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one of these phenomena ought to account for all 

 the others. 



role of size and surround. The importance of the 

 surround in motion perception is strikingly demon- 

 strated by requiring observers to equate perceived 

 velocities for two moving bands which are identical 

 except lor the fact that the opening, or frame, is 

 homogeneous in one ease and covered with dots or 

 lines in the other. Motion through the second, or 

 •structurcd' field appears faster, just as a rider who 

 enters the woods after traversing an open field seems 

 to double his speed. Similarly, movement seems 

 faster when either the frame or the mining object is 

 reduced in size, the 1, liter effect being the same as 

 one's tendency to overestimate the speed of small 

 animals, for instance .1 scurrying mouse. 



velocity TRANSPOSITION. Much less expected is 

 Brown's discovery of transposition of perceived veloc- 

 ities. The experiment, again, consists of asking the 

 observer to equate the speeds of two endless belts at 

 equal distances from his eyes; however, for one 

 belt, all dimensions are reduced, e.g. the width of the 

 opening and the size of the moving squares are half 

 those of the other. In that case, the observer will 

 approximately double the rate of motion of the smaller 

 display in order to obtain an impression of velocity 

 equal to that of the larger. 



As Wallach has pointed out (516), this result seems 

 i losely related to the outcome of experiments which 

 demonstrate relative 'constancy' of perceived veloc- 

 itv; two displays of identical size are placed at dif- 

 ferent distances from the observer, e.g. one twice 

 as far as the other. Under these conditions, observers 

 increase the objective speed of the more distant 

 display only very slighllv in order to obtain impres- 

 sions of velocity equal to the less distant display. 

 If rates of stimulus motion across the retina were all 

 that mattered, then the display twite as far away 

 should be set at double speed lor the impression of 



equality. The fad that nearly equal velocities, at 

 different distances, are perceived as equal has of 

 course considerable biological utility, even though this 



-lam J ot perceived speed is far from perfect, 



indeed, significandy less than the relative constancy 

 .I perceived size [because of this discrepancy, Wal- 

 lach 1 |i6) 1 reluctant to 'reduce 1 speed constancy 



to size 1 oust 



1 |i trly, the phenomena of speed constancy are 

 equivalent to those of transposition; the distant 

 display is proportionately smaller (on the retina, at 



least) and the higher subjective velocity thus, in 

 effect, counteracts this diminution in size. An un- 

 resolved question remains: is transposition the pri- 

 mary phenomenon [as Wallach (516) prefers to 

 assume] or is transposition a case of constancy mis- 

 applied? One could argue that with displays equi- 

 distant but differing in size, the observer performs a 

 'correction' in perceived speed that would have been 

 appropriate if the difference in the scale of the two 

 displays had been due to differences in distance. 

 If his reasoning holds, then velocity transposition 

 would be a particular instance of a whole class of 

 illusions which are actually constancies of perception 

 running off in vacuo. 



'paradoxes' of seen motion. Several other puzzling 

 aspects of motion perception, although less well 

 known, are important for any attempt at constructing 

 a physiologic theory of perceived movement. Appar- 

 ent velocity of moving objects varies not only with 

 retinal area stimulated (faster in fovea 1 regions), 

 but also within the fovea, depending on whether the 

 observer moves his eyes or not. Following an object 

 in motion with moving eves leads to impressions of 

 somewhat lower speeds, as compared with the same 

 objective motion observed while fixating at a sta- 

 tionary point. The effect is called the AubcriT'lcisi hi 

 paradox (14, 15, 126); it may be related to whatever 

 mechanisms provide us ordinarily with a stationary 

 environment when our eyes sweep over it. The Au- 

 bert-Fleischl rule was called a paradox because its 

 originators felt that pursuit movements of the eves 

 should increase the impression of motion of a moving 

 object. However, if the relative motions of objects 

 during eve movement have to be somehow counter- 

 acted, then the paradox may be lessened, it may 

 again reflect a tendency towards constancy lin this 

 case of perceived movement during motion of recep- 

 tor Structures) which overshoots to reduce an impres- 

 sion of real motion. 



Acuity (for resolution of moving targets) is mark- 

 edly diminished during attempted pursuit movements 

 of the eves, and (his the more so, the more rapid the 

 motion of the target, as shown by Ludvigh (329). 

 Ill- attributes this effect to the decreasing alignment 

 between target and eve with increasing speed (and 

 hence, Increasing blurring of the image) Maximal 

 blurring occurs during the rapid saccadic movements 

 of the eyes [e.g. from fixation to fixation in reading, 

 cf. Woodworth & Schlosberg (549, pp. 500 510)]. 

 This blurring is so marked that some ol the early in- 

 vestigators of eve movements, such as Dodge (108) 



