RESPONSES OF CORTEX TO DIRECT ELECTRICAL STIMULI APPLIED AT DIFFERENT DEPTHS

Abstract

Stimulated directly by single pulses at increasing depths from the optic cortical surface in the cat, an increasingly complex record is obtained from leads at the surface and in white matter. A shallow stimulus activates only apical dendrites, which fail to conduct downward significantly. Deeper stimulation activates a successively increasing number of brief spikes, until from a stimulus in white matter a record identical to that following optic nerve stimulation is obtained. Propagation upward therefore involves conduction in apical dendrites to give a diphasically recorded wave, and a succession of spikes from cell bodies and their axons, and conduction downward is not evident following single stimuli at any depth. Mutual facilitation of conduction upward (antidromically) in apical dendrites is evident from the fact that the negative phase signalizing the arrival of their impulses at the cortical surface is relatively small and variable following optic nerve stimuli, larger and less variable following radiation stimuli, and still larger and constant following stimuli applied within the cortex. The difference in activation in these cases corresponds to the increasing density of cortical elements excited by stimuli at these loci. On the other hand activation of apical dendrites is suppressed by a previous response when the remainder of the record assignable to cell bodies is of full magnitude. The negative wave conducted to the cortical surface is the component facilitated by strychnine, to form the "strychnine spike" of paroxysm. A discussion is presented of the nature of the record of cell units stratified in a conducting matrix, and a diagram is included presenting an analysis of the potential components of which any record of optic cortical response represents the algebraic summation.