An object's motion is always described relatively to some arbitrarily chosen reference system and the object's motion is described regarding to it. Many studies showed that the reference system of perceived motion was determined spatially and temporary, mainly depending on the following two Gestalt factors ; law of closure (Ehrenstein, 1925 ; Duncker, 1929) and law of common fate (Rubin, 1927; Johansson, 1950). When three or more small light spots moved downward (0.9° visual angle/sec.) as shown in Fig. 2, the reference system was dependent upon the spatial constellation of the spots and upon the motion common to them, i. e., the spots on both sides tended to stop their motion and to be seen stationary and other spots was mostly perceived moving in horizontal direction (Sumi and Takegaki, 1965). This was also found in the condition of monocular vision and the spots on both sides tended to be seen motionless (92%) (Table 1). When the spots observed in the condition of stereoscopic vision, the above tendency was found, too. The spots on both sides were seen motionless (96%) and the middle spots moved clearly in the horizontal direction when there was no disparity between both retinal images, and in the three dimensional direction when there was a disparity between them. The apparent stillness of the spots on both sides was observed in either way ; they were seen in the completely stationary state or in the relation of " figure-background ", in analogy to Rubin's well known terminology (Johansson, 1950). Those differences will be attributed to the relation between both mental processes of psychophysische "Welt-Vorgange" and psychophysische "Korper-Ich-Vorgange" mentioned by Metzger (1954).
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