ADAPTATION; CENTER VS. PERIPHERY 353 



ological) factors does not bear this out. But the psychic factors are 

 largely in favor of the periphery, in which movements have a saliency 

 and attention-value quite out of proportion to the clarity with which 

 they are actually discriminated. We might think that this was a com- 

 pensation for the inferior capacity of the periphery to detect movements; 

 but a moment's reflection shows that there could be no such compensa- 

 tion, any more than increasing the strength of our brightness-sensations 

 could of itself make the eye more sensitive to weaker lights. 



So, when the retinal* periphery is described as "an organ which is 

 specially adapted to see movement", we need to append: "so far as 

 psychic factors are concerned." Animals with panoramic vision, animals 

 which like the horse have greatly extended peripheries and wide visual 

 angles, and animals like the mouse with pure-rod retinae (which can be 

 thought of as 'all periphery') , are not specially equipped to discriminate 

 movement and moving objects. If in the rat's whole retina, or in the 

 ungulate's periphery, visual acuity is so low that only movingness, not 

 moving objects, can be seen, it is not that these retinal areas are designed 

 for movement-perception — rather, it is that they are too crude to afford 

 any phases of vision except movingness and brightness. What move- 

 ments the animal does pick up may startle him more than they would a 

 lizard or a man; but this is a matter of saliency, the biological need or 

 lack of need for which varies from species to species. Lizards and men 

 are better able to identify a moving object promptly, and are therefore 

 not under the necessity of treating every moving object which enters the 

 visual field as a dangerous enemy until it proves itself otherwise. 



Certainly, as Woodworth says, the brain is tuned to see motion and 

 grasps at any chance to see it. That is doubly true when the brain is 

 peering out at the world through the periphery of the retina. Even the 

 momentary stimulation of the periphery by a spot of light is said to 

 cause an impression of movement. A moving point in the periphery is 

 more visible than a line of similar length, direction, and duration. There 

 are two factors which operate to promote movement-detection in the 

 periphery. One of these is quite important, and gives a moving object 

 a sort of physiological saliency which may indeed be a large part of the 

 basis of psychic saliency. This is the great 'fatigibility' of the periphery. 

 Motionless objects in the periphery of the visual field actually tend, from 

 this cause, to disappear after a few moments. But now if one of them 

 begins to move, it is immediately seen again, since its image passes over 

 retinal areas which have meanwhile become adapted to other images but 



