Abstract
The motion and position of an object are sometimes dissociated, including famously in the motion aftereffect where a physically stationary test stimulus is perceived simultaneously in motion and not changing position. The study of other phenomena such as the motion-induced position shift or the curveball illusion has shown that motion and position can indeed be dissociated but are not fully independent. Here we are interested in the initial position and speed of a moving object. It is well-known that the initial position of a moving object is mislocated in its direction of motion (Fröhlich effect). We manipulated the duration of a line moving at 4 deg/sec and measured both its perceived initial position and its perceived speed. We used forced-choice psychophysical discriminations to another line that was presented either simultaneously, stationary, and with varying spatial offsets (position task), or sequentially and with varying speeds (speed task). Not too surprisingly, when the duration of the movement was brief, the spatial bias for the initial position was also short. The magnitude of the spatial bias increased with motion duration and reached a plateau of 0.4 deg at about 200 msec. More surprisingly, there was a large over-estimation of perceived speed for brief durations, up to 14 deg/sec for 50 msec durations. The magnitude of the speed overestimation decreased with motion duration and vanished also after 200 msec. Interestingly, for durations under 200 msec, the spatial bias in perceived initial location was almost as large as the full trajectory of the object over its duration, suggesting that the moving object was almost perceived stationary. Yet, for these short durations, there was a large over-estimation of perceived speed. Taken together, these results highlight a paradox in that the same moving object appears both almost static and moving very fast.