Motion extrapolation and velocity transposition.

A study of the effect of the size of a moving target and the extent of its visible motion on motion extrapolation is reported. Targets (a horizontal pair of dots separated by either 0.2 or 0.8 deg) moved across a 10 deg rectilinear path and were then occluded. Observers pressed a key when they thought the leading dot of a hidden target had reached a randomly specified position (0-12 deg from the point of occlusion). In experiment 1, in agreement with velocity-transposition predictions, at moderate (5 deg s-1) and rapid (10 deg s-1) velocities extrapolation times were longer for large targets than for small ones. At slow velocity (2.5 deg s-1) this effect was reversed. In experiment 2 the effect of target size at moderate velocity was found for a short (2.5 deg) visible path. However, the extrapolation time increased with shorter (2.5 deg versus 10 deg) paths. A proposed account of these effects suggests that the visual system performs a spatiotemporal scaling, according to the velocity-transposition principle, not only of visible motion but also of extrapolated motion.