CHANGES ASSOCIATED WITH FOREBRAIN EXCITATION PROCESSES 



319 



Then the ECG and SP will revert to the preinjection 

 status. Unless the thiosulfate is injected the change 

 continues for 3 to 8 min.; respiration stops and shortly 

 thereafter the heart also. Meanwhile SP undergoes an 

 opposite shift which carries it 3 to 4 mv more negative 

 than it was at the start of the experiment (fig. 3). 



We ha\e followed .SP under ether anesthesia more 

 carefully than under any other. Commencing at a 

 light level of anesthesia and deepening it gradually, 

 SP may remain relatively steady although occasionally 

 a positive shift may occur. If inhalation anesthesia 

 is carried too deep, SP may shift negatively by com- 

 parison with the preceding light anesthesia baseline. 

 However, as long as anesthesia is maintained at a 

 relatively light level, the changes it occasions in SP 

 are relatively insignificant, and induced physiological 

 variations are much like those encountered in a rela- 

 tively unanesthetized state. Finally, solutions added 

 to the cortical surface which are not isotonic may 

 disturb SP for sexeral minutes. These are diffusion 

 potential artifacts. 



Summarizing, one must emphasize that to provide 

 a stable base for measuring induced SP changes there 

 should be careful control of injury, anesthesia and 

 oxygen tension. 



EXPERIMENT.'\L STUDIES 



Spontaneous SP Changes Correlated with Interruption 

 in Usual Resting Cortical Rhythm 



It often is not possible to demonstrate SP altera- 

 tions which coincide with a change in the ECG 

 rhythm of the lightly anesthetized animal. However, 

 in the rabbit (12) we have noted repeated brief nega- 

 tive shifts amounting to 0.6 mv which coincide with 

 low voltage ECG intervals between runs of regular 

 frequency activity. Furthermore, a relatively rapid 

 increase in ECG amplitude with substitution of 

 slower for usual frequencies may show an accom- 

 panying positive SP shift. In the pentobarbitalized 

 cat a significant negative shift can be recorded during 

 and succeeding barbiturate spindles, sometimes out- 

 lasting the spindle by several seconds. 



After-effects of Evoked Responses 



Since singly evoked responses are quick transients, 

 the only SP change which could be expected to occur 

 would appear as an aftermath. We have studied such 

 after-effects in the visual cortex of the rabbit (11, 12) 

 together with the conditions leading to their summa- 



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7 

 6 

 5 

 4 

 3 

 2 

 / 





 / 

 2 

 3 

 4 

 5 



6 



+ 



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ime in minuxeb 



ici 



I Z 3 4 5 6 7 S 9 10 1/ IZ 13 14 15 lb 17 18 19 10 2J 22 2i 24 



FIG. 3. Graph of change in SP during intravenous injection of 20 ml of i per cent solution of 

 malononitrile. a, start of injection; b, ECG commences to become isoelectric; c, respiration stops. 



