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Poutasi Urale, D. Samuel Schwarzkopf; The Ebbinghaus Illusion depends on Cortical Distance. Journal of Vision 2020;20(11):225. https://doi.org/10.1167/jov.20.11.225.
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In the Ebbinghaus Illusion, the perceived size of a circle (target) depends on the size and proximity of a surrounding circular arrangement of other circles (inducers). In most depictions of the illusion, inducers larger than the target make the target appear smaller while small inducers have the opposite effect. Although originally thought of as a size-contrast effect, strides have been made in understanding the mechanistic basis of this illusion. Converging evidence has suggested that it is driven by interactions between contours, which in turn may be mediated by cortical distance within primary visual cortex. Here we directly tested the effect of cortical distance on the strength of the Ebbinghaus Illusion by using a two interval forced choice design and an adaptive staircase procedure. In Experiment 1, we varied the physical distance between illusion components and found a progressive decrease in the perceived size of the target with greater physical distance both for large and small inducers. In Experiment 2, we applied a similar method using the Delboeuf Illusion, which is a size illusion like the Ebbinghaus Illusion that uses a ring annulus instead of circular inducers. We found an effect on illusion strength comparable to that seen in the Ebbinghaus illusion, supporting a common underlying mechanism. Finally, in Experiment 3 we predicted that due to lower cortical magnification in the peripheral visual field – and thus smaller cortical distances between illusion components – targets in the Ebbinghaus Illusion presented peripherally should appear larger compared to when they are presented centrally. We tested the illusion strength when positioning the stimuli at various eccentricities and our results supported this hypothesis. Taken together, results using two methods of manipulating cortical distance both point to cortical distance as a key mediator of the Ebbinghaus Illusion.
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