RECEPTIVE-FIELD PROPERTIES OF SINGLE UNITS IN OPOSSUM STRIATE CORTEX

On the basis of their trigger-features, 98 of 127 units recorded in the striate cortex of immobilized opossums [Didelphis marsupialis aurita], under forced breathing of a nitrous oxide/O2 mixture, were classified into 5 receptive field groups. Group 1 units (20/27), responding to small stationary spots, made up of regions of opposite response type and mutual antagonism, separated by linear boundaries. The optimal discharge was elicited by a stimulus configuration consisting of rectilinear regions of opposite contrast positioned and oriented in the visual field so as not to elicit antagonism while maximizing the overlap with regions responsive to that contrast. To edges in motion these units were made up of light and dark discharge centers, the locations of which could not be predicted from the map to stationary spots. In addition to position and orientation, direction was another important stimulus parameter. Group 2 units (34/127) had uniform requirements of stimulus orientation, direction of motion or both, throughout the receptive field. Width was rarely a significant variable. Three subgroups were detected: orientation selective, directional selective and orientation-direction sensitive. Group 3 units (18/127) required stopped stimuli. In most instances (14/18) this property was attributed to a suppressive surround with relatively non-specific stimulus requirements. Oriented and non-oriented responsive receptive fields were observed. Group 3 units with no surround (4/18) responded best to properly positioned and oriented wedges, usually of 90.degree.. Group 4 units (24/127) had uniform fields with little stimulus specificity and were often responsive to diffuse light. Although not sensitive to stimulus orientation and direction, motion was frequently a requisite for optimal responses. Group 5 receptive fields (2/127) had concentrically arranged regions of distinct response type which displayed mutual antagonism. No sensitivity to orientation or direction was detected. Twenty-nine units remained unclassified. Other group distinctions were the relatively higher spontaneous activity of group 4 units and the large field sizes encountered among groups 1 and 4 when compared to group 2. Based on their properties and receptive field type distribution, striate receptive fields in the opossum apparently have a similar organization to those of other mammals.