The spatial organisation of the excitatory region of receptive fields in the cat's superior colliculus.

Abstract

1. The spatial organisation of the excitatory regions of 54 receptive fields from cells in the cat's superior colliculus was studied. Rectangular light bars, dark bars and single contrast borders were used as stimuli. Average response histograms were compiled of the spike discharges elicited from collicular units by these stimuli. 2. A cell would normally respond to the light edge as well as to the dark edge of a stimulus crossing its receptive field. It was possible to describe for a particular unit and a particular direction of stimulus movement the area within the excitatory region of the receptive field responsive to each edge of the rectangular stimuli. In a particular field, these areas were either superimposed (32 units) or spatially separated from one another (22 units). 3. In the receptive fields in which the areas responsive to different edges of a stimulus were spatially offset for stimulus movement in one direction, they also were offset for all other directions of stimulus movement. The direction of this offset changed characteristically with the direction of stimulus movement, revealing a radial symmetry in the organisation of the excitatory region of the receptive field. 4. For units with offset edge-responsive areas, this offset determined the width and contrast of the optimal stimulus for the unit. 5. Receptive fields with superimposed edge-responsive areas are also radially symmetric. The optimal stimuli for units with these receptive fields are either narrow dark slits or narrow light slits. 6. The significance of these properties of collicular cells for coding the position, size and contrast of fast moving stimuli is discussed.