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Dejan Todorovic, Ljubica Jovanovic; Is the Ebbinghaus illusion a size contrast illusion. Journal of Vision 2015;15(12):330. doi: 10.1167/15.12.330.
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© 2017 Association for Research in Vision and Ophthalmology.
In recent decades the Ebbinghaus illusion was used mainly to study issues such as the relation of action and perception or the neural basis of size perception. However, there is still no consensus on the basic question why it occurs at all. An intuitively appealing approach, based on the description of the effect (identical targets look smaller/larger when surrounded by larger/smaller figures) is that the essential cause of the illusion is size contrast. However, several studies, mostly in the nineteen seventies and eighties, have indicated a variety of other factors, such as number and distance of surrounding figures and the vicinity of their contours to targets. In three experiments we tested whether the presence, per se, of groups of similar surrounding figures contrasting in size with the targets was a relevant factor in this illusion. Experiment 1 was a control/baseline study in which the standard effect was replicated. For better control of contour positions and extents, instead of conventional circular shapes square-shaped targets and surrounding figures were used, in an arrangement similar as in the conventional display. Experiment 2 used a modified, geometrically related display in which the surrounds of the two target squares were constituted by single homogeneous frame-like figures, which encompassed the surround squares in Experiment 1 and shared half of their contours. Despite the absence of conventional size contrast there was an illusion of very similar strength and structure as in Experiment 1. Experiment 3 used a display in which the target squares were surrounded by many larger/smaller squares, but in a somewhat different arrangement than in the conventional display, including overlap of targets and a minority of surrounding figures. Despite the presence of size contrast the illusion was absent or reversed. These results suggest that size contrast is neither sufficient nor necessary for the Ebbinghaus illusion.
Meeting abstract presented at VSS 2015
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