236 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



The action currents of the optic nerve in the dark showed the nerve to 

 be " fairly quiet." A sudden exposure of the retina to illumination 

 always produced a well-marked monophasic action current discharge. 

 The average duration of one of these responses was 0.0015 second, which 

 is about the same order of magnitude as that observed in a peripheral 

 sensory nerve fiber. The frequency of the impulses was found to depend 

 on the size of the retinal image and on the intensity of illumination. 



300 



Dark 



Fig. VI-11. The action currents of the optic nerve begin after a latent period 

 (L.P.) less than 0.2 second after the light is turned on. After 3 seconds when the 

 light is suddenly extinguished a second latent period appears, followed by a rise in 

 frequency of the discharge, the "off effect." From data by Adrian and Matthews 

 [1927]. 



The results show that the discharge frequency begins after a consider- 

 able latent period (L.P. in Fig. VI-11), rises to a maximum, then 

 declines slowly. When the illumination is suddenly extinguished, it is 

 followed by a sudden less prominent frequency increase which subsides 

 gradually to zero frequency. 



The general conclusion is that, the greater the intensity of the illumi- 

 nation, the shorter the latent period of the response, the greater the fre- 

 quency of the discharge, and the greater the number of impulses in the 

 initial burst of frequencies (Hartline [1938]). 



The decline in frequency after the first burst may be correlated with 

 our experience that a dimly outlined object appears in a dark room to 

 decrease in brightness when the eye is prevented from moving while 

 looking at it. After 20 seconds or less the object disappears and the 

 field seems uniformly blurred. 



These results mean that a changing visual field should produce a 

 greater sensory experience than a stationary field; hence a movement 

 will be more readily perceived than a steady pattern of light and shade. 



When the entire retina was exposed to uniform illumination (Adrian 



