Two visual corticotectal systems in cat.

By using the method of cortical cooling we were able to deactivate specific regions of cortex and demonstrate that there are two functionally separate visual projections to the superior colliculus (SC): one from area 17-18 and one from posterior regions of the suprasylvian cortex (PSSC). Deactivation of area 17-18 depressed the activity of nearly all of the superficial lamina cells in the SC that were binocular and directionally selective. The receptive-field properties of those cells still capable of responding to visual stimuli were altered significantly so that 1) moving stimuli became poorly effective, while stationary flashed stimuli became relatively more effective; 2) directional selectivity and binocularity were minimized or eliminated; 3) the characteristic spatial summation, inhibition, and suppressive surround of the SC-receptive field were compromised; and 4) the range of effective stimulus velocities was altered. However, deep lamina cells were unaffected by the cooling of area 17-18. Deactivation of the PSSC affected deep lamina SC cells in the same way that the cooling of area 17-18 affected cells in superficial layers. Apparently there are two functional visual corticotectal systems that are largely independent of each other: one from area 17-18 (to superficial SC cells) and one from the PSSC (to deep SC cells). Both are critically important for maintaining the excitability and complex receptive-field properties of their target cells in the SC. The only exception to the complete segregation of their influences was the ability of the PSSC to affect slightly the general level of excitability of some superficial lamina cells. A small population of SC cells receiving a tonic inhibitory corticotectal influence was also located. This influence was apparent as an increase in excitability and receptive-field complexity of cells in superficial layers when area 17-18 was cooled, and of deep lamina cells when the PSSC was cooled. The presence of separate visual corticotectal influences on superficial and deep lamina SC cells is consistent with the distinct identities of these subdivisions of the SC and their different functional roles.