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Pablo Barrionuevo, Elisa Colombo, Luis Issolio; Illusory contours in a White's display. Journal of Vision 2009;9(8):355. doi: 10.1167/9.8.355.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: When the grey target of a White's display overlaps a pair of adjacent black and white stripes, an illusory contour appears as the continuation of the border between the stripes, but with the contrast polarity reversed. This effect accounts the hypothesis that when two aligned contours undergo a discontinuous change in the magnitude of contrast, but preserving the contrast polarity, the lower contrast region is decomposed into two causal layers. In order to analyze the plausibility of this hypothesis we studied the strength of the illusory effect.
Methods: Using a forced choice method with constant stimuli, we measured the luminance of the gray target half over the white stripe that had been matched in brightness to the gray target half over the black stripe which had a constant luminance value. In order to avoid aftereffects a test presentation time of 0.5 s and an ISI of 1.5s was adopted. A control experiment was made considering a similar measurement with a grey target centered over a half white-half black background. Six subjects were evaluated.
Results: As we had expected, brightness matches show a slightly but systematic increment (mean of 8.6%) in the half part of the target overlapping the black stripe. In the control experiment, we surprisingly found a similar result to the main experiment, a brightness increment in the half part of the target surrounded by a black background (mean of 7.6%). This last result indicates that the illusory contour could be also explained by mean of a depth cue introduced by the T-junction and a posterior simultaneous contrast mechanism.
Conclusion: We present a new visual illusion that consists in an illusory contour that appears in a target embedded in a White's display. The two previously proposed high level processes could explain the illusion.
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