436 



Comparative Animal Physiology 



correlation of visual field defects with small lesions in the occipital lobe;««' ^^^ 

 (2) correlation of degeneration in the geniculate bodies with the site of le- 

 sions in the retina or in the cortex (monkey"- ^-y, (3) correlation of elec- 

 trical action potentials from various parts of the cortex in response to re- 

 stricted stimulation of the retina.^^- It has been demonstratedi^*^'- ^" that the 

 retina is projected point-to-point through the geniculate body to the optic 

 cortex. The areas of the striate cortex devoted to impulses from various 

 parts of the retina have been determined by Talbot and Marshall, and are 

 shown in Figure 140. It is evident that the image formed on the retina is 

 projected to the striate cortex and thence to higher association centers. 



SUPERIOR 



Lower ^^ 

 Vertical ^^^ 

 r'Wridian 



' 1 CM. 



Sulcus 

 Lunatus 



Upper 



Vertical 



i Meridian 



\ nibLinE 



Left 

 4' Hon3on'ial 

 Meridian 



IMFERIOR 



Fig. 140. Projection of the retina on the left occipital lobe of the monkey, 

 determined by electrical methods. After Talbot and Marshall. ^*- 



The Mechanism of Color Vision 



Normal Color Vision. Color vision is the term ordinarily applied to the 

 ability of an animal to distinguish between light of various wave lengths. 

 Inasmuch as several combinations of wave lengths may appear to have the 

 same color, i.e., to give the same sensation, the discrimination termed color 

 vision is not identical with wave length discrimination. However, the dif- 

 ferential effect of wave length on the receptors is the fundamental basis of 

 color vision. 



The most obvious fundamental evidence of color discrimination is ob- 

 tained from color matching. To a normal human observer the spectrum ap- 

 pears as a series of colors varying from dim red through orange, yellow, 

 brilliant yellow-green, green, and blue, to dim violet. The colors seen by 



1 r i_ 



a normal observer can be duplicated with few exceptions by mixtures ot three 

 lights, a red, a green, and a blue light, provided each is independently ad- 

 justable in intensity. Two lights are insufficient and four are unnecessary for 

 most colors, but when a fourth light is present the matching is more perfect. 

 A normal person is said to be trichromatic, by which is meant that he can 

 duplicate the sjK'ctrum with three colored lights. 



Abnormal Color Vision. Some individuals with abnormal vision require 

 lights of three colors to match any given color, but use an intensity adjust- 



