Illusory contours in a White’s display

PABLO BARRIONUEVO; ELISA COLOMBO; LUIS ISSOLIO

Naples, Florida, USA

Congreso; VSS 2009; 2009

VSS

Purpose: When the grey target of a White's display overlaps a pair of adjacent black andwhite stripes, an illusory contour appears as the continuation of the border between thestripes, but with the contrast polarity reversed. This effect accounts the hypothesis that whentwo aligned contours undergo a discontinuous change in the magnitude of contrast, butpreserving the contrast polarity, the lower contrast region is decomposed into two causallayers. In order to analyze the plausibility of this hypothesis we studied the strength of theillusory effect.Methods: Using a forced choice method with constant stimuli, we measured the luminanceof the gray target half over the white stripe that had been matched in brightness to the graytarget half over the black stripe which had a constant luminance value. In order to avoidaftereffects a test presentation time of 0.5 s and an ISI of 1.5s was adopted. A controlexperiment was made considering a similar measurement with a grey target centered over ahalf 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 controlexperiment, we surprisingly found a similar result to the main experiment, a brightnessincrement 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 depthcue 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 appearsin a target embedded in a White's display. The two previously proposed high level processescould explain the illusion.