Human sensitivity to temporal proximity: the role of spatial and temporal speed gradients.

Estimates of temporal proximity (sometimes called time-to-collision) from random-dot flow patterns are shown to be based upon retinal speed, rather than upon changes in dot density. Neither the spatial nor the temporal gradient of motion is essential to the task, but estimates can be made from either alone. Performance is unaffected by the addition of rotational motion, suggesting that observers are capable of extracting the radial component of motion, which contains all the relevant information, from complex stimuli.