The term illusory motion, also known as motion illusion, 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. Apparent motion is the most common type of illusory motion and is perceived when images are displayed in succession at a specific frame rate such as in a movie.
Induced movement works instead by moving the background around a fixed object. Films such as Airplane! and Top Secret! use a fixed prop and move the background props to give the effect of induced motion.
Motion aftereffect occurs when we view moving stimuli for an extended period of time and then focus on a stationary object. The object will appear to move in the opposite direction of the moving stimuli.
Illusory motion in the eye
Illusory motion is perceived as movement in a number of ways. The first can manifest through the retinal image where the motion flows across the retinal mosiac. The perceived motion can also manifest by the eyes changing position. In either case, an aftereffect may occur.
Illusory motion in the brain
Using an fMRI, Roger B. H. Tootel et al. were able to identify the area of the brain that is active when experiencing illusory motion. Tootel and his colleagues had participants view a set of concentric rings that appeared to move inward and outward. Participants would experience a motion aftereffect after viewing the moving stimuli for 40 seconds. Participants showed an increased activity in the MT area of the brain.
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 to 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.
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.)
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 an "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.
In popular culture
- Goldstein, E. Bruce (2010). Sensation and perception (8th ed. ed.). Belmont, Calif.: Wadsworth Cengage Learning. ISBN 9780495601494.
- "Induced Motion". Retrieved 4/5/2012.
- "Motion Aftereffect Demo (Waterfall)". Retrieved 29 Aug 2013.
- "Visual Perception of Movement". Ann R Coll Surg Engl 33: 267–81. April 1963. PMC 2311643. PMID 14075040.
- "Letters to Nature". Retrieved 5 April 2012.