XII 



A THEORY OF COLOR-VISION 



According to the Young-Helmholtz theory, any color can be 

 matched, at least after sufficient desaturation by admixture with 

 white light, by combining in suitable proportions lights of three given 

 colors (cf. Peddie, 1922) . These three colors may be chosen arbitrari- 

 ly except that no one is to be matchable by combining the other two. 

 The fact that three primary colors are sufficient for a match of all 

 others strongly suggests, as Helmholtz brought out, that retinal ele- 

 ments of three distinct types are involved in the perception of color. 

 This interpretation has not gained universal acceptance by investi- 

 gators, two of the objections being that anatomical studies fail to 

 differentiate three types of receptor, and that the degree of acuity 

 with monochromatic illumination is too high to be accounted for by 

 only one-third the total density of elements. Hence theories have 

 been proposed to yield quantitative predictions of the discriminal pre- 

 cision in judging color-differences in particular without postulating 

 three distinct types of receptor (e.g. Shaxby, 1943). However, the 

 case here is analogous to the case of discrimination of intensity-dif- 

 ferences in general: different intensities, and also different colors, 

 can occasion qualitatively different responses so that at some place 

 in the neural pathway from receptor to effector the locus of the a 

 must be capable of varying with variation of the stimulus. This state- 

 ment is practically self-evident since obviously different final path- 

 ways are involved in reaching the different effectors, so that the only 

 question is where the variation occurs — centrally, along the afferent 

 pathway, or along the efferent pathway. Wherever this may take 

 place, the method demands explanation. But the statement is also a 

 consequence of the well known Muller's law of specificity, as is clearly 

 brought out by Carlson and Johnson (1941). 



Hence the neural mechanism which mediates any discriminal pro- 

 cess must provide for a segregation of the neural pathways affected 

 by different stimuli. In the case of a simple stimulus characterized 

 by a single parameter, S , there needs to be only a one-dimensional 

 array of synapses reached by neurons from the receptor in such a 

 way that when S has one value the resultant a is positive at one set 

 of the synapses, and when the value of & is changed sufficiently the 

 resultant a is positive at a different set of the synapses. A mechanism 

 capable of bringing this about was described in chapter ix. When the 

 stimulus is complex and requires three parameters, S v (i = 1 , 2 , 3), 



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