G04 



RADIATION BIOLOGY 



concludes that he has demonstrated a basically trichromatic mechanism 

 for color discrimination in the cat.^ 



On the basis of the work just described, Granlt assumes that in most 

 cases his microelectrode makes contact with a retinal third-order neurone 

 that is served by several receptor elements. When rod cells alone are 



0.400 



0.4 50 



0.500 0.550 



WAVE LENGTH, U, 



0.600 



0.650 



Fig. 13-14. Spectral-sensitivity curves for the cat, as analyzed by Granit (1947, 

 p. 311). Ordinate: relative sensitivity, evaluated for an equal quantum intensity 

 spectrum. Abscissa: wave length of stimulating light. Curve P is from corrected 

 values for visual-purple absorption. The other curves are those which would be 

 obtained from receptors containing visual purple alone (curve p) and from these 

 receptors in combination with modulator (Af) receptors (curve u) after selective 

 adaptation to filtered light (red, green, or blue filter). 



involved, typical scotopic dominator characteristics are revealed by the 

 responses of the nerve fiber. A mixture of rods and cones permits the 

 shift from a scotopic to a photopic dominator as the level of illumination 

 is raised. The cone cells differ in their wave-length sensitivities, but the 

 presence of several different ones within the region of convergence on the 



^ Behavioral evidence has not revealed that cats are capable of color discrimination. 

 Although negative evidence of this sort is never conclusive (Granit, 1950b), it may be 

 pointed out that only by selective adaptation have the photopic modulators revealed 

 themselves. If a cat receives no more information from its optic nerve fibers than is 

 recorded in these experiments, the animal must be color-blind most of the time. 



