356 ADAPTATIONS TO SPACE AND MOTION 



first case the blur of movingness is attached to the object, while in 

 the second case the object is clearer — therefore less 'moving'; and also 

 the blur of the shifting background, being out of focus to begin with 

 (since the eye is accommodating for the object) is less prominent in 

 consciousness. Still, even with steady fixation, movements seem two or 

 three times as fast when seen peripherally as they do in direct vision. 



Not all perceptual factors promote saliency, however — some have a 

 reverse effect. If two lights are flashed simultaneously just once, one 

 being seen centrally and the other peripherally, they appear to flash in 

 succession with the central light leading the other. The latent period of 

 perception is thus longer in the periphery. A French worker has studied 

 the whole sensorimotor reaction time with central versus peripheral vision 

 and with the motor elements constant. A chronoscope was used, whose 

 indicating hand could be started moving by the experimenter and stopped 

 by the subject's pressing a key as soon as he was aware of the movement. 

 On the average, the whole reaction time with central observation was 

 0.170 seconds, while at 90° in the periphery it was extended to 0.327 

 seconds. This means that an object would have time to move farther in 

 the periphery, as compared with the center of the field, before the 

 individual could take any motor action upon the matter. 



On the whole, it would seem that the periphery exercises its greatest 

 usefulness in movement-perception by instigating the reflex 'eye-jump' 

 which calls the visual axis over to aim at the locus of the movement. 

 This reflex is very strong, even in civilized man, who should theoretically 

 have very few primitive fear-reflexes left. It may not occur when the 

 peripheral movements are expected or at least not unexpected; but a 

 man in strange surroundings will inevitably turn his eyes — the first time, 

 at least — to see foveally a waving window-curtain or what not, which 

 has 'caught his eye' peripherally. 



Stroboscopic Movement versus Real Movement — There are many 

 kinds of apparent movements — perceptions of movement where the ob- 

 ject to which the motion is attributed is actually stationary. Most of 

 these have their basis in movements of the eyes, or of the head, of which 

 the subject is unaware — the visual axis swinging, though the subject 

 believes his fixation to be constant. Any disturbance of egocentric local- 

 ization, as in vertigo and intoxication, results in a swimming apparent 

 movement of the whole field. In another category are after-images of 

 motion, where both the eye and the apparently moving object are station- 



