PERCEPTION 



1641 



will appear longer than the distance from B to C. 

 Analogous effects can be obtained by producing 

 tactile apparent motion on the skin (198, 498). 



TACTILE AND AUDITORY APPARENT MOTION. Any 



physiologic theory of stroboscopic effects must take 

 account of their occurrence in other sense modalities 

 beside the visual one, and of their presence in lower 

 animals. Nonvisual apparent motion effects in man 

 are best established for tactile stimuli applied serially 

 in separate places on the body surface (35, 36, 81 , 

 498). Under appropriate conditions, observers report 

 the stages of phenomenal succession, optimal motion 

 and simultaneity, depending on time intervals, 

 spatial separation, pressure employed and Other 

 factors. Some experimenters had no difficulty in con- 

 firming Korte's 'laws' for touch [e.g. Burtt (81)], 

 but others found more exceptions to these rules in 

 touch than in vision (36, 225, 4141. Training and 

 attitudes seemed to play a considerable role in some 

 of the tactile effects, particularly in apparent motion 

 from one hand to the other (35, 36). Must experi- 

 menters, however, observed a maximal shortening 

 in the perceived distance of the two contacts with 

 optimal movement (414); in one of the earliest 

 studies, von Frey (498) obtained shortening oi up 

 to 25 per cent. 



Apparent motion likewise exists in audition, 

 although the experimental facts for that modaliiv 

 are somewhat less clear. Burtt (80 1 simply used two 

 sound sources in a dark room; by varying their dis- 

 tance and intensities, and the interval between them, 

 he elaborated a set of relations between 1, 1 and / 

 analogous to Korte's laws for vision. Others, including 

 Scholz (414), were less successful in this respect. 

 Further experiments are needed, particularly since 

 the stimuli for apparent auditory movement can 

 be made much more compelling In using a stereo- 

 phonic arrangement employing dichotic clicks. 

 By continuous shifting of their time or intensitv rela- 

 tions below the duality threshold, one can obtain 

 particularly convincing forms of apparent auditory 

 movement (506). Note that the minute time dif- 

 ferences that are effective here are not perceived as 

 such, but are heard as changes in localization or as a 

 movement in the case of a continuous change. 



STROBOSCOPIC EFFECTS IN SUBHUMAN SPECIES. For the 



visual modality at least, stroboscopic effects have been 

 demonstrated convincingly in animals below man. 

 Monkeys react to moving pictures (e.g. of other ani- 



mals) as if to a real scene (259). In birds, a puzzling 

 structure within the eye (the pecten) has been claimed 

 to serve as a shadow caster which converts any con- 

 tinuous movement across the retina into successions 

 of stimuli (343). By producing a lattice of shadows, 

 the pecten could in fact enhance retinal on-and-off 

 effects for patterns transported across the photore- 

 ceptors (96). If this is true, then 'real motion' for 

 birds would be just as discontinuous as stroboscopic 

 motion within the range of their pecten. 



For fish, there are two experiments showing re- 

 sponses to apparent motion under conditions where 

 the same perceptual effects occur in man (206). 

 Thus, Gaffron (138) utilized the tendency of min- 

 nows (Phoxinus laeuis) and sticklebacks [Gasterosteus 

 aculeatus) to follow a moving striped pattern. The 

 test pattern was rotated with variable speed behind a 

 sectored disk spun at constant speed (20 rps). When 

 the striped pattern reached a speed where the direc- 

 tion of movement of the stripes seemed to reverse 

 itself for human observers, the fish turned around and 

 swam in the direction of the 'illusory 1 motion. Still 

 more telling are the experiments by von Schiller 

 (509). He trained minnows [Phoxinus) to discriminate 

 between screens with .1 vertically moving; dot and 

 those with a stationary dot; then he introduced two 

 dots, alternating at varying rates, and established 

 that the minnows could be trained to discriminate 

 what were, lor man, simultaneous and successive 

 phases of apparent motion, on the one hand, from 

 real motion, on the other. However, the fish could 

 not maintain the discrimination when time rela- 

 tions of the apparent motion stimuli were adjusted 

 to produce what was, for man, the optimal stage of 

 apparent movement, for minnow as lor man, op- 

 timal apparent motion seems equivalent to real 

 motion 



'-'" It is puzzling th.it convincing reports of apparent motion 



|ic nrption are lacking lor invertebrates (.allien 138) failed 

 to obtain such effects in dragonfly larvae and housetlies under 

 experimental conditions she had used successfully with tish. 

 Further work with cephalopods and with invertebrates with 

 compound eves is needed Some ot the studies on motion 

 perception in lower vertebrates also require repetition and 

 extension, for instance the much quoted experiments by Beniuc 

 il . Lissmann I ;-■-, and Brechei (60) on Bella splendens (the 

 Siamese fighting fish). As they stand, these studies do not bear 

 on motion perception as such, but on perception of flicker and 

 fusion. The range of parameters explored does not permit the 

 conclusion that fusion thresholds for intermittent visual stimula- 

 tion are higher in Bella than in man, nor do the results suggest 

 that Bella "sees all motions at half the speed seen by man" 



I. Mil, I 



