The barberpole illusion is a visual illusion where the turning of a diagonally striped pole makes it appear as if the stripes are moving up or down vertically. When a horizontally oriented barberpole is rotated, the stripes appear to move left or right. The barberpole is commonly found outside barber shops; hence the origins of its name.
Our continued interest in such phenomena, and the fascination they have for the viewer, reflect both the endless creativity and the appreciation for creativity that are to be found in human nature.
In 1929, psychologist J. P. Gilford noticed the paradoxical motion of the 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.
A barber pole with spiral stripes rotates around its vertical axis, so the colors move horizontally, but the stripes appear to move upwards vertically. Both a rotating, spiral-striped pole and a horizontally/vertically moving series of diagonal lines behind an aperture will create the illusory perception of vertical (or sometimes horizontal) movement of the stripes.
Rather than being perceived as rotating, the viewer will have the illusion of ascending or descending stripes (depending upon the direction of spin). The eyes use the visual cues where the stripes end at the sides of the pole to override any visual depth cues, and therefore the stripes appear to move vertically or horizontally rather than spin. Perception of motion is biased in the direction of the longer (in this case, vertical) axis; stripes on horizontal poles appear to move horizontally and vertical stripes move vertically (see Example One)
The barberpole effect is often shown as the movement of bars behind an aperture, or opening. A series of lines move vertically or horizontally back and forth behind the opening, and the effect is the same as that of the spinning barberpole. 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 (Example Two), the perceived direction of movement is usually perpendicular 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 that move vertically. This stronger motion signal forces us to perceive vertical motion. Functionally, we perceive a moving pattern as a rigid surface moving in one direction.
Because we have only limited information, the actual motion of the line has many possibilities. Donald Hoffman has explained this "aperture problem" as the choice of our visual system to construct the smallest motion. This choice is motion orthogonal to the line that moves.
The barberpole illusion is found most often outside barber shops, although its use is declining.
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