Temporal thresholds and reaction time to changes in velocity of visual motion

Abstract

A random dot pattern moved at a velocity V 1 . The velocity then increased or decreased abruptly to another value V 2 for some time and again returned to V 1 . The temporal threshold, i.e. the duration of V 2 that was necessary to detect the change was measured. Thresholds for the detection of the same velocity increment, V 2 = 2 · V 1 , were shorter when the baseline velocity V 1 increased from 1 to 8 deg/sec (Expt 1). The temporal threshold decreased as the velocity contrast (V 2/ − V 1)(V 1 + V 2) increased from 0.33 to 0.77. The thresholds for the detection of velocity decrements were in general longer than those for the detection of increments (Expt 3). In Expts 2 and 4 the random-dot pattern moved with velocity V 1 , which abruptly increased or decreased to V 2 , without returning to V 1 . The reaction time to the change was measured for the same velocity pairs as those used in the temporal threshold measurements. There was a good correspondence between changes in the reaction times and changes in the thresholds under the various conditions. The data are interpreted on the basis of two hypotheses: higher velocities are detected by mechanisms that respond more rapidly; and integration of velocities occurs when temporally-adjacent motions are presented.