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| Paper IPM / Cognitive / 11235 |
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| Abstract: | |
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Kanizsa (1979) reported a size illusion in which a figure bounded by an occluding edge looks larger than the same figure not bounded by an occluder. Brooks et al (2004 [Abstract] Journal of Vision) 4(8) 722a, http://journalofvision.org/4/8/722/ showed that partial modal completion theory might explain this illusion. According to this theory, the occluded region appears larger because the visual system fills in a thin strip along the occluded border. Earlier studies of the time course of perceptual completion showed that modal completion occurs within 100 - 200 ms, and shorter presentations leave the fragmented figures uncompleted (Ringach and Shapley, 1996 Vision Research 36 3037 - 3050). On the basis of this finding, modal completion theory predicts that occlusion illusion should be perceived better in presentations longer than 100 - 200 ms when completion has almost occurred rather than presentations shorter than these times. In the present study, this prediction is evaluated by estimating occlusion illusion in different presentation times. In each trial, after fixation-cross presentation, the occluded semi-circle (the occluder was a square) and unconcluded semi-circle were shown for 80 - 280 ms (in steps of 40 ms) and then immediately a mask was presented [it has been shown that masking can interrupt perceptual completion (Rauschenberger and Yantis, 2001 Nature 410 369 - 372)]. Observers then had to report which semi-circle was bigger. Contrary to modal-completion-theory prediction, observers were more likely to report the illusion in shorter presentations. This finding is not in agreement with modal completion theory and implies that the modal completion mechanism cannot properly explain the occlusion illusion. Current results point out that faster mechanisms must be in operation for occlusion illusion perception.
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