ELECTRICAL PHENOMENA IN VISION 



605 



retinal fiber is likely to make it a photopic dominator that has a rather 

 broad spectral range of sensitivity. A predominance of red-sensitive 

 cones will, however, render the fiber particularly responsive to red; in 

 this case it is called a "red modulator." Green and blue modulators of 

 various sorts are also found by the indirect means of selective adaptation 

 just described. 



100 



0.400 0.450 0.500 0.550 0.600 0,650 0.700 



WAVE LENGTH (^« ) 



Fig. 13-15. Average modulator sensitivity curves of the cat, obtained by selective 

 adaptation {Granit, 1947, p. 312). Filled circles: red modulator curves resulting from 

 selective adaptation to blue or green light. Open circle.s: green modulator curves 

 obtained after adaptation to red or green light. Half -filled circles: blue modulator 

 curves obtained after adaptation to red or green light. 



CONCLUSIONS WITH REGARD TO THE RESPONSES 

 OF OPTIC NERVE FIBERS 



The following conclusions may be drawn with regard to the response of 

 optic nerve fibers: (1) The optic nerves of some primitive eyes appear to 

 be made up of true sensory fibers without synapses. They reveal the 

 responses of the end organs, uncomplicated by the effects of convergence 

 or interaction. (2) Light and dark adaptation in single sensory cells of 

 invertebrates are similar in detail to these processes as observed in higher 

 animals, including man. (3) Some variability in the responsiveness of a 

 single end organ is associated with a recovery process following the dis- 

 charge of each impulse. Random variability is also present. (4) Verte- 

 brate optic nerve fibers are those of third-order neurones, the retinal gan- 

 ghon cells. Three fundamental types of response to light are exhibited 

 by such fibers (on-, on-off-, and off-responses). (5) Each fiber typically 

 serves a relatively large receptive field of the retina. Spatial summation 

 is readily demonstrated within this area. (6) Specific color sensitivity 

 appears to characterize some of the vertebrate retinal fibers, but only 

 very indirectly is it possible to infer the color-responding properties of 

 the corresponding receptor cells. 



