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Lothar Spillmann, Walter H. Ehrenstein, Baingio Pinna; Cognitive theory fails to explain illusory form and brightness enhancement. Journal of Vision 2002;2(7):353. doi: https://doi.org/10.1167/2.7.353.
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Purpose: To study the role of “cognitive hypotheses” in the formation of illusory figures, such as the Ehrenstein illusion (1954). Top-down approaches (Gregory, 1972; Rock, 1972) focus on the incompleteness of the inducers and the need for a perceptual postulate occluding an unlikely stimulus. Consistent with this assumption, illusory form and inducing stimuli appear to be separated in depth. We show that brightness enhancement and illusory contours emerge also when the inducers are perceptually complete and are coplanar with the inducing stimuli. Method: Nine Ehrenstein patterns were made from alphabetical letters instead of radial lines. Capital letters with terminators (e.g., L, I) and without (O, D) were used and letters were oriented either towards or away from the central area (gap). Magnitude estimation was used to quantify the strength of (i) the illusory contour, (ii) brightness enhancement, and (iii) depth segregation. Three groups of fourteen subjects participated in the experiments. Results: Strong illusory figures and brightness enhancement were perceived without the need for apparent occlusion, amodal completion, or depth segregation between the illusory surface and the inducers. Conclusion: This result logically weaken cognitive explanations of illusory figures and instead favors a low-level, bottom-up explanation in terms of a neurophysiological mechanism in the cortex (Dresp and Bonnet, 1991). How such a mechanism would operate to produce illusory contours and brightness enhancement from physical contours is unclear.
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