Looking Backwards or Moving Forward? A Novel Motion After-effect Reflects the Mechanisms Underlying Human Motion Perception

Prolonged exposure (adaptation) to a stimulus drifting at a constant speed can bias the perceived speed of stimuli subsequently presented in the same location. Following adaptation, the speed of a test stimulus drifting at an identical or slower rate than the adaptor will generally be underestimated, while the speeds of test stimuli drifting faster than the adaptor will be overestimated. These biases can be used to infer the nature of the neural mechanism(s) that encode motion. It has recently been proposed that a combination of two distinct mechanisms, one tuned to direction and another that is non-directional, can best account for post adaptation biases in perceived speed. To assess this possibility, the non-directional mechanism was selectively adapted, as was a combination of the directional and non-directional mechanism. Doing so revealed that the non-directional mechanism is not tuned for movement per se, but for simple local rates of change (or temporal frequency). The directional mechanisms, however, was tuned for stimulus speed. The results suggest that outputs from these two mechanisms are summated, and based on the summated biases obtained from Experiment 1, we were able to predict a novel motion phenomenon – a dynamic motion aftereffect that is seen to move in same direction as adaptation