510 RADIATION BIOLOGY 



order neurones (ganglion cells) have ceased to function. This observation 

 was made by the use of a microelectrode inserted into the retina of a rabbit 

 in such a way as to reach the bipolar cell region. (5) Electrical stimu- 

 lation of the optic tract reveals that conduction is possible even after 

 responses to photic stimulation are lost. This suggests that retinal ele- 

 ments are inactivated sooner than optic nerve fibers are during anoxia. 

 (6) In the geniculate region of the rabbit, responses of the postsynaptic 

 elements are lost in the early stages of anoxia at a time when optic-tract 

 activity has not yet been seriously impaired. (7) The striate area of the 

 cortex is most susceptible to anoxia. The initial effect is to fuse the 

 normally small and uncoordinated deflections into a monophasic wave 

 of much greater size. This effect, together with the subsequent dis- 

 appearance of all cortical responsiveness, leads to the conclusion that 

 mechanisms of summation and inhibition are among the first to be 

 affected by anoxia. 



ELECTRICAL STIMULATION OF THE VISUAL SYSTEM 

 VISUAL EFFECTS OF ELECTRICAL STIMULATION 



The visual system is of course capable of being aroused by electrical 

 stimulation at any point from the retinal receptors to the centers in the 

 brain. Variations in the location and intensity of this inadequate stimu- 

 lation produce concomitant changes in the apparent location, brightness, 

 and hue of the resulting "phosphenes." It has also been reported that 

 the brightness and hue are influenced by the polarity of the stimulating 

 current. Light blue phosphenes are commonly reported for stimulation 

 in which an electrode placed in contact with the front of the eye is posi- 

 tive, whereas dark- or reddish-yellow ones result from current flowing in 

 the opposite direction. Alternating current produces visual flicker, and 

 it is even possible to induce flicker by placing the head within a strong 

 magnetic field of alternating polarity. For the most part no attempt has 

 been made to locate the site of action of any of these subjective effects. 



PHYSIOLOGICAL EFFECTS 



Physiological studies have also been made of responses to electrical 

 stimulation. Bishop (1933) studied the speed of conduction along fibers 

 in the optic nerves of the frog and the rabbit. He concluded that there 

 were three fiber groups. He speculated on the possibility that the fastest 

 of these may perhaps be associated with spatial form discrimination, leav- 

 ing the slower ones to mediate brightness. 



Following the pioneer studies of Waller (1900), Granit and Helme 

 (1939) undertook to study the changes that electrical stimulation may 

 cause in the wave form of the electroretinogram. They reported that 

 both negative and positive components of the response to light (PII and 



