124 Neural Aspects of Vision /7 : 2 



forms of color blindness than did the tricolor theories. Most of the 

 opponents theories attempted to assign retinal distinctions to three 

 different photosensitive pigments or did not specify in any detail how 

 the retina actually produced these responses. The theory discussed in 

 the next section presents a model which includes both a tricolor mechan- 

 ism in the rods and cones and also an antagonist mechanism, which it 

 assigns to the neurons of the retina. As in all antagonist theories, it 

 assumes that the brain carries out or duplicates the antagonistic action 

 when it receives different impulses from the two eyes or contradictory 

 signals from one eye. 



2. Cellular Mechanisms 



The tricolor and antagonist theories were originally based almost 

 exclusively on psychophysical evidence. There is considerable other 

 information available in terms of which any theory of vision must 

 ultimately stand or fall. The evidence from histology, electrophysi- 

 ology, biochemistry, and communication must all be included before 

 a theory of vision can be considered complete. The scheme described 

 in the following pages was developed by Talbot in an attempt to syn- 

 thesize these diverse lines of evidence into a model which would be con- 

 venient both to use and to form a basis for designing additional ex- 

 periments. It is used in this text as a convenient scheme in terms of 

 which many different types of phenomena may be described. 



Talbot started with the idea that any proposed scheme of color vision 

 must contain at least three different color receptors, although only two 

 types, the rods and the cones, are known. Talbot assumed that the 

 receptors included two types of cones indicated by 8 and i in Figure 2, 

 as well as rods indicated by p. These are connected to cell bodies 

 labeled a for rods and b for the cones. (The letters on these and the 

 other cell bodies discussed are those assigned by Polyak in his book, 

 The Retina.) 



The three types of receptors, 8, t, and p, are assumed to have different 

 absorption spectra. The receptor p is postulated to contain the pigment 

 rhodopsin (visual purple) whose spectral absorption peak is in the blue- 

 green at 497 nux; this type of receptor is used for blue vision in this theory. 

 The cone i is assumed to have iodopsin whose spectrum has an absorption 

 peak near the green at 562 m/x. (Actually, Talbot desires i to represent 

 red, so he has had to add a contribution from p and 8 labeled dz in the 

 figure.) The third receptor 8 is a green-absorbing cone of exact nature 

 unspecified. Talbot suggests this might be a modified form of rhodopsin, 

 "daylight rhodopsin." The necessity of this 8 cone for which there is 



