THE PERCEPTION OF SPACE 697 



usually turned so that an object of attention is brought within the 

 binocular field ; even in a Primate such as Tarsius, the large eyes of 

 which are practically immobile, ocular movements are largely taken 

 over by movements of the neck which can rotate through an angle 

 of 180° so that the animal can look directly behind without incon- 

 venience (Fig. 846). 



UNIOCULAR AND BINOCULAR VISION 



It has often been implied, and indeed said, that animals with 

 laterally placed eyes and panoramic vision and with a total chiasmal 

 decussation cannot fuse the two uniocular fields ; the logical implica- 

 tion is that two separate uniocular impressions are appreciated so that 

 the only alternative to rivalry or diplopia in the binocular field would 

 be that suppression alternates between the two retinae. Partial 

 decussation of the sensory paths and the projection of each half-field 

 onto the same hemisphere has thus been taken as the anatomical basis 

 of fusion and stereoscopy. Such a view is without firm foundation. It is 

 our universal experience that visual impressions from our semilunar 

 uniocular fields, the afferent fibres from which suffer complete decussa- 

 tion and are relayed to separate hemispheres in the cortex, are in- 

 extricably mixed without rivalry with those from the binocular field 

 and form a unity with them ; the sensory impression is comparable 

 to our unitary appreciation of an object such as a pencil when touched 

 with the fingers of each hand. There is no reason why the uniocular 

 fields of animals cannot be fused to form a single perceptual whole 

 even although they are appreciated by different halves of the brain just 

 as, if we close one eye, the two segments of the resulting uniocular 

 field are seen as one although they are perceived by the synthesis 

 of the activity of different cortical hemispheres. The whole behaviour 

 of Vertebrates, the preference for binocular vision when visual accuracy 

 is required, and the extreme accuracy in spatial judgments of bifoveate 

 vision in a chameleon or a bird of prey justify the conclusion that, 

 despite total decussation at the chiasma, the Vertebrates below 

 Mammalia enjoy binocular single vision with a considerable degree of 

 depth perception and stereopsis in the overlapping parts of the fields 

 owing to the appreciation of binocular parallactic clues. In all 

 Vertebrates, whether they are provided with a complete or partial 

 decussation of the optic nerve fibres, binocular vision is a percejJtual 

 process, the singleness of which represents the product of a synthesis 

 which is built upon already elaborated uniocidar sensations. 



The historical evokition of these ideas is interesting. The structural 

 hypothesis, depending on the direct continuity of the neural apparatus particu- 

 larly as seen at the chiasma, was taught by Galen ^ and elaborated by Isaac 

 ' De usu partium corporis humani. 



