Comparison of the motor responses with the electrical alterations of cerebral cortex and cord induced by repeated electrical stimulation of the motor cortex show two factors for facilitation and two for extinction.
There is negative afterpotential with decreased threshold (facilitating), followed by positive afterpotential with increased threshold (extinguishing), both maximal at site of stimulation. There is electrical afterdischarge, hyperactivity in reverberating chains (facilitating), followed by deficiency of spontaneous action potentials, hypoactivity (extinguishing), both demonstrable in structures functionally related to the focus stimulated.
By appropriate choice of the physical characteristics of the stimulus, the spatial and temporal distribution of these factors may be made so dissimilar that their part in determining the size of response is distinguishable.
With stimulation designed to avoid motor afterdischarge, local threshold changes associated with D. C. potentials predominate at the site; but even during extinction there, facilitation with electrical afterdischarge appears at neighboring motor foci.
With subthreshold stimulation designed to produce motor afterdischarge at threshold, prolonged electrical afterdischarge appears not only throughout that subdivision of the sensori-motor cortex (except area 6) but also in the ventral white and gray of the corresponding enlargement of the cord, and is associated with facilitation. Subsequently cortical action potentials are practically gone in the areas involved and extinction is demonstrable.
The local rise in threshold associated with positive afterpotential is dependent only on the actual firing of the nerve cells there, and, as afterdischarge signifies prolonged, frequent firing of nerve cells wherever it occurs, similar threshold changes with positive after potential supervene, which may account for the loss of spontaneous activity. Thus the factors themselves may be interrelated.
The spatial separation is not absolute. All four factors can be demonstrated electrically at the site of stimulation. By setting up subthreshold background stimulation of five pulses per second and, through the same electrodes, superimposing short periods of stimulation of high pattern and pulse frequency, it is possible to demonstrate the influence of all four factors, and arrive at some conclusions as to their significance.
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Keywords: Cortex, Show, Factors, Potentials, After Discharge, Stimulation, Extinction, Triumph, Brain, Memory
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