Visual velocity discrimination: Effects of “blanking” the stimulus

Abstract

Weber ratios were obtained for visual velocity discrimination under two stimulus conditions which differed only with regard to the presence of visible continuous motion. Discrimination of velocity under normal conditions was compared with discrimination under "blanked" conditions. In the latter, Ss saw only the initial and final positions of a target separated in time by intervals comparable to the duration of the motions seen under the former condition. Difference thresholds for blanked motion are approximately double those for continuous motion over a wide range of velocities. Proble .. When two stimulus velocities are compared by a human observer, the difference threshold is not a Simple function of the values of distance and time of the respective stimulus movements. Stimulus size, shape and luminance; topographical features of the movement field; and the presence of correlated temporal or spatial cues all influence the difference threshold (Brown, 1961; Mandriota et aI, 1962). The human observer need not see the entire path of motion to estimate velocity. He may interpolate or extrapolate on the basis of the velocity he has seen. No direct evidence seems to be available, however, concerning how well relative velocity discriminations might be performed when observers see nothing but the total spatial extent and duration of the displacement of an object. Such information may be valuable in specifying velocity discrimination baselines for the human observer. The present study investigated the effects on velocity discrimination of "blanking" continuous motion. In other words, test stimuli were chosen bearing specific distance/time relationships to each other which were outside of the range of phi movement. Judgments were performed regarding the relative velocities of these stimuli. The precision of these judgments was then compa red with judgments under identical conditions except for the inclusion of continuous motion (i.e., ordinary visible movement). Method Two female Ss with emmetropic vision viewed successive pairs of velocities generated by moving spots of light on a cathode ray tube. Ss reported whether the second member of the pair was "faster" or "slower" than the first. The stimuli were presented under two conditions referred to as "Heterodimensional" and "Heterodimensional-blanked.' ,