Connections of striate cortex in the prosimian, Galago senegalensis.

Efferent and afferent connections of primary visual cortex, Area 17, were determined in a prosimian, Galago senegalensis, by autoradiographic methods after injections of 3H-proline or 3H-HRP. The cortical connections of Area 17 with Areas 18 and MT were homotopic and reciprocal. Projections from Area 17 terminated largely in layer IV and somewhat in layer III of both Areas 18 and MT. Most of the cells projecting to Area 17 were located in layer V of Area 18 and layer VI of MT. Subcortical projections included the reticular nucleus of the thalamus, where columns of label corresponding to injection sites were found in the caudal fourth of the nucleus. Projections to the lateral geniculate nucleus were along lines of isorepresentation and were in register with the cells projecting back to the injection site. The parvocellular layers were less densely labeled than other layers by the transport of 3H-proline, while concentrations of label were noted on the dorsal and ventral margins of the nucleus and in interlaminar regions between the internal parvocellular and magnocellular layers and between the two magnocellular layers. The pattern of terminations in the pulvinar complex suggested functional subdivisions. We have divided the inferior pulvinar into a large central nucleus, IPc, with topologically organized input from Area 17; a smaller medial nucleus, IPm, with a second pattern of input from Area 17; and a dorso-posterior nucleus, IPp, without input from striate cortex. The superior pulvinar likewise appears to have several subdivisions. One of these, a "central" nucleus of the superior pulvinar, SPc, receives topologically organized projections from Area 17. SPc is about the same size as IPc and is organized as a mirror image of IPc. Thus, both IPc and SPc represent the lower visual quadrant medially and the upper visual quadrant laterally; central vision is represented along the common border for both nuclei, while peripheral vision is represented dorsorostrally in SPc and ventrocaudally in IPc. Finally, the superficial grey of the superior colliculus receives topologically organized input from Area 17.