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Satoru Suzuki, Timothy Sweeny, Marcia Grabowecky; Simultaneous shape-contrast and global assimilation effects in the perception of aspect ratio. Journal of Vision 2009;9(8):897. doi: 10.1167/9.8.897.
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© ARVO (1962-2015); The Authors (2016-present)
During brief stimulus presentation shape perception remains in a “labile” state in which features are susceptible to perceptual distortion (e.g., Suzuki & Cavanagh, 1998). Shape distortion effects are typically examined using adaptation paradigms with sequential stimulus presentations. Here we present two aspect-ratio related distortions that occur when two ellipses are briefly (40 ms) and simultaneously presented. We measured perception of individual ellipses by randomly post-cueing one of them and having observers indicate the perceived aspect ratio of the post-cued ellipse. We call the first effect a “simultaneous shape-contrast effect.” That is, the aspect ratios of briefly presented ellipses appear to “repel” each other. For example, if a slightly tall ellipse and a taller ellipse are simultaneously flashed, the less tall one appears flatter and the taller one appears even taller. This effect is quite non-retinotopic in that it occurs across a distance of at least up to 7Â° and whether the ellipses are presented within a single quadrant, across quadrants, or across visual hemifields. We call the second effect a “global assimilation effect.” That is, when the two flashed ellipses are horizontally organized, the individual ellipses appear flatter than they actually are, whereas when the ellipses are vertically organized, the individual ellipses appear taller than they actually are. Interestingly, flat ellipses tend to be resistant to this global assimilation effect. In summary, in brief viewing two simultaneously presented aspect ratios appear to repel each other and global organization is incorporated into perception of individual shapes. These results are consistent with the idea that shape perception emerges from rapid neural processes involving both lateral and hierarchical interactions.
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