the inferior olive and the prepositus hypoglossi nucleus. Following injections of diamidino yellow in one structure and fast blue in the other, a significant number of retrogradely labeled neurons projecting to either target were observed dispersed in the nucleus of the optic tract. Three populations of labeled cells were found: one which projected to the inferior olive, a second to the nucleus prepositus hypoglossi, and a third which projected by means of a bifurcating axon to both of these structures. Quantification of these results reveals that 72% of the total number of labeled neurons are labeled by the IO injection, the remaining cells being labeled by the NPH injection. Double-labeled neurons represent more than 7% of the total number of the labeled cells. Tentative inferences as to the electrophysiological properties of the nucleus of the optic tract are discussed in the context of the optokinetic system.