CHANGES ASSOCIATED WITH FOREBRAIN EXCITATION PRf)f:ESSES 



3^3 



decreasing the excitability of the substrate. By use of 

 surface-positive applied polarization, a paroxysm 

 can be initiated in the rabbit at a significantly lower 

 intensity than is required to produce a paroxysm 

 with surface-negative polarization. Such a paroxysm 

 is also accompanied by a positive .SP change. This, 

 likewise, is supplanted by a negative one after the 

 paroxysm disappears. If a paroxysm is initiated by 

 sufficiently strong surface-negative polarization, that 

 paroxysm is also related to a surface-positi\'e SP shift 

 which develops in reaction to the immediately pre- 

 ceding surface-negative applied polarization. 



More recent studies (16) have re\ealed an SP 

 change accompanying a cortical paroxysm induced 

 by ventroanterior thalamic stimulation opposite to 

 the one initiated by the methods cited abo\e (stim- 

 ulation of relay nucleus and polarization). In this 

 instance the .SP shifts negatively durina; the high 

 \oltage discharge and then positively in the post- 

 paroxysmal depression period (fig. 10). A cortical 



PIG. 10. D.C. change accompanying a cortical paroxysm 

 induced by repetitive stimulation of ventroanterior thalamic 

 nucleus in the cat. A. Negative d.c. shift with 20 per sec. stimu- 

 lation. .\ 3 sec. strip of record has been omitted between A and 

 B; with continuation of stimulation there is an increase in 

 negative d.c. change. C. Upon cessation of stimulation (white 

 dot) high voltage paro.xysmal activity is in evidence and the 

 steady potential remains shifted negatively. D. Return of 

 steady potential to the prestimulatory base line with termination 

 of paro.xysm. E. Positive shift of steady potential in the post- 

 stimulatory isoelectric period. Vertical line of right angle in 

 right lower corner represents 500 mv; horizontal line, i sec. 

 Positive is up. 



FIG. II. .SP shift accompanying cortical paro.xysm induced 

 by thiocarbohydrazide in rabbit. .Straight white lines are base 

 lines from which shifts in SP are read. Positive is up. A. 15 min. 

 after intravenous injection of 30 mg of thiocarbohydrazide. 

 SP commences to shift negatively. B. 10 sec. later SP shifts 

 more negatively and paroxysmal activity begins. C. 6 sec. 

 later paro.xysmal activity continues and SP remains shifted 

 negatively. D and E. As high voltage discharge breaks up and 

 stops, SP shifts back to the base line. The tracing nearest the 

 base line in D is the same paroxysm recorded at lower ampli- 

 fication on the b beam of the oscilloscope. This beam is set at 

 lower gain in order to record the full excursions of larger SP 

 shifts. In the 10 sec. interval between C and D, the a beam 

 threatened to move off the tube face and therefore the b beam 

 was turned on. In all other strips only the a beam is shown 

 There is a 10 sec. interval between D and E. 



paroxvsm initiated by the intravenous injection of 

 convulsive drugs such as thiocarbohydrazide and 

 pentylenetetrazol (Metrazol) is accompanied by a 

 similar d.c. change as shown in figure 1 1 (Goldring, 

 S., P. \'anasupa & J. L. O'Leary, manuscript in 

 preparation). Other workers have also demonstrated 

 SP shifts accompanying paroxysmal activity, van 

 Harreveld & Stamm (43) found a negative SP shift 

 with cortical paroxysm produced by faradic stimula- 

 tion of the cortical surface or intravenous injection of 

 pentylenetetrazol, and Liberson, using the guinea 

 pig, found SP shifts accompanying induced par- 

 oxysmal discharge in the hippocampus (29, 44). 



D.C. Changes Which Accompany Spreading Depression (5D) 



In 1944 Leao (24) discovered a depression of the 

 usual cortical rhvthms of the rabbit which spreads 



