Spatial and temporal properties of illusory figures

The best known example of illusory figures is the Kanizsa triangle consisting of three disks with a sector removed. The disks and sectors are arranged so that they form the corners of a triangle. Although the sides of the triangle are not physically present, they are clearly visible to the observer. In this study the effect of sequential presentation of the inducing disks on an illusory (Kanizsa) triangle was investigated. The task of each subject was to find, by the method of adjustment, the longest critical duration that allowed perception of the illusory triangle produced by presenting the three inducing disks sequentially. We varied the sizes and/or separations of disks. An illusory triangle produced by flashing the three inducing disks simultaneously for 33 msec served as a comparison stimulus. Our experiments showed that increasing the inducing disk size or reducing the inter-disk distance increased critical duration. The result means that the shorter the illusory contour to be induced the longer the critical duration and vice versa. Thus, if the inductive disks are less separated in space they can be more separated in time and vice versa. These findings seem to agree with the suggestion that illusory contours emerge from the synchronization of gamma-waves emitted by the neurons in the visual cortex.