Academic journal article Attention, Perception and Psychophysics

Stereoscopic Depth and the Occlusion Illusion

Academic journal article Attention, Perception and Psychophysics

Stereoscopic Depth and the Occlusion Illusion

Article excerpt

In the occlusion illusion, the visible portion of a partly occluded object appears larger than a physically identical nonoccluded region. Stereoscopic displays allowed for a direct test of the apparent-distance hypothesis. In Experiments 1A and 1B, we measured both the perceived size and the perceived depth of partly occluded targets when the binocular disparity of both targets and occluders was varied. Stereoscopic occlusion greatly increased perceived target size but not perceived target distance. A reduced illusion was still present when the target was stereoscopically in front of the abutting rectangle, however. Experiments 2A and 2B showed similar results, even when the occluding figures were illusory rectangles that formed no explicit T-junctions. Experiment 3 showed that an unexpected negative size illusion on control trials was primarily due to adaptation to the occlusion illusion on other trials. The present findings eliminate apparent-distance explanations of the occlusion illusion but are consistent with other hypotheses, such as partial modal completion and selective dimensional expansion.

Kanizsa and Luccio (1978; translated in Kanizsa, 1979) first reported an illusion in which a figure that is perceived as partly occluded (the target) appears larger than a physically identical region that is fully visible (the standard). This illusion was later christened the occlusion illusion, because of the key role played by evidence of occlusion (Palmer, 1999; Palmer, Brooks, & Lai, 2007). Figure 1A shows a typical example in which a half-circle target abuts a rectangle along one edge, such that the display is perceived as a rectangle partly occluding a full circle. Under these circumstances, the partial circle appears to be substantially larger than the optically identical standard shown next to it.

Palmer et al. (2007) investigated this illusion in an attempt to determine the conditions under which it occurs and to test the predictions of two hypotheses about its cause. They found that the size of the illusion, which was sometimes as large as 20% in area, varied with the amount of evidence of occlusion, in that stronger evidence of occlusion produced larger illusions. They also examined two possible explanations-the partial modal completion hypothesis and the apparent-distance hypothesis-and found stronger support for the former.

Palmer et al. (2007) argued that the occlusion illusion is of great potential interest because partial modal completion is a new mechanism underlying size illusions. Partial modal completion postulates that the illusion occurs because the visual system fills in a small strip of the partly occluded target surface along the occluding edge. The key idea is that, when an object is perceived as occluded, not only is there the well-known and much-studied phenomenon of amodal completion of the target behind the occluder (i.e., perceptual completion without a corresponding sensory experience in the relevant modality; see Kanizsa, 1979), but there is also a small but measurable extension of sensory experience along the occluding edge for the partly occluded surface. Note that this explanation implies that the occlusion illusion is actually an illusion of both size and shape, because the target is perceived as though it were less occluded than it actually is.

There is an even more obvious explanation of the occlusion illusion in terms of apparent distance, however, that we attempted to rule out in the present experiments. The apparent-distance hypothesis explains the occlusion illusion as yet another size illusion caused by applying the size-distance relation with an erroneous perception of distance, akin to the famous moon illusion (Kaufman & Rock, 1962; Rock & Kaufman, 1962). The logic of this explanation is as follows. When the target is perceived as occluded by the rectangle, it is necessarily at least somewhat farther away than the occluding rectangle. If the occluding rectangle is perceived as located in the same depth plane as the standard, the target must also be at least somewhat farther away than the standard. …

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