Single unit responses to moving spots in the superior colliculus of the cat's isolated midbrain.

The midbrain was isolated from the pons and forebrain by two brainstem transections, pretrigeminal and "preoptic", inclined at 32 degree or 35 degree to the vertical plane. The preoptic transection passed along the rostral border of the optic tract, leaving it and the chiasm intact. Thus, direct visual input into the isolated tissue was maintained. Single unit responses to a moving spot were investigated in the superior colliculus. This stimulus evoked vigorous responses in most cells. Of the 70 tested units, 25 showed direction-selectivity, 48 speed-selectivity and 47 reacted to diffuse flash. Compared to cats with only pretrigeminal transection only the number of direction-selective units was reduced. The reduction was similar to that found previously in cats with ablated visual cortex or visually deprived from birth. Thus, the cells of the isolated superior colliculus show a striking degree of functional integrity.