ELECTRICAL PHENOMENA IN VISION 613 



by "contrasting" spectral stimuli, is taken as evidence for color-contrast 

 effects occurring at a retinal level. Even in the cat, then, it appears 

 that a peripheral mechanism exists for simultaneous contrast and after- 

 image phenomena, since the on-effect for one color may be similar to the 

 off-effect for another. Finally the specific enhancements of certain spec- 

 tral regions by polarization are taken to mean that the cat is also in 

 possession of a peripheral mechanism for wave-length discrimination. 

 This mechanism is presumably based upon facihtation and inhibition in 

 the horizontal pathways of the retina. 



EFFECTS OF LIGHT ON ELECTRICAL EXCITABILITY 



In the experiments just mentioned the effects of electrical stimulation 

 were studied by the use of test flashes of hght. The opposite type of 

 experiment may also be performed, namely, the use of test stimuli of an 

 electrical nature to study the effects of visual stimulation by light. 

 Motokawa (1949a,b,c,d, 1950) has carried out a series of studies of this 

 nature on human subjects. His usual procedure is to stimulate the eye 

 with a 2-sec flash of light. Then, after varying intervals of time in the 

 dark, a test stimulus consisting of direct current is appUed for 0.1 sec 

 with the aid of large electrodes on either side of the eye. A study is 

 made of the intensity of electrical stimulation necessary to arouse a just 

 noticeable phosphene or visual effect. 



It is found that, as a result of the flash of hght, electrical sensitivity is 

 enhanced. This enhancement builds up to a maximum value within a 

 few seconds after the flash of light and thereafter declines fairly rapidly. 

 The interval of time from the disappearance of the light to the moment 

 of maximum electrical sensitivity is called by Motokawa the "crest time." 

 This crest time is found to be relatively independent of the intensity of 

 hght used in the stimulating flash. It does depend, however, upon the 

 wave length of the light. Specifically the crest time decreases from about 

 3 sec at a wave length of 420 m/x to about 1 sec at 670 m^t. Motokawa 

 relates the form of this function to that of a psychophysical curve of 

 wave-length discrimination and draws the conclusion that "the physio- 

 logical basis for hue discrimination lies in the periphery, as has been 

 assumed in the theories of color vision." 



In further experiments Motokawa has found that the degree of enhance- 

 ment of electrical excitability increases with the logarithm of the intensity 

 of the preceding flash of light in a manner suggestive of the usual critical- 

 flicker-frequency curves. Enhancement also increases as the flash is 

 moved in from the periphery to the fovea. Furthermore it is possible to 

 determine the degree of enhancement as a function of the wave length 

 of the stimulating light. When this is done, using an interval of 1 sec 

 from the flash to the electrical test stimulus, the resulting curve looks 



