ELECTRICAL PHENOMENA IN VISION 607 



in the slower components. Still later the a activity is reduced or abol- 

 ished by strychnine, and spontaneous diphasic spikes begin to appear in 

 trains. During and immediately after these spikes the cortex is not 

 excitable by optic nerve impulses. It is therefore concluded that the 

 spontaneous activity and response activity are similar with respect to 

 their cellular origin and functional characteristics (Bartley et at., 1937). 



2. Area, duration, and intensity of stimulation all have the power to 

 influence the impHcit time (time to peak) of the first cortical-response 

 wave. Bartley (1941) has noted that an increase in any or all of these 

 factors leads to a reduction in response time. With respect to the area 

 factor, he found evidence that two effects were operating independently — 

 stray light and neural interaction. 



3. The cortical a rhythm and cortical-response activity were found by 

 Bartley (1941) to be closely interrelated. The arrival of optic nerve 

 impulses might produce any effect ranging from abolition of existing a- 

 waves to the initiation of these waves if they were originally absent. 

 These effects chiefly depended upon the frequency and phase relations 

 between the spontaneous and evoked potential waves. 



4. Localization on the cortex of two separate visual areas has been 

 studied by Thompson et al. (1950). They have mapped the areas repre- 

 senting the various portions of the retinal fields. The wave forms of 

 responses from the two areas are similar. 



5. The wave form of the cortical response has been analyzed by Chang 

 and Kaada (1950). These investigators noted six component deflections 

 in the typical cortical response to electrical stimulation of the optic nerve 

 in the cat. They attributed the earliest component to the optic-nerve 

 volley itself, the next three to separate geniculocortical connections, the 

 fifth to intracortical neurones, and the sixth to cortical internuncial cells. 

 They also considered the possibility that the three geniculocortical paths 

 may relate to a three-color visual mechanism. 



THE SEQUENCE OF EVENTS FROM RETINA TO CORTEX 



Of particular interest are investigations in which an attempt is made 

 to relate events occurring in a lower station to those of a higher one in 

 the visual pathways. Adrian (194(3), for example, has made simultaneous 

 recordings of the retinal action potential and the optic nerve responses in 

 the rabbit and cat. He concludes that, although there are many points of 

 disagreement between the two forms of recordings, " . . . . the response 

 of the eyeball seems to be a reasonable guide to the performance of the 

 receptor mechanisms." Adrian has also obtained simultaneous records 

 of the action potential of the retina and the responses from the striate 

 area of the brain of the monkey {ihid., p. 35). He has found that blue 

 light, which is very effective in arousing the action potential, is much less 

 so for arousing a cortical response. Red light has the opposite charac- 



