The strength of an illusion of curvature created by an equilateral triangle, square or pentagon inscribed in a circle has been measured in the 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 less illusory effect than the straight ones. Similar distortions of the stimuli have been observed in the output of a neurophysiological model of spatial frequency filtering of images, and the computed curves resembled those of the experimental data, in respect to their shape and quantitative values.