Effect of orthogonal adaptation on the perceived velocity of multidirectional random dot stimuli at different speeds

Motion adaptation generates strong aftereffects in the opposite direction. Here, we investigated the effect of orthogonal adaptation on perceived velocity across different speeds. Human observers adapted to a broadband grating, moving upwards at 6°/s. Then, a test stimulus was presented, which consisted of random dots moving at directions following a gaussian distribution centered at the orthogonal direction to adaptation (leftwards or rightwards). Participants manually matched the velocity of unidirectional dots arranged in a ring around the adaptation area with the perceived velocity of the test stimulus. Due to the aperture problem, the ambiguity of the adaptor’s velocity should produce an asymmetry in the adaptation of velocity channels: faster speeds should be adapted for directions away from upwards but not slower speeds. To test this prediction, test stimuli were presented at speeds both faster and slower than the adapting speed. Our results showed a strong effect of repulsion away from the presented velocity (downwards and slower). The effect on speed was stronger at the adapting speed and weaker elsewhere, consistent with a non-directional adaptation (Stocker and Simoncelli, 2009). In contrast, the effect on direction was uniform across speeds. We present a computational model that can explain this discrepancy