Photoreception 



439 



sponsible for the sensation of brightness but is not responsible for color dis- 

 crimination. The fibers designated as 2 and 3 carry impulses from single cones 

 which have different spectral sensitivity curves with peaks in the blue, green, 

 yellow, or red (B, G, Y, R). These are responsible for the abilitv to discrim- 

 inate between wave lengths and are called "modulators." The theory as out- 

 lined here is called the "dominator-modulator" theory of color vision."' "^ The 

 electrical evidence obtained by Granit was recorded with electrodes at posi- 

 tions designated as Ei to E4. 



A composite theoretical curve of the tertiary neurones of the human retina, 

 adjusted to fit the data on perception, is shown in Figure 143. For various 

 reasons it seems better to assume four rather than three tvpes of modulators. 



Fig. l43. Synthesis of the human photopic luminosity curve, D, after Coblentz and 

 Emerson (1917). B, G, and R represent blue, green, and red fundamental sensation 

 curves. The R curve is composed of two modulator curves, My and Mr. From Granit." 



Four have been well demonstrated for the frog and three for the cat, and 

 even more may be present. However, four will explain most of the data on 

 human color vision and color blindness. For instance, if, in Figure 142, the 

 red- and green-sensitive modulators are absent or non-functional, the person 

 would be unable to distinguish red from green, both of which would appear 

 gray, but the visibility spectrum determined by the dominator would be 

 normal; this is the condition in deuteranopia. Furthermore, if, in addition, 

 all of the red-sensitive cones were non-functional, the visible spectrum would 

 be shortened; this is the condition in protanopia. Tritanopia is subject to a 

 comparable explanation. 



SUMMARY 



Light (visible and ultraviolet) is an important quality of the environment 

 which permits animals to locate food, to escape from predators, and to make 



