Effects of superior colliculus inhibition on three-dimensional visual motion processing in the lateral suprasylvian visual area of the cat.

PURPOSE To determine whether visual inputs from the tectothalamocortical pathway influence three-dimensional motion processing within the lateral suprasylvian (LS) area of the cat. METHODS Tungsten microelectrodes were used for recording visual-evoked potentials (VEPs) from the LS area of 4 cats. Random dot stereograms were used as visual stimuli. Three-dimensional, motion-triggered VEPs were recorded from the LS area. Each motion sequence consisted of an abrupt onset of motion disparity with a 2 degrees amplitude followed by an abrupt offset and a stationary phase of 900 ms. The velocity of the motion disparity was varied in eight steps from 10 degrees to 400 degrees per second. The onset of motion disparity was used as the trigger for recording the VEPs. Single or multiple injections (two to three) of muscimol were made mainly into the rostral superior colliculus (SC). The amplitudes of the VEPs before and after the muscimol injection were compared. RESULTS A large negative wave ( N1) with an implicit time of 92.7 +/- 13.5 ms (mean +/- SD, n = 98) was recorded consistently. The amplitude of N1 was significantly larger on stereovision of motion disparity than on either binocular vision of two-dimensional lateral motion or monocular vision, indicating that N1 contains neurons sensitive to motion disparity. The amplitude of N1 was not altered by muscimol injection into the SC at velocities < or =50 degrees/s. On the other hand, the amplitude of N1 was reduced to 66-71% of that observed before muscimol injection at velocities > or =75 degrees/s. CONCLUSIONS These findings suggest that the LS area processes three-dimensional motion inputs via two parallel pathways, the geniculostriate pathway and the tectothalamocortical pathway, at velocities of motion disparity > or =75 degrees/s, while the three-dimensional motion inputs project to the LS area only via the geniculostriate pathway at velocities of motion disparity < or = 50 degrees/s.