Abstract

When the contrast of a clearly suprathreshold, one-dimensional, sinusoidal grating was weighted with a circular, two-dimensional Gaussian window, the shape of the grating was found to be elliptical: It was flattened in the direction of grating bars. We studied this shape illusion by measuring how much the spatial dimensions of the Gaussian window along or across the bars had to be adjusted in order to make the perceived shape circular. Our experiments showed that the grating appeared circular when the diameter of the two-dimensional Gaussian window along the bars was about 1.25 times the diameter across the bars. The illusion was found to be independent of spatial frequency, grating area, and viewing distance. Neither was it affected by peripheral viewing at eccentricities over the 020 tested. The magnitude of the illusion was the same for horizontal, vertical, and oblique gratings and in monocular and binocular vision. However, the illusion disappeared when the number of grating bars was small or the grating contrast was near the threshold.

© 1990 Optical Society of America