Illusory contours and spatial neglect.

It has been controversial whether the perception of illusory contours arise from higher level cognitive mechanisms that require attention or from early preattentive visual processes. We studied three patients with left spatial neglect who were unable to detect the left inducers of Kanizsa illusory figures in a same/different judgment task but nonetheless showed implicit perception of the figures in a midpoint judgment, in that they made identical bisection for figures with illusory or real contours but very different bisection for other spatially discontinuous figures that did not yield illusory filling-in. Grouping and filling-in mechanisms can thus occur without explicit detection of, or attention to, the inducing features, consistently with the hypothesis that they involve preattentive visual processes.