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Karen B. Schloss, Francesca C. Fortenbaugh, Eli D. Strauss, Stephen E. Palmer; CSI Berkeley Episode II: Perceptual organization and selective attention. Journal of Vision 2010;10(7):1173. doi: 10.1167/10.7.1173.
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© ARVO (1962-2015); The Authors (2016-present)
Last year we described the Configural Shape Illusion (CSI), in which the shape of a rectangular target is distorted by an attached region, or “inducer” (Palmer, Schloss, & Fortenbaugh, VSS-2009): the target's perceived aspect ratio changes toward the aspect ratio of the whole configuration. We also showed that the illusion increases as grouping increases, due to connectedness, proximity, lightness similarity, hue similarity, and shape similarity. We now show that CSI magnitude is an inverted U-shaped function of inducer height that scales with overall target size, increasing rapidly as the height of the inducer increases from zero and then diminishing slowly, but never reversing in sign, as its height increases further. Because grouping strength was previously shown to affect CSI magnitude, we also measured perceived grouping between target and inducers of different sizes. The grouping function was qualitatively similar to the CSI function, and when it was scaled by target size, the correlation between grouping strength and CSI magnitude was 0.91. We suggest that the CSI is caused by the inability to selectively attend to the target to the extent that it is grouped with the inducer, such that the size and shape of the global configuration influence the perceived size and shape of the target. We tested this hypothesis using a Stroop-like interference task in which participants were asked to categorize the target as taller-than-wide or wider-than-tall, when the aspect ratio of the whole configuration (target plus inducer) was either consistent or inconsistent with the target's aspect ratio. The pattern of reaction times was consistent with Stroop interference: response times slowed when the global and target aspect ratios were inconsistent (taller/wider or wider/taller), but no facilitation when they were consistent (taller/taller or wider/wider). The results support a role for selective attention in causing the CSI.
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