Psychophysical measures of illusory form perception: further evidence for local mechanisms

Detection thresholds for a small light spot were measured at various distances from configurations (Kanizsa squares and other) consisting of white inducing elements on a dark background. Threshold distributions as a function of target position, number, size and spacing of contrast inducing elements were established. The data show that thresholds are elevated when the target is located close to one or more inducing element(s). Furthermore, threshold elevations diminish with increasing distance of the target from the configurations, increasing spacing and decreasing size of their inducing elements. When the target is flashed upon an illusory contour, no threshold elevation is observed in any of the conditions tested. Within incomplete illusory figures (only half of the square visible), the threshold gradients show the same tendencies. The present observations add further empirical support to the idea that illusory figures are built up by way of local mechanisms at early stages of processing.