Behavioral evidence for the perception of Kanizsa illusory contours in pig-tailed Macaque Monkeys (M. nemestrina)

Illusory (subjective) contours as described by Kanizsa are readily perceived by most human observers. Many investigations into the neural correlates of illusory contour perception are conducted in non-human primates. These neurophysiological studies presuppose that non-human primates “see” the illusory contours evident in human perception. However, there is no definitive behavioral evidence to support that supposition. The purpose of this study was to quantitatively investigate whether macaque monkeys (Macaca nemestrina) in fact show evidence for Kanizsa illusory contour perception. Using a zero-delay similarity matching-to-sample (S-MTS) procedure, two juvenile monkeys were initially trained on an orientation discrimination with simple forms. They were then transferred to a combined S-MTS + visual search procedure where the potential matching stimuli were “hidden” within one of two fields of random non-contour inducing “pacman” elements; the subject's task was to correctly identify the field that contained the match. Using the same procedure, subjects were then tested with six contour-induced Kanizsa illusory shapes as potential matches, which were embedded in the fields of random elements. Both subjects reached the criterion performance level of 80% or better with the six illusory shapes demonstrating clear evidence of Kanizsa figure perception. We also measured thresholds for performance on this task by varying the support ratio for the illusion. Thresholds for each subject (S1: 27% and S2: 41%) were similar to those obtained in 8-month old human infants (37%) as well as human adults who are typically not tested with support ratios below 25%. These results provide the first quantitative behavioral evidence that monkeys perceive Kanizsa illusory contours similarly to humans. In addition, we show that they can use illusory forms to discriminate shape and orientation. Finally, these findings enhance our understanding of the similarities in visual perception between macaque monkeys and humans, strengthening the justification for studies of shape and boundary processing in macaques.