BINOCULAR SINGLE VISION 323 



If a fusion of right- and left-eyed images in itself creates the impres- 

 sion of solidity, then the question of whether total-decussation animals 

 have stereopsis or not hangs simply upon the question: do they have 

 singleness of vision in the binocular field? For if we complete either 

 Figure 120 or Figure 119b by indicating the psychic act of fusion we 

 derive Figures 121a and b respectively; and in them it would seem that 

 the final result is the same — the fusion of the whole right-eyed view 

 of the object with the whole left-eyed view. Even if we close one eye, we 

 are still seeing with both halves of the brain. We still effect a jimction of 

 these bilateral activities. There is no reason why a fish cannot do like- 

 wise. The fusibility of images in the two sides of the brain into a 'median' 

 image cannot conceivably depend upon the character of the optic chiasma. 



If solidity crops out phenomenally in the case represented in Figure 

 121a, why not also in the case of total decussation shown in Figure 121b? 

 If stereopsis depends only upon the fusion of the right kind of images, 

 and we find reason to believe that animals with total decussation do have 

 fusion and singleness in their binocular fields, then (since we know their 

 images are of the right kind — i.e., right- and left-eyed) we must look for 

 an explanation of partial decussation other than the firmly-rooted tradi- 

 tional one that without partial decussation there could be no fusion and 

 hence no stereopsis. 



The Case for Singleness in Animals — Let us consider a fish, which 

 of course has total decussation and which we will suppose to have no 

 binocular field at all. He sees a mouse on the bank. He can look at the 

 mouse with one eye, or turn his body and look at it with the other. In 

 either case he certainly sets but one mouse, and he has no binocular par- 

 allactic cue to its distance and no impression of its solidity other than 

 that afforded by monocular cues. But now an owl, who also has total 

 decussation but who moreover has a wide binocular field and convergent 

 foveal lines of sight, also looks at the mouse. Is it reasonable to suppose 

 that the owl sees two mice? If so, must he aim his talons half-way between 

 the two 'mice' in order to seize the mouse — or if not, which 'mouse' 

 shall he aim for? 



Eye-minded species have certainly done everything they could do to 

 gain binocular vision, by making evolutionary modifications of their static 

 facial and ocular anatomy. Quite apart from the enormous aid it affords 

 to bathopsis in intelligent animals which might be able to get along with 

 only monocular cues to distance, binocular vision has a great advantage 

 over monocular in any animal, as we shall see. But whatever the gain 



