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.

We investigated how the visual-motor system recruits a novel visual feedback cue for a manual control task. We presented conditions in which an arbitrary cue (color) was coupled with task-relevant feedback (position or velocity), and measured the effect of the novel cue on performance. Participants used a joystick to keep a moving horizontal line centered on a display under velocity or acceleration control dynamics. Participants normally rely primarily on line position feedback for velocity control and line velocity feedback for acceleration control. The novel color cue was coupled with either line position (becoming red as it deviates from center) or line velocity (becoming red as it moves faster). For velocity control, performance error was smaller and response gain was larger when the novel color cue was coupled with line position than when it was coupled with line velocity. Conversely, for acceleration control, performance was better when color was coupled with line velocity than with line position. Our findings show that the visual-motor system can recruit a novel arbitrary cue to improve active control performance, but the effectiveness of the novel cue depends on its relationship to the feedback appropriate for control dynamics.