Privileged Directions for Subjective Contours: Horizontal and Vertical versus Tilted

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Abstract
Subjective contours and brightness enhancement in Ehrenstein-like situations are affected by pattern orientation. If a classic Ehrenstein pattern (with four inducing elements for every gap at intersection points) is observed, a number of anomalous illusory patches usually appear in these gaps, but if the same pattern is observed tilted by 45° the patches disappear and it is possible to see an illusory grid of horizontal and vertical ‘streets’. These two perceptual results are mutually exclusive. In a Koffka-cross variant of this pattern, the illusory patches, which are usually square, appear more rounded in the tilted pattern. All these results were confirmed in two experiments by means of a magnitude estimation procedure. It is suggested that the formation of a subjective contour is easier along horizontal and vertical directions and more difficult in an oblique direction, and that this phenomenon, as well as other visual acuity oblique effects, depends in part on the basic functioning of the visual system at the level of sensation.