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The barberpole illusion is a visual illusion that reveals biases in the processing of visual motion in the human brain. When a diagonally-striped pole is rotated around its longer axis, so that the stripes are moving in the direction of the pole's shorter axis, it nonetheless appears the stripes are moving in the direction of its longer axis. Motion perception is biased in the direction of the longer axis. If the pole is reduced to a circle, the bias is removed and the true motion of the contour is obvious.
In 1929, psychologist J.P. Guilford noted the paradoxical motion inherent in the perceived motion of stripes on a rotating barber pole. The barber pole turns in place on its vertical axis, but the stripes appear to move upwards rather than turning with the pole.
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 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 to ensure that we perceive a moving pattern as a rigid surface moving in one direction.
Individual motion-sensitive neurons in the visual system 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. Neurons have been identified in the visual cortex of ferrets, the activity of which may correspond to the perception of patterns such as barber poles.
- See also: Strange loop
- ↑ Guilford, J.P. "Illusory Movement from a Rotating Barber Pole." American Journal of Psychology 41, 686-687.
- ↑ (2002). A new variant of the barberpole effect: Psycholphysical data and computer simulations. PSIHOLOGIJA 35 (3-4): 209–223 UDC 159.937.075.
- ↑ Hoffman, Donald D. 2000. Visual Intelligence: How We Create What We See. W. W. Norton. ISBN 0-393-31967-9
- ↑ (July 15, 2003)Rethinking How the Brain Sees Visual Features: Duke neurobiologists study brain's visual-processing region.
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