348 ADAPTATIONS TO SPACE AND MOTION 



only in a visual field which has pattern. The perception of a real move- 

 ment does not depend solely upon a displacement of an image on the 

 retina, but upon a displacement relative to the images of other objects. 

 Visual orientation in space becomes as imperfect as auditory, as soon as 

 visual space is greatly emptied of reference-objects. 



Motor Factors in Movement-Detection — It might seem that all of 

 the 'objective' factors in movement perception should be purely sensory, 

 but there are certain ones which are chiefly motor in character — notably, 

 the 'gyroscopic' action of the involuntary eye movements, under the con- 

 trol of the membranous labyrinth. This action tends to preserve the abso- 

 lute orientation of the eyeball in space so that — as Erich Sachs puts it — 

 "the head rotates around the eye" during the dynamic maintenance of 

 equilibrium. This maintenance of ocular orientation makes toward a con- 

 stancy of the visual field, whereas voluntary eye movements are designed 

 to exchange the field and fixation-point for new ones. 



If the eyes always turned with the head instead of automatically 

 'against' the head, the swimming of the visual field in a wholesale 'appar- 

 ent movement' would conceal from the animal small real movements 

 within the field. So many more parallactic relative movements would 

 take place, that actually-moving objects would be harder to spot. The 

 gyroscopic stabilization of the eye is a means of combatting the rela- 

 tivity of motion — by keeping the visual field still, the animal can better 

 know what moves, when, and where. 



Another motor phenomenon whose sensory accompaniments aid in 

 movement-detection is the 'saccadic' eye movement. This is the type of 

 voluntary eye movement which we make to change our point of fixation. 

 During involuntary movement of the eyes, and during pursuit move- 

 ments, we see continuously. But it is a striking fact, more than a little 

 hard to believe, that we do not see at all during saccadic movements. 

 Some sort of switch is opened in the brain, until the movement is com- 

 pleted. Then, vision returns. One simple proof of this is the fact that it 

 is impossible to see the eyes in voluntary motion in a mirror. Another is 

 Dodge's experiment: look at an object through the narrow apex of a 

 paper cone, then look to one side of the aperture and sweep the line of 

 sight across it. You will see nothing of the object, through the aperture, 

 unless the line of sight stops upon it. We read a line of print not con- 

 tinuously but by jerks, seeing the words only in the moments when the 

 eye is at rest. The fewer stops one makes per line, the faster a reader he 



