Not physically present contours can yield illusory motion.

We demonstrate a perceptual effect whereby contours not physically present in a visual scene can yield striking illusory motion. The not physically present contours are paths of invariant contrast polarity (CP). For example, when a square checkerboard composed of dark and light square checks with small black and white discs covering the vertices is put in lateral motion, there is the striking perception of vertical expansion/contraction. Such a checkerboard has (not physically present) diagonal paths of CP presentation with vertical components. However, when a square checkerboard made up of square black and very light checks with gray discs of luminance intermediate to the checks is put in lateral motion, no expansion/contraction is seen. For this checkerboard the vertical components of paths of CP preservation cancel each other out, predicting the lack of perception of vertical expansion/contraction. We also discuss how not physically present contours can explain previously described effects and suggest new effects to be explored.