Visual motion aftereffects: differential adaptation and test stimulation.

The local motion adaptation at the basis of the motion aftereffect (MAE) can be expressed in a variety of ways, depending upon the structure of the test display [Wade et al. (1996). Vision Research, 36, 2167-2175]. Three experiments are reported, which examined the characteristics of the test display and of the local adaptation process. In Experiment 1, MAEs were recorded in the central of three test gratings but their directions depended on the location of the centre relative to the adapting gratings. The effects of adapting motions in different directions were examined in Experiments 2 and 3, in which one or two adapting gratings were presented above or above and below a fixation cross. The upper grating always received the same (leftward) direction of motion during adaptation, and the lower grating was: moving in the opposite direction, stationary, moving in the same direction, or absent. The results indicate that no MAE is visible in the upper grating when a single test grating is observed experiment 2) and only occurs with two test gratings following differential adaptation between the upper and lower gratings (Experiment 3). Thus, the MAE occurs as a consequence of adapting restricted retinal regions to motion but it can only be expressed when differentially adapted regions are also tested.