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Barberpole illusion

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 vertical axis (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).

Explanation

Rather than perceiving a rotational motion, the viewer will have the illusion of seeing ascending or descending stripes (depending upon the direction of rotation). This illusion occurs because the perception of motion is biased in the direction of the longer (in this case, vertical) axis.

A bar or contour moving behind an aperture provides ambiguous information about the direction of movement (see aperture problem). In the case of contours or figures seen within an aperture, our visual system regards the surface as occluding stripes that move behind the occluder. This is because the figure contains numerous T-junctions (where the ends of the line meet with the inside of the aperture), which suggest that the surface is behind, and not bounded by, the aperture. This depth cue, however, is in conflict with binocular disparity, which indicates that the diagonal stripes are on the same depth plane and therefore part of the same object.

The shape of the aperture tends to determine the perceived direction of motion for an identically moving contour. Thus, 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.

ee also

*Barber's pole for the modern device exploiting this illusion
*Motion perception

* [http://www.aceviper.net/Optical2/barber_pole_illusion.htm external animation]

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