Composition of geniculostriate input ot superior colliculus of the rhesus monkey.

1. In order to examine the composition of the geniculostriate input to the superior colliculus, microelectrode recordings were undertaken in this structure of the rhesus monkey while parvocellular or magnocellular laminae of the LGN were reversibly inactivated by injecting minute quantities of lidocaine or MgCl2. 2. The inactivation of magnocellular laminae disrupted the visually driven activity of most cells in the topographically corresponding areas of the colliculus, but not in the superficial retinotectal recipient zone. The inactivation of parvocellular lamina had no effect on the visually driven activity of collicular cells. 3. Several controls were carried out to rule out the possibility of intervention with fibers of passage. We ascertained that the LGN injections did not affect the direct retinotectal pathway by comparing the effect of such inactivation with the effect produced by reversibly cooling visual cortex. These two manipulations yielded similar results: cells in the most superficial regions of the superior colliculus were unaffected by both cortical cooling and by magnocellular injections, while below this region the response of collicular cells was reduced or eliminated in both cases. 4. These results suggest that the indirect visual pathway to the superior colliculus via cortex is activated selectively by the broad-band system, which is relayed through magnocellular LGN. The color-opponent system does not appear to have a corticotectal input sufficient to drive collicular cells independently.