Identity imposition and its role in a stereokinetic effect.

Lines of constant curvature, a circle or a straight line, have no distinguishable parts. Yet they are perceived as if they did. When they move and intersect, they are perceived to slide across each other as if one of them had parts that can be seen to move in relation to the other line. With no such parts present in stimulation, they are products of perception. It was found that a third line of constant curvature, the helix, is also seen to slide when two helixes intersect and are in motion. Another manifestation of perceived identical parts is that rotating circles and similar shapes are perceived not to rotate even when cues for rotation are present. Furthermore, changes between merely perceived identical parts can result in apparent depth. Evidence is presented that such depth, known as the stereokinetic effect, results from kinetic depth-effects that are based on perceived identical parts instead of on actually identical parts, and that depth is seen when the intervals between such perceived parts change length and orientation simultaneously.