MACAQUE-V1 NEURONS CAN SIGNAL ILLUSORY CONTOURS

WE describe here a new view of primary visual cortex (V1) based on mea surements of neural responses in V1 to patterns called 'illusory conto urs' (Fig. 1a, b). Detection of an object's boundary contours is a fun damental visual task. Boundary contours are defined by discontinuities not only in luminance and colour, but also in texture1-3, disparity4 and motion5-7. Two theoretical approaches can account for illusory con tour perception. The cognitive approach emphasizes top-down processes8 ,9. An alternative emphasizes bottom-up processing. This latter view i s supported by (1) stimulus constraints for illusory contour perceptio n10-14 and (2) the discovery by von der Heydt and Peterhans15-17 of ne urons in extrastriate visual area V2 (but not in V1) of macaque monkey s that respond to illusory contours. Using stimuli different from thos e used previously15,16, we found illusory contour responses in about h alf the neurons studied in V1 of macaque monkeys. Therefore, there are neurons as early as V1 with the computational power to detect illusor y contours and to help distinguish figure from ground.