438 



Comparative Animal Physiology 



Granit used the technique of placing microelectrodes on single tertiary 

 neurones of the retina before they enter the optic nerve (Fig. 141). Then he 

 stimulated the area of sense cells connected to this fiber with light of known 

 wave length and intensity and varied the intensity at each wave length until 

 he determined the threshold of the fiber for that particular wave length. In 

 this manner he was able to obtain sensitivity curves for individual visual 

 units. Each unit consisted of the tertiary fiber and all sense cells and secon- 

 dary neurones from which it received impulses. It was found that the units 

 were not all the same. Some gave both the normal scotopic (dark-adapted) 

 and photopic (light-adapted) sensitivity curves as the state of adaptation was 

 appropriately changed, and this indicated that the same tertiary fiber received 

 impulses from both rods and cones. Other tertiary fibers apparently received 

 impulses only from cones. The sensitivity curves from exclusively cone units 

 also seemed to be of several types, some with a narrow range and a peak in 



RCX3S 



CONES 



I 2 3 



E NON- FUNCTIONAL IN PBOTANOPIA 

 X NON-FUNCTIONAL IN DEUTERANOPIA 



Fig. 142. An explanation of color vision and color blindness based on the findings of 

 Granit and his collaborators and on the histology of the retina. Ei, Ea, Ea, Et, recording 

 electrode positions; B, blue-sensitive cones; G, green-sensitive cones; Y, yellow-sensitive 

 cones; R, red-sensitive cones. From Jahn."* 



the blue, others with a narrow range and a peak in the green, yellow or red 

 and still others with a wide range and a peak which coincided with the nor- 

 mal photopic visibility curve. 



The above typesi of data can be explained by assuming that there are 

 cones with narrow sensitivity curves with peaks in the red, yellow, green, 

 and blue, and that these are connected both singly and in combination to 

 tertiary neurones, as demonstrated morphologically by Polyak.^-" The con- 

 nections are shown diagrammatically in Figure 142. The fiber designated as 1 

 is connected to numerous rods and numerous cones; this fiber has two sen- 

 sitivity curves, one the normal photopic and one the normal scotopic, de- 

 pending on the state of adaptation. It is called a "dominator" and is re- 



