Extrapolation of motion path in human visual perception.

According to Newton’s First Law of Motion. a physical object moving at uniform velocity in one direction will persevere in its state of uniform motion unless acted upon by an external force to change that state (Newton. 1687). Smce the visual system has evolved to process information from the physical world. one might expect to find a similar principle of “inertia” in the visual perception of moving objects. Using dot displays (Fig. 11 we have found that any object which moves in one du-ection at uniform velocity will tend to be perceived as continuing its motion in that direction (Ramachandran and Anstis. 1981). This might be regarded as a perceptual equivalent of Newton’s first law. If two spatialI> separated spots of light (Fig. la) are presented to the retina in rapid succession the spot will appear to move from the first point to the second. as commonly seen m neon advertisement signs (Korte. 1915; Kolers. 1971: Anstis. 1970. 1978; Julesz. 1971: Burt and Sperlmg. 19X1 ). If a single spot is follow,ed h! two flanking spots (Fig. I b) which appear on either side of it simultaneously. it is almost always seen to “split” and to move simultaneously in opposite directions (Ullman. 1980). This predilection for splitting can be ovcrcomc by placing one of the flanking spots nearer to the tirst spot. in which case it will always attract the apparent motion. We shall call this the “proxunit!” rule. FIgtIre Ic shows a matrix of dots (Gengerelli. 1948) forming the four corners of a diamond. This display (ah well as subsequent ones described in this paper) ~a’\ generated on ;I pCphosphor CRT using an “Apple 2” microcomputer and viewed from a distace of I m. The dots were arranged in a diamond with oblique sides because a square array with vertical sides shows an unwanted preponderance of vertical rather than horizontal apparent motion. possibly because of inter-hemispheric delays across the visual midline. The sides of the square subtended 1 and the dots themselves were about 4min of arc in diameter. The number by each dot refers to the time at which it is presented. If u and h. the sides of the square. are of equal length the display will be ambiguous and always seen as clearly bistable. The two possible per-