Barberpole illusion

[2] In 1929, psychologist J.P. Guilford informally noted a paradox in the perceived motion of stripes on a rotating barber pole.

In 1935, Hans Wallach published a comprehensive series of experiments related to this topic,[4] but since the article was in German it was not immediately known to English-speaking researchers.

[6] 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.

This illusion occurs because a bar or contour within a frame of reference provides ambiguous information about its "real" direction of movement.

The perceived direction of movement relates to the termination of the line's end points within the inside border of the occluder.

An example of the barberpole illusion. The grating is actually drifting downwards and to the right at 45 degrees, but its motion is captured by the elongated axis of the aperture.
The barber's pole is commonly found outside barber shops .
In this example the motion of the grating is identical to that in example 1, but the aperture is isotropic .