The effect of orientation and motion of enclosed texture on induced movement

Optical expansion or contraction of a vertical grating induces rotation of an oblique line or bar superimposed upon it. Three experiments are reported in which the oblique (45 deg clockwise) bar was defined by parallel line segments that were varied in orientation and motion. In Experiment 1 subjects estimated the degrees through which the boundaries of the bar appeared to rotate during 2 sec expansion of the vertical grating background. The induced rotation declined with increasing relative orientation between the texture elements and the background; virtually no effect was reported when they were orthogonal. The texture elements were stationary in Experiment 1 whereas they expanded or contracted in Experiment 2: when their motion was in-phase with the background induced rotation of the boundary occurred irrespective of relative orientation. Expansion or contraction of the texture elements alone did not result in induced bar rotation (Experiment 3). It was concluded that induced movement at the boundaries between two patterns is reduced when they differ in static orientation and in the directions of motion due to zooming.