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HANDBOOK OF PH^SIOLOGV 



NEUROPHYSIOLOGY I 



caution \'astola used the chopper technique at a fast 

 rate of interruption to prove that the SP between the 

 indisidual transients also shifted negatively. 



Effects uf Stimulation at a Distance Along a 

 Mullisynaplic Path Upon Transcortical SP 



Dondey & Snider (8) recorded SP from the cerebral 

 cortex in much the same fashion as outlined herein, 

 using animals prepared under ether anesthesia and 

 maintained under a'-tubocurarine (15 mg per kg). 

 Besides confirming the findings reported previously 

 concerning the relation between the appearance of 

 cortical paroxysm and of positive SP shift, and be- 

 tween the postictal silent period and negative shift, 

 they studied the effect of fast frequency cerebellar 

 stimulation (200 and 300 per sec, 10 to 30 v.). Such 

 stimulation induced a positive shift in cortical SP last- 

 ing for as long as 50 sec. and becoming as large as 5 

 mv. Suppression of cortical spindles occurred during 

 the shift and was the principal criterion for the effi- 

 cacy of the cerebellar stimulation. Under the same 

 circumstances slow frequency stimulation, ranging 

 between 10 and 20 per sec, induced a negative SP 

 shift in the cortex which might last as long as 70 sec. 

 and reach 4 mv in amplitude. With the lower frequency 

 stimulation the ECG did not change as significantly 

 and spindles might occur throughout the recording. 

 In both the instances of fast and of slow stimulation, 

 SP recording from the nucleus ventralis lateralis of 

 the thalamus showed oppositely polarized effects. Don- 

 dey & Snider found in addition that fast frequency 

 stimulation of the cerebellum might prevent the ex- 

 pected negative shift which occurs at the end of cor- 

 tically induced paroxysmal discharge; instead posi- 

 tivity continued much beyond the cessation of the 

 paroxysm. 



Injury Potential Components 



D.C. changes with injury were reported h\ Walker 

 et al. (47) and Meyer & Denny-Brown (37, 38). K.em- 

 pinsky's (23) study of the distribution of SP 

 change associated with experimental vascular occlu- 

 sion of the middle cerebral artery in the cat con- 

 clusively demonstrates that a significant component 

 of such injury effect is a demarcation potential across 

 the zone of injury in the white matter. He used pia- 

 ventricular, subcortical-ventricular and transcortical 

 leads simultaneously. The prompt, sustained negative 

 shift which he obtained in the center of the cortical 



area of distribution of the vessel could be recorded 

 in the subcortical-ventricular and the pia-ventricular 

 combinations but not in the cortex-subcortex one. 

 When recorded simultaneously at different cortical 

 loci, the magnitude of the shift decremented toward 

 the periphery of the ischemic region. 



Human Studies I'smg Scalp Recording 



In a study by Goldring et al. (19) it was shown that 

 d.c changes can also be recorded from the human 

 scalp during electroconvulsive therapy. The difiiculty 

 which arises in the interpretation of these and other 

 d.c. changes recorded from the scalp with non- 

 polarizable electrodes is the complication introduced 

 by d.c. changes which occur in the skin. Of the results 

 reported in the literature the negative shift with 3 

 per sec. spike and dome discharges (5) seems most 

 free of this criticism. 



DISCUSSION OF ORIGIN OF STE.ADY POTENTI.iiLS 



Starting from a base of transcortical voltage meas- 

 urement called steady potential (SP), evidence thus 

 far accumulated supports the existence of d.c. con- 

 comitants of conventionally recorded cortical excita- 

 tion processes. The d.c. changes which correspond 

 have also been obtained from some subcortical centers. 

 In one such nucleus (the lateral geniculate) it seems 

 clear that there is a close association between d.c. 

 change and the neural excitation process (46). In 

 the cortex d.c. changes occur subsequent to brief 

 transients such as evoked responses, strychnine and 

 veratrine spikes, and during and after repetitive ones 

 like barbiturate spindles or recruiting responses. 

 Thus, bv means of a d.c. amplifier the electrical sign 

 of excitation at a cortical point can be recorded trans- 

 corticalK' significantK' after the acti\ity obtained with 

 a condensor-coupled amplifier has disappeared. 



The evidence is yet insufficient to decide whether 

 such d.c. changes are simply a prolongation of the 

 same neural process which occasions a neuron's dis- 

 charge or are analogous to the after-potentials of 

 peripheral nerve. If the latter, evidence for metabolic 

 causation needs to ije considered. There are also in- 

 dications, liut no proof, for excitability change occur- 

 ring simultaneou.slv with certain after-effects, and 

 this also needs close investigation. If excitability 

 change does occur, the analogy with peripheral 

 nerve after-potentials becomes much closer. 



