Background motion and the perception of shape defined by illusory contours.

A Kanizsa triangle is usually generated by placing three circular tokens, with deleted wedges, at the apexes of an equilateral triangle. If the token angles do not each subtend 60 degrees, a Kanizsa triangle may still be evident, but with illusory contours that appear to be curvilinear. We investigated whether this curved-contour distortion in shape can be nulled by radial background image motion utilizing a two-alternative forced-choice procedure. We report that a test Kanizsa figure with concave illusory contours appeared to form a perfectly regular equilateral triangle when it was superimposed on a globally expanding pattern. Conversely, a test Kanizsa triangle with convex illusory contours was perceived as regular when it was superimposed on a globally contracting pattern. This distortion effect was most distinct for fast dot speeds and was greater for contracting motion. Additionally, the effect was observed regardless of a polarity difference between background dots and tokens. However, shape distortion was not evident when the Kanizsa figure was defined by "real," luminance-defined, contours. Our findings support the conclusion that background image motion plays an important role in the perception of shape, especially when there is an "insufficiency" in the position information that specifies the shape's contours.