The spatial substructure of visual receptive fields in the cat's superior colliculus

Although the direction selective properties of the superficial layer cells of the cat's superior colliculus have been extensively studied, the mechanisms underlying this property remain controversial. With the aim to understand the mechanism(s) underlying directional selectivity of collicular neurons we examined the substructure of their visual receptive fields. 1. The strength of cell responses and the direction selectivity indices varied in relation to the location of the tested region within the receptive field and the amplitude of stimulus movement. 2. Decrease of the amplitude of motion resulted in a decrease of direction selectivity index both in the group of direction-selective cells and in the group of cells classified as direction nonselective but with a directional bias. 3. The decrease of direction selectivity for small amplitude movement resulted mainly from increase in the magnitude of response in the nonpreferred direction of movement. 4. These results suggest that the receptive fields of most collicular cells are composed of subregions with different response profiles and indicate that inhibitory mechanisms dictate direction selectivity of collicular cells.