You can find similar aftereffects when you stare out the front window of a moving vehicle for a couple of minutes and then stop: the landscape suddenly seems to slowly move away from you for some time; or when you stare out the widow of a train: when it stops the landscape appears to slowly move in the opposite direction. Aftereffects of this type can have any direction: when you stare at a rotating image and the rotation stops, the image appears to rotate in the opposite direction. Depending on the stimulus, this can be a smooth motion or it "jumps" forward and back. This is called the **direction aftereffect** (DAE), "whereby prolonged exposure to a moving pattern affects the perceived direction of subsequent motion" (Clifford, 2002). This is an effect of [perceptual adaptation][1], where the brain compensates for a supposed error in perception. The mechanism behind this phenomenon was tested by Schrater and Simoncelli (1998): > Adaptation to a moving visual pattern induces shifts in the perceived > motion of subsequently viewed moving patterns. Explanations of such > effects are typically based on adaptation-induced sensitivity changes > in spatio-temporal frequency tuned mechanisms (STFMs). An alternative > hypothesis is that adaptation occurs in mechanisms that independently > encode direction and speed (DSMs). Yet a third possibility is that > adaptation occurrs in mechanisms that encode 2D pattern velocity > (VMs). We performed a series of psychophysical experiments to examine > predictions made by each of the three hypotheses. The results indicate > that: (1) adaptation-induced shifts are relatively independent of > spatial pattern of both adapting and test stimuli; (2) the shift in > perceived direction of motion of a plaid stimulus after adaptation to > a grating indicates a shift in the motion of the plaid pattern, and > not a shift in the motion of the plaid components; and (3) the 2D > pattern of shift in perceived velocity radiates away from the > adaptation velocity, and is inseparable in speed and direction of > motion. Taken together, these results are most consistent with the VM > adaptation hypothesis. * * * Sources: - Clifford, C. W. G. (2002). Perceptual adaptation: Motion parallels orientation. *Trends in Cognitive Sciences, 6*, 136-143. doi:[10.1016/S1364-6613(00)01856-8][2] Available online at http://psy.mq.edu.au/vision/~peterw/corella/315/clifford.pdf - Schrater, P.R. and Simoncelli, E.P. (1998) Local velocity representation: evidence from motion adaptation. *Vision Research, 38*, 3899-3912. doi:[10.1016/S0042-6989(98)00088-1][3] Available online at: http://www.cns.nyu.edu/pub/lcv/schrater97.pdf [1]: http://en.wikipedia.org/wiki/Perceptual_adaptation [2]: http://dx.doi.org/10.1016/S1364-6613%2800%2901856-8 [3]: http://dx.doi.org/10.1016/S0042-6989%2898%2900088-1