Segregation of motion aftereffect following adaptation to transparent motion

Characteristics of motion aftereffect (MAE) following adaptation to two dot-populations moving in orthogonal directions at different speeds are reported. A test stimulus also consisted of two populations of randomly moving dots whose speeds were equal to the two adaptation speeds. In every pair of adaptation speeds we employed, the results of experiments showed that (1) when the contrast of the test stimulus was high, a transparent (bidirectional) MAE was perceived; (2) when the test contrast was low, a unidirectional MAE was perceived. Test contrast, thus, determined whether two MAE-components generated by the two adaptation speeds were segregated or not. These results suggest that speed channels in our visual system are multiple.