The barberpole illusion is a visual illusion 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 (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.
The barber's pole is commonly found outside barber shops.
In 1929, psychologist J.P. Guilford informally noted a paradox 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. Guilford tentatively attributed the phenomenon to eye movements, but acknowledged the absence of data on the question.
In 1935, Hans Wallach 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.