90 THE VISUAL PROCESS 



Binocular color mixture has been mentioned above, in the instance of 

 the binocular fusion of red and green into yellow. Its existence is fatal 

 to any theory which places the color-vision mechanism entirely in the 

 periphery of the visual apparatus — that is, in the retina. There is no 

 color-sensation, which can be produced by mixing two lights in one eye, 

 that cannot be duplicated by supplying the two lights, independently, 

 one to each eye. If color-mixture can be made centrally, one wonders 

 whether all color-mixtures, even monocular ones, may not always be syn- 

 thesized centrally. To suppose so necessitates believing that the optic 

 nerve fibers can simultaneously carry several separate 'primary' kinds 

 of information, which are integrated into a perceptual whole only after 

 reaching some level in the central visual apparatus. To account for 

 binocular color-mixture (and, it can be allowed to account also for 

 monocular mixture) a multiple synthetic mechanism must exist centrally. 

 But it would seem difficult for any one photochemical substance in the 

 cone to be able to give rise to more than one kind of optic nerve impulse. 

 To account for the transmission of simple primary impulses along the 

 optic nerve, when the retina is being illuminated by such a mixture as 

 purple, there must also be a multiple, differentially responsive analytical 

 mechanism in the periphery. 



The binocular synthesis of mixed colors and white results in sensa- 

 tions identical with those aroused monocularly by the same stimuli. One 

 reason for this could be that the vision of even one eye by itself is 

 actually carried out through the binocular (fusion) 'center'. This sounds 

 roundabout and improbable, but there is considerable evidence for it. 

 It is difficult to explain otherwise why things look no brighter to us 

 when seen with two eyes than with only one. The functioning of one eye 

 can affect the way things are seen with the other eye. To give only one 

 example: the convergence of a shielded eye causes an apparent lateral 

 movement of a spot of light seen, in a darkroom, only by the other eye 

 — especially when the non-seeing eye happens to be the individual's 

 master or dominant eye. The brain is so accustomed to ascribing most 

 of vision to the dominant eye, that it can be deceived into supposing 

 that eye to be seeing even when it is not, and thus 'sees' the spot of light 

 move in just the way it would have to, to remain visible to the dominant 

 eye during the latter's rotation. The brain is confused as to which eye 

 is seeing what, which could only be possible if the two eyes always 

 formed a team even when only one member of the team works. 



