Local direction of edge motion causes and abolishes the barberpole illusion.

The perceived direction of motion of a one-dimensional grating is measured in straight-edged rectangular and indented rectangular apertures. It is shown that the perceived direction of motion of the pattern is largely determined by the directions of motion at the edges, rather than by the aspect ratio or global shape of the aperture. The edge motion vectors appear to be calculated at a remarkably local scale. The barberpole illusion is abolished when indentation size equals or exceeds one-quarter of the grating period. This critical size is scale invariant with grating period and corresponds well with a quadrature model of motion perception.