ACTION POTENTIALS IN THE OPTIC NERVE 237 



and Matthews [1927]), the action current discharge in the optic nerve 

 might consist of a series of regular pulses which usually varied between 

 15 and 5 per second. The frequency of the pulses varied with the light 

 intensity and was found to decline as exposure continued. 



They concluded that the intermittent discharges observed in the 

 optic nerve may be attributed to nervous interconnection between the 

 ganglion cells, but not to intermittent responses of the rods and cones to 

 the continuous illumination. They also found that an increase in the 

 size of the illuminated area paralled the effects of an increase in the inten- 

 sity of the illumination. This, they decided, must be due to a nervous 

 summation of the excitation from different points. 



On extinction of the illumination (" off effect," Fig. VI-11) a latent 

 period again develops comparable to that preceding the initial discharge 

 lasting from 0.2 to 0.5 second. This latent period of the off effect is 

 followed by a renewed outburst of impulses which lasts for about 1 

 second. It is difficult to say what causes this second, less violent, burst 

 of impulses. 



Whether the decline in frequency after the " on effect " is due to 

 fatigue or to adaptation is still a matter of opinion. The impulses may 

 vary in frequency but not in size, and the intervals between consecutive 

 impulses cannot be shorter than the absolute refractory period of the 

 fiber and must be long enough to allow for its complete recovery. 



The electrical impulses detectable in the whole optic nerve when the 

 retina is stimulated by light must represent the composite results of the 

 activity of several thousand fibers. For instance, Hartline [1938] found 

 that in 20 per cent of the isolated single fibers the initial burst of impulses 

 of high frequency was followed by a much lower discharge frequency, 

 which lasted throughout the duration of the illumination. When the 

 illumination was extinguished, this discharge stopped. About 50 per 

 cent of the fibers of the whole optic nerve responded with a short burst of 

 impulses of high frequency when the light was turned on, but transmitted 

 no impulses as long as the intensity of illumination was not changed. 

 In about 30 per cent of the fibers no impulses appeared at all during 

 illumination, but they showed a vigorous discharge of impulses when the 

 light was extinguished. 



Apparently it is the composite effect of these three groups of frequen- 

 cies that is detected when the optic nerve as a whole is under examina- 

 tion. The burst of frequencies drops to half value in about a tenth of a 

 second, so that a flicker whose frequency is greater than 10 per second 

 should give the impression of continuous illumination. The motion- 

 picture exposure frequency of about 12 frames per second will, as we 

 know, cause no visual flicker. 



