The time it takes to detect changes in speed and direction of visual motion.

We studied the ability of human observers to detect abrupt changes in velocity of motion of a random dot pattern. The pattern moved horizontally for 0.9 s at velocity V0, then changed to V1 either in speed, or in direction for a time T and returned to the initial motion. The threshold duration for detection of the change was measured for initial speeds of 2, 4, 8 and 16 deg/s. The time to detect a velocity reversal was equal to that for detection of an increase in speed by a factor of three. The time to detect an abrupt cessation of motion was equal to the time for detection of an increase in speed by a factor of two. The time to detect a direction change, the speed being constant, decreased gradually with increasing angle between V0 and V1 from 12 to 180 degrees and with increasing V0; the detection time was a function of (V1-V0) almost independent of the value of V0. This finding supports the hypothesis of Dzhafarov et al. (Percept Psychophys 1993;54:373-750), that the visual system effectively reduces the detection of velocity changes (from V0 to V1) to the presumably more simple detection of a motion onset, from 0 to (V1-V0). The characteristics of the detection process in the cases of uni- and two-dimensional velocity changes are discussed.