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THE PERCEPTION OF MOVEMENT 



From the biological point of view the two critical functions of 

 vision are the control of the movements of the individual and the 

 perception of the movement of objects in the outside world ; it is for 

 this reason that eyes are found essentially in actively moving animals 

 while m. those leading a sedentary existence they degenerate. ^ 

 The fundamental visual sensations are therefore the perception of 

 light and of motion ; the perception of form and of colour are 

 accessory. In the human eye the latter two are essentially the 

 prerogative of the recently evolved central area and the periphery 

 of the retina is primarily concerned with the former; so in the wide 

 panoramic field of the lower Vertebrates the perception of movement 

 is the most important aspect of the animal's visual experience. Even 

 in creatures so lowly as the larvae of Amby stoma, Moore and Welch 

 (1940) obtained an association by training between food and movement 

 or between food and light, and experimenting on the frog, Hijla, Pache 

 (1932) found that recognition of forms such as triangles, circles or crosses 

 depended essentially on the occurrence of some movement. It is well 

 known that Am])hibians such as the frog or Reptiles such as turtles, 

 lizards and snakes appear not to see motionless prey, just as the 

 rabbit in flight will collide with a motionless man. The eyes of the 

 lower Mammals can see little else beyond light and movement, while 

 Schmid (1936), studying the visual performance of the dog, concluded 

 that the recognition of a moving object was possible at a much greater 

 distance than of the same object when stationary (900 compared with 

 585 metres). 



From the physiological point of view the perception of movement 

 depends on two factors— the fineness of the retinal mosaic and the 

 persistence-time of vision. When the visual elements are few the 

 retinal area served by a single optic nerve fibre is la]?ge ; in such a 

 " coarse-grained " retina an image must travel a considerable distance 

 before it excites the sensory elements associated with another optic 

 nerve fibre so that a small movement may not be appreciated. Simi- 

 larly, if the physiological effect of stimulation persists for a long time, 



1 p. 721 



S.O.— VOL. T. 



