Anomalous spiral aftereffects: a new twist to the perception of rotating spirals.

Stationary spirals viewed after inspecting rotating sectored disks appear to rotate and to expand or contract radially, even though the rotating disks contain no perceptible components of radial motion. Moreover, the relative directions of illusory rotation and radial motion observed in these instances are ‘impossible’ under the stimulus constraints normally imposed by the geometry of a spiral under rotation: the stationary spirals appeared to expand/contract in directions opposite to those normally observed under conditions of actual spiral rotation, and under conditions of illusory spiral rotation in classical spiral aftereffects.