A jittered squares illusion and a proposed mechanism.

A new visual illusion is reported in which randomly positioned squares that are perfectly aligned with the horizontal and vertical appear slightly rotated about their midpoints ('jittered') relative to each other. Possible mechanisms for the illusion have been explored in a series of three experiments. Experiment 1 showed that, unlike the Münsterberg illusion, the Jittered Squares illusion persists at isoluminance. Experiment 2 indicated that the degree of rotation from vertical and horizontal, of rows and columns of squares in which the edges of individual squares remain perfectly aligned with vertical and horizontal, changes the perceived strength of the illusion such that the strongest effect is found at rotations of +/- 10 degrees to +/- 15 degrees. Experiment 3 revealed that the illusion is dependent upon the spatial extent of the gaps between the squares, such that it becomes weaker as the separation increases. On the basis of the findings it is suggested that the jittering results from the extraction of obliquely oriented contours by an integrator mechanism when the difference in orientation, between the edges of the individual squares and the global orientation of a contour made up of a number of edges of individual squares, is not too dissimilar. It is proposed that a mechanism such as simultaneous orientation contrast between these global contours and the vertical and horizontal edges of the individual squares causes the squares to appear rotated away from the orientation of the extracted contours, leading to the perceived jittering.