The Orthogonal Motion Aftereffect

A recent model of motion perception suggests that the motion aftereffect (MAE) is due to an interaction across all directions, rather than just opposite directions (Grunewald, 1995 Perception 24 Supplement, 111). According to the model, the MAE is caused by the interaction of broadly tuned inhibition and narrowly tuned excitation, both in direction space. The model correctly suggests that, after adaptation to opposite directions of motion, no MAE results. Unlike other accounts of the MAE, this model predicts that, after adaptation to opposite but broadly defined directions of motion, a MAE orthogonal to the inducing motions is observed.

We tested this counter-intuitive prediction by adapting subjects to two populations of dots, whose average motion vectors were opposite, but which contained motion vectors deviating slightly (up to 30°) from the average direction. During the subsequent test phase, randomly moving dots were displayed. Subjects were asked to indicate whether they perceived any global motion during this phase, and if so, they were asked to indicate the perceived motion axis by aligning a line. Subjects were tested on four pairs of directions: vertical, horizontal, and the two diagonals. In all four conditions subjects reported seeing an MAE, and the axis that they indicated was always orthogonal to the inducing motions (ANOVA: p<0.001, accounted for 95% of variance). This experiment confirms the predictions made by the model, thus further supporting the interaction across all directions of narrowly tuned excitation and broadly tuned inhibition.