Illusory motion

The term illusory motion, also known as motion illusion, is used to define the appearance of movement in a static image. This is an optical illusion in which a static image appears to be moving due to the cognitive effects of interacting color contrasts and shape position.

Another type of motion illusion that causes an optical illusion is when a moving object appears to be moving in a path other than what is perceived by the brain. An example of this can be demonstrated by placing a colored filter over ones eye of the observer, and swinging a ball back and forth in front of them. To the observer the ball appears to be swinging in a circular motion.^{[citation needed]}

Occurrences

Theoretical example with the Illusion of motion: It appears that the sky is falling, while the reality is that the ocean is rising.

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.

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.)

Gallery

Examples of motion illusions.

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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.

In popular culture

The American Indie Rock band Animal Collective used an optical illusion of this type on the cover of their award-winning 2009-album Merriweather Post Pavilion.

See also

• Akiyoshi Kitaoka