Abstract

Since many centuries, artists have been puzzled by the problem of representing depth and motion – the four-dimensional world – on a two-dimensional, static surface. Many inventive and creative steps eventually led to a very peculiar use of motion illusions by some Op artists in the 20th century, creating vivid sensations of movement from simple black and white patterns, and thus setting the stage for a lively scientific debate about possible visual processing mechanisms. Evidence is reviewed combining perceptual judgements of motion illusions and observations of eye movements with computer simulations of the induced optic flow. This work suggests that the frequently executed small involuntary saccades would generate an incoherent distribution of motion signals that resemble the perceptual effects experienced when viewing such patterns. The combined experimental and computational evidence supports the view that the illusion is caused by involuntary image displacements picked up by low-level motion detectors, and also suggests that coherent motion signals are crucial to perceive a stable world. Similar phenomena can be observed in other configurations of simple geometrical patterns, which open new possibilities to create new dynamic illusions and study motion processing in the human brain.