7: 1/ Neural Aspects of Vision 123 



green but not blue. In this category, the tritanopes see purplish blue and 

 greenish yellow as gray, whereas the tetartanopes see all blue and yellow 

 as gray. 



Several types of monochromasy exist. In one type, called cone 

 blindness, only rods are present in the retina. This type of monochro- 

 mats show a loss of acuity; they retain the scotopic luminosity curve only. 

 This strongly supports the connection between the rods and the scotopic 

 vision. 



To further complicate matters, there are various inbetween defi- 

 ciencies, such as protanomalous trichromasy, in which the red and blue- 

 green sensitivities are markedly less than normal, but all three colors are 

 necessary for matching hues. Almost any combination the reader can 

 imagine is known to occur in humans. 



Two general types of theories of color vision have been maintained in 

 the past. One of these, the tricolor theory, was supported historically by 

 Young, Helmholtz, and Maxwell. The other type of theory, the oppo- 

 nents or antagonist theory, has appealed -to many psychologists; its many 

 variations are each associated with a person's name such as Hering, 

 Ladd-Franklin, or Adams. The scheme presented in this text is essen- 

 tially that developed by the biophysicist, Talbot, who emphasized that 

 both theories have some elements of truth. His detailed picture makes 

 more use of the specific structures of the retina than do most of the other 

 theories. 



Briefly, the tricolor theories assumed that there were in the retina 

 three pigments, B, G, and R, having maximum absorptions in the blue, 

 green, and red regions respectively. These pigments were postulated 

 to exist in separate receptors which sent impulses to the brain producing 

 responses B', G', and R'. According to this theory, the brain "com- 

 puted" yellow and white from G' and R' at high luminosities and white 

 from B' at low luminosities. The original forms of the tricolor theory 

 had difficulties with several types of color blindness and with white- 

 black vision. Even the best refinements failed to use the detailed neuron 

 structure of the retina. This last oversimplification seems most mis- 

 leading. (See Figure 10, Chapter 2, page 42.) 



In contrast to the tricolor theories, which attempted to assign a 

 minimum of types of retinal actions, the antagonist (or opponents) 

 theories regarded the retina as the basis of considerably more complex 

 actions. The opponents theories postulated that there were six retinal 

 responses which occurred in antagonistic pairs. Excitation leading to 

 any single response was supposed to suppress the action of the other 

 member of the pair. These six retinal responses were identified as 

 blue-yellow, red-green, and black-white. Various forms of the oppo- 

 nents theories had less trouble explaining black-white vision and several 



