Contextual influence on orientation discrimination of humans and responsesof neurons in V1 of alert monkeys

Contectual influence on orientation discrimination of humans and responses of neurons in V1 of alert:monkeys. J. Neurophysiol. 83: 941-954, 2000. We s tudied the effects of various patterns as contextual stimuli on human orien tation discrimination, and on responses of neurons in V1 of alert monkeys. When a target line is presented along with various contextual stimuli (mask s), human orientation discrimination is impaired. For most V1 neurons, resp onses elicited by a line in the receptive field (RF) center are suppressed by these contextual patterns. Orientation discrimination thresholds of huma n observers are elevated slightly when the target line is surrounded by ort hogonal lines. For randomly oriented lines, thresholds are elevated further and even more so for lines parallel to the target. Correspondingly, respon ses of most V1 neurons to a line are suppressed. Although contextual lines inhibit the amplitude of orientation tuning functions of most V1 neurons, t hey do not systematically alter the tuning width. Elevation of human orient ation discrimination thresholds decreases with increasing curvature of mask ing lines, so does the inhibition of V1 neuronal responses. A mask made of- straight lines yields the strongest interference with human orientation dis crimination and produces the strongest suppression of neuronal responses. E levation of human orientation discrimination thresholds is highest when a m ask covers only the immediate vicinity of the target line. Increasing the m asking area results in less interference, On the contrary, suppression of n euronal responses in V1 increases with increasing mask size. Our data imply that contextual interference observed in human orientation discrimination is in part directly related to contextual inhibition of neuronal activity i n V1. However, the finding that interference with orientation discriminatio n is weaker for larger masks suggests a figure-ground segregation process t hat is not located in V1.