Independent efferent populations in the nucleus of the optic tract: an anatomical and physiological study in rat and cat.

The efferent projections of the nucleus of the optic tract (NOT) and dorsal terminal nucleus of the accessory optic system (DTN) to the contralateral NOT-DTN, ipsilateral inferior olive (IO), ipsilateral nucleus prepositus hypoglossi (NPH), and ipsilateral dorsal lateral geniculate nucleus (LGNd) were examined in pigmented rats and in cats by using anterograde and retrograde tract tracing, as well as extracellular recording and electrical stimulation. Anterograde tracing in the rat revealed a dense termination field of NOT-DTN efferents throughout the homologous contralateral territory. In both species three different cell populations, projecting to the contralateral NOT-DTN, ipsilateral IO, and ipsilateral LGNd, respectively, were distinguished by means of multiple retrograde tracing. No clear topographical segregation of the different NOT-DTN relay cell populations was observed. On the other hand, a large proportion (at least 60%) of NOT-DTN neurons projecting to the ipsilateral NPH were found to bifurcate upon the IO in the rat. Electrophysiologically, NOT-DTN neurons projecting to the IO were identified by their directionally selective responses. Such neurons were never activated by electrical stimulation of either the contralateral NOT-DTN or the ipsilateral LGNd. Neurons antidromically activated from the contralateral NOT-DTN could not be activated from the ipsilateral LGNd. Thus, in both cat and rat the NOT-DTN includes at least three independent relay cell populations. As a consequence, the NOT-DTN must serve functions additional to the generation of eye movements during optokinetic nystagnus, a function subserved by the directionally selective NOT-DTN cells.