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Stefano Guidi, Sergio Roncato, Oronzo Parlangeli; Selectivity for contrast polarity in contour integration revealed by a novel tilt illusion. Journal of Vision 2011;11(11):1056. doi: https://doi.org/10.1167/11.11.1056.
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© ARVO (1962-2015); The Authors (2016-present)
We studied a novel illusion of tilt inside a checkerboard pattern due to the role of contrast polarity in contour integration. The preference for binding of oriented contours elements having same contrast polarity, over binding of opposite contrast polarity ones (CP rule), has been firmly assessed by previous research, and it is phenomenically shown by several visual illusions. In three experiments we have investigated how the binding effect is affected by luminance contrast value, relatability of contour elements, and distance among them. Experiment 1 showed that the effect is stronger when the luminance contrast values of the elements are more similar, and when their absolute values are higher. In Experiment 2 the strength of the illusion was found to decrease steeply as relatability of the elements decreased. The CP rule effectiveness, thus, seems to depend on good continuation, and to be overcome when this is broken. It might be that the contrast polarity signals intensity decreases in the association field the more the relative orientation of the contour elements deviate away from linearity. In Experiment 3 we estimated the distance threshold and it was found to be smaller than the one found for other illusions, arising with collinear fragments. This seems to show that the contrast polarity signal inside the association field of a contour unit looses strength more steeply along non collinear orientations, than collinear ones.
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