Corticostriatal and corticotectal projections from visual cortical areas 17, 18 and 18a in the pigmented rat

Regions of the visual cortex and the deep layers of the superior colliculus (SC) have been suggested to be functionally linked via an 'indirect' pathway through the basal ganglia. The present report demonstrates projections from the striate (area 17) and extrastriate (areas 18 and 18a) visual cortex in Long-Evans hooded rats to the striatum (ST) and SC with anterograde (biocytin) and retrograde (WGA-HRP and fluorescent dyes) tracers. Biocytin injections into each cortical area produced markedly different patterns of labeling in ST and SC. Injections into area 17 resulted in a dense labeling in the superficial layers of SC, with little labeling present in the deep layers of SC or ST. In contrast, injections into area 18a, which produced marked labeling in the deep layers of SC with moderate in the superficial layers, resulted in dense labeling in the caudal two-thirds of the dorsal region of ST; injections into area 18 produced numerous terminals mainly in the deep layers of SC and in the rostral two-thirds of the dorsal aspect of ST. WGA-HRP injections into ST resulted in numerous retrogradely labeled pyramidal neurons in areas 18 and 18a, but only a few labeled neurons were observed in area 17. Corticostriatal neurons in extrastriate visual cortex were distributed predominantly in layer V, with smaller numbers in layers II and III, whereas corticotectal neurons were located only in layer V of both the striate and extrastriate visual cortex. Although corticostriatal and corticotectal neurons were intermingled in layer V, no double-labeled neurons were observed following injections of different fluorescent dyes into ST and SC. It appears, therefore, that: 1) the major source of visual input to both ST and the deep layers of SC in the rat arises not from the primary visual cortex but from extrastriate visual areas; and that 2) the projections from the extrastriate visual area to ST and SC originate from different populations of corticofugal neurons.