122 Neural Aspects of Vision \1 : I 



spectral density at all wavelengths) is matched by equal amounts of the 

 three standards. Symbolically, this last may be stated as 4 



|*780 m.n /*780 my /»780 mji 



*x d* = y*d\ = z k dX 



J380 van J380 mu J380 mp. 



Any colored light can be analyzed spectrophotometrically to give its 

 spectral density E K . This is defined so that the total energy E is given by 



E = E x dX I or in other words E x = -p- J 



Many spectral densities E A will give the same sensation. To specify the 

 sensation, three numbers, X, Y, Z, are needed ; in terms of the artificial 

 standards above 



X = J x k E K dX Y = j y k E k dX and Z = \ z A E A 



dX 



where all three integrals are evaluated from 380 nux to 780 mju. 



These color-matching experiments are based on human response. 

 Because they require subjective information, similar experiments are 

 difficult to perform on laboratory or wild animals. Nonetheless, con- 

 siderable evidence indicates that many vertebrates, and even insects, have 

 color vision. However, the cat, whose eye is anatomically more like the 

 human's than is the eye of any other animals except the primates, is 

 believed to lack color vision. (The primate eyes are all so similar that 

 the anatomist, Polyak, in discussing the retina lumps together humans 

 and other primates in all his diagrams.) Since so much of the available 

 data on color vision comes from humans, most attempts to explain color 

 vision on cellular levels emphasize human vision. 



In the past, one of the major factors considered in testing any theory 

 of color vision was its ability to account for various types of color defects. 

 People whose color vision is normal are called trichromats since they need 

 three colored lights to match the hues. Those needing only two colored 

 lights are called dichromats and those with no color distinctions, mono- 

 chromats. 



Four different types of dichromasy are known. Persons with two of 

 these distinguish only blue and yellow. In this category, the group 

 protanopes identifies red and blue-green colors as gray and has low 

 luminosity sensitivity in the red. In contrast, the deuteranopes have a 

 normal luminosity sensitivity in the red but identify greens and purple 

 reds as gray. The other two types of dichromats distinguish red and 



4 These integrals are usually written as extending over the wavelength range 

 from zero to infinity. However, x x , y x , and z* are zero at all wavelengths outside 

 of the range 380 m/x to 780m^t. 



