CHANGES ASSOCIATED. WITH FOREBRAI.N EXCITATION PROCESSES 



321 



in figure i (15, 16). It also persists significantly after 

 tiie period of stimulation. Oscillographic recording 

 shows that there is a significant SP negative change 

 after even the first spike of a series, and that this 

 change is summated with the after-effects of succeed- 

 ing higher amplitude spikes as the recruiting series 

 continues. The persistence of the SP negati\ity after 

 the stimulus is turned off indicates that it is truly an 

 after-effect disturbance comparable to that which 

 follows single evoked responses. Increasing the 

 stimulus frequency to between 6 and 20 per sec. (the 

 voltage and shock duration remaining constant) 

 increases the amplitude and duration of the negative 

 SP change. At 20 per sec. a further increase in ampli- 

 tude and duration of SP negati\ely occurs if the 

 stimulus duration is increased from o.i to i .0 msec. 



SP After-effects of Strychnine and Veratrine Spikes 



The effect of strychnine has been studied in the 

 rabbit and the cat (12, 13, 14). In the rabbit a 0.05 

 per cent strychnine solution applied to the cortex 

 may be of sufficient strength to suppress the spon- 

 taneous component of the ECG; a minor 0.3 to 

 0.4 mv negative SP shift appears within 10 min. 

 following the application of the drug to the cortical 

 surface. The strychnine spikes which occur sporad- 

 ically at this time show no detectable after-effect. 

 However, a solution sufficiently strong to occasion 

 intermittent repetitive spike paroxysms (0.5 per cent) 

 causes a positive after-effect to appear following each 



random spike which appears between the paroxysms. 

 With application of a crystal of strychnine, negative 

 after-effects follow each instead. A o. i per cent 

 strychnine sulfate solution applied to the cortex of 

 the cat also occasions a minor negative SP shift, and 

 here after-effects of strychnine spikes have a negative 

 polarity. When the single spikes occur in rapid suc- 

 cession the negativities associated with individual 

 spikes summate (fig. 6). The SP change which occurs 

 during a strychnine activated paroxysm will be re- 

 ferred to later. 



X'eratrine hydrochloride (10^^) applied to the 

 cortex of the pentobarbitalized cat unstabilizes SP 

 almost immediately, resulting in one or more 5 to 

 1 5 mv negative shifts from each of which SP may 

 recov-er significantly (13, 14). A plot of several such 

 shifts following \eratrine application shows a down- 

 ward negati\e drift, and the SP does not again reach 

 its value at the start of the experiment. Finally, how- 

 ever, it stabilizes upon a plateau. At that time the 

 initial positive phase of the evoked response becomes 

 significantly prolonged and spikes of dominantly 

 positive polarity appear spontaneously, or may be 

 initiated (as in the visual cortex) by turning the room 

 lights on or off. Each evoked response and each such 

 spike is accompanied by a positive after-effect which 

 may endure for 5 to 10 sec. (fig. 7). Barbiturate 

 spindles also come to show a principally positive 

 polarity in the ECG, and they, too, show a positive 

 polaritv SP change which persists significantly after 

 the end of the spindle (13, fig. 8). If strychnine is 



.v«,^^W^Il'> 



Rg. 6 



Fig. 7 



,UH)to|W^MjVui/V^ A 



,wv%1 



. ' ' I:200/iV 



I- I I sec 1 ^ 



200/1 V- 1\__ 



I sec 



ZOO juW- 





FIG. 6. Effect of 0.1 per cent strychnine sulfate solution 

 applied to cortical surface of cat under pentobarbital anes- 

 thesia. / and 2 indicate ECG and SP records, respectively. 

 A. Individual spontaneous strychnine spikes with negative after- 

 effects. B. Occurrence of a cluster of spontaneous strychnine 

 spikes with summation of the negative after-effects. [From 

 Goldring & O'Leary (14).] 



Fic. 7. Effect of application of veratrine hydrochloride 

 (io~') to surface of the cortex of the cat under pentobarbital 

 anesthesia. / and -' indicate ECG and .SP records, respectively. 

 A. Single spontaneous positive veratrine spike with a long 

 positive after-effect. B. A series of such spikes with positive after- 

 effects. [From Goldring cl? O'Leary (13).] 



