According to a model for fonn and motion perception proposed by Johansson (1964), every two-dimensional change in the proximal stimulation is projected out as a motion in depth. The amount of perceived depth motion can then be predicted from the projective relationship between the proximal change and the projected motion. This prediction was tested in a series of experiments by using squares that continuously changed their sizes as stimuli, and measuring perceived distance of motion in depth. The obtained relationship between perceived and predicted distance of motion was curvilinear for all Ss. Furthermore, the majority of the Ss underestimated the motion systematically, the remainder overestimated it. Thus, the prediction given in the model could not be verified. However, an alternative projective relation based on the assumption that a fixed proportion of the change is not projected out as a motion but perceived as a change of size agreed quite well with the data, both with distance judgments and with judgments of perceived change of size.
According to a model for motion and form perception proposed by Johansson (1964), every two-dimensional change in a changing proximal stimulation is projected out as a motion in depth. The model assumes that the amount of perceived relative motion (the fraction between the perceived amount of motion of the object and the perceived initial distance to the object) is determined only by the amount of relative change (the fraction between the absolute amount of change and the initial size). The aim of the present study was to test this hypothesis by studying the effect of some other variables on perceived relative motion in depth. As stimuli. continuously shrinking and growing squares were used. No effects were found when varying the absolute amount of change. Neither did the rate of change influence the perceived relative motion in any important way. The only variable that gave rise to strong and systematic effects on perceived relative motion "'as the initial distance to the perceived object. The greater the initial distance, the less relative motion was perceived.
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