7'24 HANDBOtm OK I'HNSIOLOGV ^ NEHROPHYSIOKOCY 1 



FIG. 8. Records of cat optic cortex to show minor spike series. ;. Total corte.x, stimulation of 

 radiation above the geniculate. 2. Same, but stimulation of the optic nerve; negativity failed to 

 dc\elop in a weak response, and between each pair of spikes there occurs a minor disturbance. 

 3. Radiation stimulus; potentiometer balance to accentuate first minor spike. 4. Like .:? (in a differ- 

 ent cat), a subma.ximal response of total cortex thickness showing a succession of spikes at half the 

 intervals of major sequence. 5. Balanced record showing first and second minor spikes. 6 and 7. 

 .Stimulus at the radiation, different fractions, similarly balanced. 5 to 7. From same cat as / to 3. 

 8. Weak response like 4 (from a different cat), showing double sequence. Major spike intervals 

 marked on 4 and 8 were obtained from other records of the respective preparations of fonn of 

 record 1. g. Total cortex, submaximal stimulus to radiation, like -', first and second minor spikes 

 recorded. 10. Balanced record accentuating second of these. 11. Maximal response, lower ampli- 

 fication, same cat. 12. Total cortex. 13. Record as in 12 but balanced to accentuate the first and 

 second minor spikes. 14. Same as /j but at half the stimulus strength and recorded at twice the 

 amplification. [From Bishop & Clare (23).] 



pression period. Negativity representing antidromic 

 conduction via apical dendrites toward the cortical 

 surface appears to be depressed more easily by prior 

 acti\ity than the other features of response. 



In the period of depression, the base line slowly 

 became negative during which responses to second 

 stimuli were diminished. Following the depression, the 

 positive phase of the specific cortical responses to 

 second stimuli returned to normal amplitude. 



Clare & Bishop, from these and other results, con- 

 cluded that most of the features of the cortical re- 

 sponse can be attributed to the excitability cycle of 

 the cortex itself. On the basis of this information on 

 the cortical cycle and Marshall's (55) analysis of the 

 geniculate cycle, cortical responses to optic nerve 

 stimulation were studied. 



The degree of depression in the geniculate response 

 to a second stimulus \aries greatly from one animal 

 to another as well as to varying intensities of optic 

 nerve stimulation. Aside from the possibilitv that some 



ol the \ariatioii may be due to anesthesia differences, 

 the variability may arise from the contextual or back- 

 grotmd excitation upon which the activation is super- 

 imposed. 



If two stimuli are delivered very close together, a 

 single suprama.ximal response may occttr. Even when 

 an initial maxiiual stimulus is invoK'ed, there seem to 

 be a number of elements that were not activated but 

 only possibly excited subliminalh . These can be 

 acti\ated by the second stimulus, hence producing a 

 supermaximal response to the paired shocks. This 

 principle is more pronounced when the shocks are 

 submaximal. The facilitation period soon ends and 

 beyond it a period of depression ensues. It may last 

 as long as 5 sec. 



We may inject here the idea that this period may 

 possibly bring about perceptual (\isual) end results 

 simtilating adaptation in the retina. Some of the many 

 visual experiments labeled those of adaptation have 

 to do with short term effects. The various experiments 



