– David Phillips and Priscilla Heard: “The Shrink and Swell Illusion”. University of the West of England. UK
Author description: The illusion is that the objectively static sides of a V-shaped window appear either to expand or to contract horizontally. Portraits within the window, expanded at the top and squashed at the bottom into the V-shape, rise or fall at constant speed. The illusion may arise because the V-shape of the window presents powerful linear perspective, suggesting depth, whereas the moving patterns within it only indicate change in size, and look flat. Illusion vanishes if this “optic flow” within the window moves not only with horizontal shrinkage and expansion, but also with vertical acceleration, typical of movement in depth.