FACTORS IN MOVEMENT-DETECTION 349 



can be; and yet, the fewer stops one makes, the more time one is actually 

 seeing the words. 



If one holds the eyes motionless in the orbits and turns the head from 

 side to side, vision is then continuous and one experiences the same 

 'swimming', or apparent movement of the whole field, that occurs during 

 vertigo or intoxication when the reflex eye movements (during which 

 vision is also continuous) are occurring in such an abnormal way that 

 the 'gyroscope' is wobbly. But now if the head be kept still, and the eyes 

 swept voluntarily from side to side through the same angle as before, the 

 field does not swim because vision occurs only at the multiple stopping- 

 points of the eye's discontinuous rotation. 



Here again, in the suppression of vision during saccadic eye move- 

 ments, we have a mechanism for maintaining a constancy of the direction 

 of objects, so that if one of these should move, we will be better able to 

 notice it. There seems to be no other reason why this kind of 'suppres- 

 sion' ever evolved. The chances are that in some animals whose eyes 

 move but Uttle or not at all, a similar suppression takes place during head 

 movements, such as the perpetual fore-and-aft movement of a walking 

 pigeon's head. 



Beebe describes an experience he has had, while helmet-diving in 

 shallow water, which demonstrates strikingly the relativity of movement 

 for animals and their dependence upon a constant visual field for the 

 recognition of movements. The movements of the water were causing 

 the bottom vegetation to sway slowly to and fro. As long as Beebe 

 swayed his body with the plants, the many fish in the neighborhood 

 ignored his presence. But when he stood erect and motionless, the fish 

 were immediately curious about him and came over to investigate. 



Sensory Factors in Movement-Detection — Given a situation in 

 which the background and motor factors are conducive to the perception 

 of a movement, and do not tend to conceal it among apparent move- 

 ments or to create a 'referred' movement (i.e., cause the motion to be 

 attributed to the wrong object), there are two principal sensory fartors 

 which come into play. These are visual acuity and the persistence-time. 

 It is upon the value of each of these, in a given animal, that the demar- 

 cations between imperceptible and slow, medium, and fast perceptible 

 movements will establish themselves for that kind of animal. 



The dependence of movement perception upon visual acuity does not 

 at first glance seem to be very direct. We can see an object, if it is in 



