The strength of an illusion of curvature created by an equilateral triangle, square or pentagon inscribed in a circle has been measured in psychophysical experiments. The arcs of the circle looked as if they were bowed outwards in the stimuli of various sizes, but, at a fixed diameter of the circumscribed circle, the triangles produced the strongest and the pentagons the weakest illusion. The strength of the illusion augmented with the stimulus diameter. Concave and convex sides of the inscribed figures caused a less illusory effect than the straight ones. The illusion strength is greater in the presence of the luminance contrast between the inscribed triangle or circumscribed circle and the background than in the presence of color contrast. These data might be interpreted as a result of different sensitivity of chromatic and achromatic vision to spatial frequencies. Similar distortions of the stimuli have been observed in the output of a neurophysiological model of spatial frequency filtering of images, and the computational curves resembled those of the experimental data in respect to their shape and quantitative values.

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