Illusory contours

Illusory contours are perceived in the absence of any real physical luminance gradients between inducers that are spatially discontinuous (Fig. 1a,b).1–3 It has been controversial whether the perception of these subjective contours arise from higher level cognitive mechanisms that depend on attention and serial search4–6 or from lower level parallel processing in the early automatic, preattentive stages of vision.7–9 In support of lower level processes, neurophysiological studies in monkeys showed that neurons in the visual cortex (areas V1 and V2) respond to illusory contours as well as to real contours of identical orientation.10,11 However, unlike subjective contours that are induced by offset gratings, Kanizsa-style illusory contours elicit responses only from V2 neurons and not from V1 neurons.11,12 Nevertheless, recent evidence suggests that even this type of subjective figure can be detected without focal attention or serial search.8 In patients with unilateral neglect who may fail to deploy attention to the contralesional side of space, Kanizsa-style illusory figures provide a special opportunity to investigate the role of attention in the perception of illusory contours. We carried out two experiments in neglect patients to investigate whether the spatial discontinuous inducers of Kanizsa figures might group together despite unilateral neglect and generate subjective filling-in of surfaces bounded by illusory contours; and whether the perception of illusory surfaces themselves might be dissociated from the explicit detection, or awareness, of their inducers. Patients and Methods