Visually perceived motion in depth resulting from proximal changes. I

According to a model for form 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.