Illusory motion

The term illusory motion is used to define the appearance of movement in a static image.

Occurrences
Illusory motion can occur in different circumstances.
 * Stroboscopic images - Where a series of static images are viewed in sequence at a high enough rate that the static images appear blend into a continuous motion. An example is a motion picture.
 * Optical art (Op art.) - Where artists use simple black and white patterns that create vivid illusions of motion, known as optical flow.

Causes
The reasons why illusory motion occurs are subject to debate. Many mechanisms and models have been proposed.

Stroboscopic images
Rotating objects can appear stationary under strobe light, also they can appear to be counter rotating. This second effect can occur in daylight, such as the apparent counter-rotation of wheels. Because of the illusion of counter rotation in constant light, it is reasonable to assume that the eye views the world in a series of still images, and therefore the counter-rotation is a result of under-sampling (aliasing.) This theory has however received a strong counter-argument. A simple demonstration to disprove the idea is to view an apparent counter-rotation (that of a rotating drum) in mirror image. Subjective reports reveal that the counter-rotation appears in only one of the images (either the real or mirrored image when both are viewed simultaneously.)

Optical art
Apparent motion in optical art has been suggested to be caused by the difference in neural signals between black and white parts of an image. While white parts may produce and 'on - off' signal, the black parts produce an 'off - on' signal. This means for a black part and a white part presented simultaneously, the 'on' part of the signal is separated in time, possibly resulting in the stimulation of motion detectors.

Another explanation is that after images from the retina (the McCullough effect) cause a moire that is hard to identify.