The barberpole illusion is a

visual illusion Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; thei ...

that reveals biases in the processing of visual motion in the human brain. This visual illusion occurs when a diagonally striped pole is rotated around its

vertical axis A Cartesian coordinate system (, ) in a plane (geometry), plane is a coordinate system that specifies each point (geometry), point uniquely by a pair of number, numerical coordinates, which are the positive and negative numbers, signed distance ...

(horizontally), it appears as though the stripes are moving in the direction of its vertical axis (downwards in the case of the animation to the right) rather than around it.

History

In 1929,

psychologist A psychologist is a professional who practices psychology and studies mental states, perceptual, cognitive, emotional, and social processes and behavior. Their work often involves the experimentation, observation, and interpretation of how indi ...

J.P. Guilford Joy Paul Guilford (March 7, 1897 – November 26, 1987) was an American psychologist best remembered for his psychometric study of human Intelligence (trait), intelligence, including the distinction between Convergent thinking, convergent and Div ...

informally noted a paradox in the perceived motion of stripes on a rotating

barber pole A barber's pole is a type of sign used by barbers to signify the place or shop where they perform their craft. The trade sign is, by a tradition dating back to the Middle Ages, a staff or pole with a helix of colored stripes (often red and wh ...

. The barber pole turns in place on its vertical axis, but the stripes appear to move upwards rather than turning with the pole. Guilford tentatively attributed the phenomenon to eye movements, but acknowledged the absence of data on the question. In 1935,

Hans Wallach Hans Wallach (November 28, 1904 – February 5, 1998) was a German-American experimental psychologist whose research focused on perception and learning. Although he was trained in the Gestalt psychology tradition, much of his later work explored t ...

published a comprehensive series of experiments related to this topic, but since the article was in German it was not immediately known to English-speaking researchers. An English summary of the research was published in 1976 and a complete English translation of the 1935 paper was published in 1996. Wallach's analysis focused on the interaction between the terminal points of the diagonal lines and the implicit aperture created by the edges of the pole.

Explanation

This illusion occurs because a bar or contour within a frame of reference provides ambiguous information about its "real" direction of movement. The actual motion of the line has many possibilities. The shape of the aperture thus tends to determine the perceived direction of motion for an otherwise identically moving contour. A vertically elongated aperture makes vertical motion dominant whereas a horizontally elongated aperture makes horizontal motion dominant. In the case of a circular or square aperture, the perceived direction of movement is usually

orthogonal In mathematics, orthogonality is the generalization of the geometric notion of ''perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...

to the orientation of the stripes (diagonal, in this case). The perceived direction of movement relates to the termination of the line's end points within the inside border of the occluder. The vertical aperture, for instance, has longer edges at the vertical orientation, creating a larger number of terminators unambiguously moving vertically. This stronger motion signal forces us to perceive vertical motion. Functionally, this mechanism has

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...

to ensure that we perceive a moving pattern as a rigid surface moving in one direction. Individual motion-sensitive neurons in the

visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...

have only limited information, as they see only a small portion of the visual field (a situation referred to as the "aperture problem"). In the absence of additional information the visual system prefers the slowest possible motion: i.e., motion orthogonal to the moving line.Hoffman, Donald D. 2000. Visual Intelligence: How We Create What We See. W. W. Norton. The neurons which may correspond to perceiving barber-pole-like patterns have been identified in the

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and ...

ferret The ferret (''Mustela furo'') is a small, Domestication, domesticated species belonging to the family Mustelidae. The ferret is most likely a domesticated form of the wild European polecat (''Mustela putorius''), evidenced by their Hybrid (biol ...

Auditory analogue

A similar effect occurs in the

Shepard's tone A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition principle, superposition of sine waves separated by octaves. When played with the bass Pitch (music), pitch of the tone moving upward or downward, it is referred ...

History

Explanation

Auditory analogue

See also

References

Notes

External links