The effects of contrast on beta movement

Abstract

The study was an investigation of some of the stimulus conditions known to affect beta movement. Beta movement is the designation for the apparent movement which is perceived when two lights are alternatively turned on and off at a critical rate. The exact value for that rate has been shown to depend upon a number of stimulus factors. Two circular lights were used to produce beta movement for the six male subjects who had trained to participate in the study. The training sessions were employed to allow the subjects to establish a criterion for beta movement which they could apply with consistency. Following the training sessions, each subject was tested under a variety of conditions in which the experimenter manipulated the following variables: exposure time of the lights, separation distance between the lights, intensity of the lights, and contrast (the ratio of the intensities of target light and surround). The intensity values of the surround were chosen to maintain constant contrast ratios over four levels of target intensity. Higher-order variables such as visual angle and velocity have been shown to be important for the perception of movement. Of interest was the possibility that another higher-order variable, contrast, might also be important in accounting for changes in beta movement. In the study the subject controlled the temporal interval between the two lights. of 20 msec to a high of 550 This interval ranged from a low msec. The subjects made two judgments in each trial. The one at the lower end of the temporal interval range marked the point at which apparent movement was first seen to begin, and the one toward the upper end of the range marked the point at which movement could no longer be seen. These measures of upper and lower thresholds of beta movement were the primary data analysed. In addition, a difference- score measure, designated as range, and the variability of the settings obtained in each of the stimulus conditions was examined. The results indicated that contrast was the most significant variable in terms of accounting for changes in beta movement thresholds. As contrast increased both lower and upper thresholds increased significantly. The increase in the upper threshold values was greater than the increase in the lower threshold values, with the consequent effect that the range measures also increased. The analyses of the variability measures indicated that as contrast increased, the subject's judgments of threshold also increased in variability. The findings of the present study were discussed with respect to an hypothesis of perceived brightness as related to Korte's laws for beta movement. The possibility for simplification of those laws through the use of higher-order variables is indicated.