318 ADAPTATIONS TO SPACE AND MOTION 



These doubled images are a sign that the object is not at the distance 

 of accommodation-and-convergence, and their appearance and disappear- 

 ance (though we are ordinarily totally unaware of them) is a minor 

 binocular cue to distance. Another kind of double vision also demon- 

 strates, and more dramatically, our dependence upon corresponding 

 points : If one eyeball is pressed and wiggled by a finger placed against 

 the lower lid, the image on its retina is displaced from the set of points 

 corresponding to those under the image in the other eye, and the visual 

 field seems to split and become two fields, one of which slides around 

 over the other as the finger is wiggled. We can unify the two fields only 

 by allowing the eye to go back into its natural position, which is one in 

 which corresponding points are stimulated by the object upon which the 

 attention is fixed. Even Barrett's Australian patient (v.s.) with his re- 

 markable ability to dissociate the two eyes at will, had continuous 

 'diplopia' or double vision while doing so. 



Diplopia is simply the seeing of one object in two directions at once. 

 Each point on each retina has its 'local sign' of direction. To take the 

 centralmost point for example : when this point receives the image of an 

 object, the brain sees that object in the direction in which, so to say, the 

 brain thinks the eye is aimed. Having given the neck muscles and the 

 extra-ocular muscles certain orders, the brain thinks it knows where the 

 eye is pointing. But if we move the eyeball passively, with a finger-tip, 

 the brain is deceived — the object in space has not moved, but it is now 

 imaged on a different spot on the retina which has a different local sign 

 of direction. This spot is now actually aimed along the same straight- 

 forward line in which the fovea was pointing a moment ago. The brain 

 does not know this, for the muscles have not been told to turn the eye. 

 So, the brain sees the object in a new direction, different from that in 

 which it is seeing it with the other, undisturbed eye. This new, second 

 direction is the one in which the object would have to lie to be imaged 

 where it is on the retina, if the fovea were still pointing dead ahead. 



Now, if both cerebral hemispheres, looking through both eyes, are to 

 see a single object at the same place in space — fuse it, in other words — ■ 

 the object must be imaged upon corresponding points in the two retinae. 

 This is only another way of saying that the two retinal areas receiving 

 images of the object must have the same local sign of direction. Whence 

 arise these all-important corresponding points of the retinas — which, 

 except for the foveae themselves, are no fixed anatomical points at all, 

 but pair up in ever-shifting combinations as the fixation is aimed here 



