PERCEPTION 



'639 



two spots of light in a dark room are engaged in 

 independent periodic motions, e.g. along straight 

 paths at right angles to each other. The resultant 

 perception does not image the 'true motions,' but 

 depends crucially on their phase relations. Johansson 

 suggests (241) that the observer performs an analysis 

 into component vectors. Such analyses can be noticed 

 in natural settings, e.g. when we see a friend waving 

 to us from a moving train. The actual motion of the 

 hand is sinusoidal but is perceived as a combination 

 of linear progression (of the train) and up-and-down 

 motion (of the waving hand). 



It is tempting to speculate on the neural mecha- 

 nisms that might be required in extracting such com- 

 ponent vectors from complex motions. Is ii the same 

 mechanism that generates complementary apparent 

 motions on inspection of regular stationary or moving 

 patterns (336), or negative afterimages of movement? 

 The problem of attributed or induced motions has 

 considerable generality. Motion is usually perceived 

 as carried by objects, and object character implies 

 either an ability to move relative to a stationary 

 surround or (in the case of stationary objects) 10 be 

 seen in relative motion as the perspective of the ob- 

 server changes. So considered, problems of motion 

 perception are continuous with those of object rec- 

 ognition and of spatial organization [see above, and 

 Gibson (154, 156)]. 25 



Autokinetii Effects 



Perhaps the most striking instance of motion per- 

 ception in the absence of objective movement (of 

 stimuli and observer) is the phenomenon of auto- 

 kinesis; a single spot of light in a dark room, after a 

 few seconds of inspection, appears to engage in erratic 

 excursions (84, 284). The direction and extent of 

 these motions are influenced by mam factors, includ- 

 ing posture, expectation or even suggestion (425); 

 but knowledge of the illusory character of the move- 

 ments does not abolish them. The movements stop, 

 however, as soon as the surrounding room is made 

 (even faintly) visible. Apparently, a single spot with- 

 out visual framework is not an adequate stimulus 



25 Beyond this, study of relative motions of two dots in 

 homogeneous iields has suggested to Michotte 1347) that there 

 might he psychophysical correlates for perception of causality. 

 If one dot slowly approaches another stationary dot, then stops, 

 and, if then the formerly stationary dot begins to move, the 

 compelling impression for most observers is that of an impact 

 which launches the second dot on its path. These demonstra- 

 tions are similar to those dealing with 'physiognomic' qualities 

 of movements carried by abstract geometric figures (189). 



for spatial localization (284); the observer cannot 

 even point at such a dot with his finger (411). Eye 

 movements can affect extent and direction of auto- 

 kinetic motions but cannot explain them (175). 

 The phenomenon casts serious doubts on theories of 

 spatial localization invoking simple retinal 'local 

 signs.' 



Stroboscopk Motion 



The principle of moving picture projection is a 

 succession of stimuli directed at discrete retinal loci. 

 This principle was known early in the nineteenth 

 century to physicists like Faraday (120 1 and Plateau 

 (382) who constructed stroboscopic devices generating 

 apparent motion by serial presentation of static pat- 

 terns. These effects were considered a 'peculiar class 

 of optical deceptions, 1 errors ol judgment rather than 

 basii sensations. Only Exner (119) argued forcefully 

 for their basic sensory character; he pointed out that 

 the compound eves of invertebrates might lie organs 

 of motion perception pal excellence an argument not 

 lost on his distinguished nephew, Karl von Frisch. 

 It was clear to Exner that an adequate hypothesis 

 about tin- neural mechanism underlying motion 

 perception should encompass all forms of 'illusory,' 

 as well as 'real 1 motion. This hope was heightened 

 by Wcrtheimer's justlv famous study (536) of appar- 

 ent movement which gave rise to Gcstalt psycholouv . 



Wertheimer simplified the stimulus conditions. 

 Instead of continuously repeated displacements (as 

 in the usual stroboscope), he presented a sintilc pair 

 of targets (luminous dots or simple lines 1, a and b, 

 separated by a variable time interval, /, and by a 

 variable angular distance, s. With long intervals 

 (over 200 msec), liis observers reported succession. 

 They saw first <i, then b, each in its place. With 

 short intervals (less than 30 msec), they perceived a 

 and b simultaneously, flickering in their respective 

 positions. With intervals intermediate between 30 

 and 200 msec, the observers reported movement, 

 with optimal movement at about 60 msec, where 

 neither a nor b were seen, but a single object moving; 

 smoothly over the track defined by a and b. At in- 

 tervals longer than those for optimal motion, various 

 partial motions were obtained; it was in this range 

 that Wertheimer's subjects noticed 'pure motion,' 

 an impression of disembodied movement from a to 

 b without the impression of a moving object. He 

 called this particular phenomenon 'pure phi'— 

 the first of a number of Greek letter designations for 



