CHANGES ASSOCIATED WITH FOREBRAI.N' EXCITATION PROCESSES 



327 



After-effects appear to sum algeliraically, giving 

 different polarity under different conditions of excita- 

 tion. As used here the term algebraic summation 

 implies something more than the simple cancellation 

 of opposing equal forces. Such an explanation might 

 suffice for the absence of detectable after-effect early 

 in the course of some experiments, but a somewhat 

 more complicated interpretation is needed to explain 

 all of the observed phenomena of after-effect summa- 

 tion during repetiti\e processes. An increasingly 

 negative after-effect during summation might thus 

 be due either to exaggeration of a negatively directed 

 tendency, or to suppression of a positively directed 

 one and vice versa. The plasticity of interaction be- 

 tween the two forces is exemplified in the summation 

 of cortical evoked response after-effects induced by 

 repetitive stimulation of the opposite optic nerve, as 

 compared to that of the lateral geniculate nucleus. 

 Optic nerve stimulation in our experience can cause 

 only a summation of positive after-effect when applied 

 at maximum, if at all; most usually the summation is 

 that of negative after-effect. Geniculate stimulation 

 produces summated after-effect consistently at stimu- 

 lus values below maximum. Another indication that 

 the same neuronal firing can produce negative after- 

 effect in one situation and positive in another lies in 

 the comparison between the summed negative after- 

 effect of clusters of strychnine spikes, and the positive 

 summation which accompanies the high voltage 

 paroxysm that develops intermittently in the same 

 strychninized corte.x. 



The unifying concept which promises the most aid 

 in harmonizing knowledge of the electrical signs of 

 the quicker transients with those of the slower d.c. 

 concomitants of neural excitation is that of Libet & 

 Gerard (30). These writers postulated a polarization 

 gradient along the vertically oriented cortical neurons. 

 For each neuron this gradient would extend from the 

 surface dendritic expansions in the plexiform layer to 

 a deeper level, even to layer \T, where the soma-axon 

 junction is situated. Further evidence relating 

 polarization gradients to neural excitation is obtained 

 by the simple expedient of changing the charge dis- 

 tribution along the pyramidal cells artificially. This 

 affects significantly the positive and negative com- 

 ponents of evoked potential, surface-negative polari- 

 zation accentuating the positive phase, surface-positive 

 polarization the negative phase. 



Other support for Libet & Gerard's view is to be 

 found in the change in visual esoked response during 

 the cycles of intense negative d.c. shift which char- 



acterizes the veratrinized cortex. The effect of vera- 

 trine applied to the cortical surface is to depolarize 

 the terminal dendritic brushes of the cortical neurons 

 (13, 14), and from this depolarized superficial region 

 intense waves of depolarization appear to spread 

 downwards over the cortical dendrites, progressively 

 engulfing the cortical neurons from above. The 

 initial positive phase of evoked response is due to the 

 successive activation of groups of neurons, each ex- 

 cited after synaptic conduction. The summation pro- 

 duced is signified by the four fast spikes which appear 

 successively higher upon the rising phase of the evoked 

 potential. During the intense negative veratrine shift 

 these spikes are removed successively from above 

 downwards, and the order of their removal corre- 

 sponds with the expectancy from their positions in the 

 cortical depth as obtained by the null point of meas- 

 urements of Bishop & Clare (i). 



The changes in charge distribution may actually 

 be more complicated than is suggested by the results 

 of these experimental studies. Besides the situation in 

 which the charge at the two ends might change in 

 opposite direction to produce an SP effect, a d.c. 

 change might also ensue if both ends changed in the 

 same direction unequally. Such a concept is now sus- 

 ceptible to experimental proof only from the limited 

 aspect of charge distribution along the neuron's ex- 

 terior. If interior differences of potential exist be- 

 tween the superficial dendrites and the cell soma, it 

 will require microelectrode recording to reveal them. 



SUMM.\RY 



Present methods of recording the d.c. potential 

 across the cerebral cortex are presented. These re- 

 quire detailed attention to oxygen tension, anesthesia 

 and injury, and necessitate stable electrodes for re- 

 cording purposes. The SP across the cortex remains 

 relatively steady under good experimental conditions 

 and serves as a base line for examining slower con- 

 comitants of neural excitation. In several situations it 

 has been shown that such SP concomitants do relate 

 to the quick transient phenomena conventionally 

 recorded from the cortex. Significant summation of 

 d.c. change associated with single transients can 

 appear during repetitive phenomena. The best prom- 

 ise of relating d.c. and transient phenomena is in 

 terms of the polarization theory of Libet & Gerard 



C30). 



