Influences of local and global motion on perceived position
26.445, Saturday, May 11, 2:45 - 6:45 pm, Orchid Ballroom
Peter J. Kohler1, Leif H. Harder1, Peter U. Tse1; 1Psychological and Brain Sciences, Dartmouth College, Hanover, NH
The perceived position of a briefly flashed stimulus can be shifted in the direction of nearby motion (Whitney & Cavanagh, 2000). We used a horizontally translating diamond stimulus (Lorenceau & Shriffrar, 1992) to examine how different global motion interpretations of ambiguous local motion affect motion-induced position shifts. Two dots were flashed as motion direction reversed, such that the dots appeared shifted in opposite directions. Subjects adjusted comparison stimuli to report perceived dot positions. We found that the perceived motion direction influenced the direction of the position shift, even when the overall motion in the image was kept constant. Surprisingly, although the effect of global motion was highly significant, the shift direction did not completely follow the motion direction: Even when the global motion percept was clearly vertical or horizontal, the shift was far from aligned with the motion direction. Others have found that high-level motion can cause position shifts in the absence of any local motion signal (Shim & Cavanagh, 2004), but our results suggest that there can be local, as well as global, contributions to motion-induced position shifts. We are currently conducting a second set of experiments, to further investigate the contributions of local and global motion to the position shifts. Motion integration takes place over a ~150ms period (Pack & Born, 2001), and by modulating the dot timing relative to motion transients, we can allow motion integration to finish before the dot is presented. This allows us to determine if the local motion contribution to the shift occurs because motion has not yet been fully integrated at the time of the dot presentation, or if there are inherent local contributions to motion-induced position shifts. Determining the contributions of local and global motion to motion-induced position shifts is an important step towards understanding the neural mechanisms underlying the phenomenon.