Abstract
Prolonged viewing of motion produces illusory movement in stationary displays, called the motion aftereffect (MAE). During motion processing, outputs of direction-selective mechanisms are combined to produce a global percept. Where in this hierarchy are MAEs produced? If MAEs arise early, then adaptation to different directions may be controlled by distinct direction-selective mechanisms. We tested this by examining whether MAEs in one direction eliminate MAEs in another direction or simply temporarily mask them. Subjects fixated while attending a sine-wave grating moving either leftward or rightward (direction "AB"). Following 10-minutes of viewing, the motion reversed (to direction "BA"), and every 4 seconds a static pattern appeared and subjects indicated their perceived direction of illusory motion. The BA motion continued until subjects perceived MAEs in the AB direction (on average 24 sec). Then, in a final test phase, the static stimulus was presented for 90 seconds, while subjects continued to report their MAE every 4 seconds. Subjects’ final test phase MAE initially reflected the recent BA adaptation (i.e MAE in AB direction). Within 10 seconds, however, the MAE of all subjects (n=7) reversed direction to reflect the original, longer-term AB adaptation (i.e. in BA direction), which then persisted until the end of testing. The simplest account of our data is that adaptation in the two directions is controlled by distinct mechanisms. Following the long AB and shorter BA adaptations, both mechanisms were active, but BA adaptation was momentarily stronger. Because the BA adaptation was shorter, however, it decayed more quickly than the AB adaptation during the final test phase, unmasking the remaining AB adaptation and allowing its MAE to spontaneously recover. These findings suggest that adaptation occurs in early direction-selective stages of motion processing, and illustrate the visual system’s ability to adapt to short-term changes while retaining information about longer-lasting visual conditions.
Meeting abstract presented at VSS 2012