Abstract
The direction of motion of dots moving 20–60 degrees above horizontal is known to be perceived more vertical than the stimulus by about 10 degrees. This effect is called reference repulsion (RR). To understand what processes give rise to this illusion, we studied the relationship between reference repulsion and the motion aftereffect (MAE). Observers were shown several different directions of motion (0, 30, 150, 180, 210, and 330 degrees, counter-clockwise from rightwards motion) and were instructed to indicate the perceived direction of motion of the stimulus before adaptation (RR), and the perceived direction of motion of a stationary dot field after adaptation (MAE). As expected, observers misperceived the direction of motion before adaptation and thus experienced reference repulsion. Furthermore, the MAE was not exactly opposite to the stimulus direction. Rather, it was biased in the direction of RR. Thus, the MAE is subject to RR. Two different models could account for these data. In the first model adaptation mechanisms act first and adapt to stimulus motion, and then repulsion acts on the adapted signals. Alternatively, first repulsion takes place, followed by adaptation. To distinguish between these models we made use of the fact that for most observers RR is not the same for opposite directions of motion. We performed a correlation analysis between RR and MAE data by either pairing conditions in which the stimulus went in the same direction, or by pairing conditions in which the percept went in the same direction. We find that the correlation between RR and MAE is strongest when the percepts are aligned. This suggests that repulsion mechanisms act on the perceived MAE, not the adaptation motion. In other words, motion adaptation mechanisms act before reference repulsion mechanisms. These results suggest that reference repulsion occurs at a higher processing stage, after motion has been detected and adaptation has taken place.
Supported in part by the Whitehall Foundation