Abstract
Potentials evoked by brief electrical pulses applied to the median nerve, were recorded from the frontal, central and vertex regions of human subjects. Averaging and spectral analysis showed that the responses contained time-locked components ranging in frequency from 0.75 to 4 c/sec and extending over periods as long as 3500 msec after the stimulus. There was no significant difference between evoked potentials induced by stimuli which were accompanied by a twitch in the muscles innervated by the median nerve and evoked potentials induced by stimuli which caused no muscular response. Stimulation by pulses separated by random intervals caused an increase in the amplitude of the late components over a broad frequency spectrum while periodic stimulation caused an increase in amplitude in much narrower ranges. When the frequency of stimulation was progressively increased from 0.25 to 5 pulses/sec it was found that the late oscillations could be “driven” at specific frequencies which caused an increase in the amplitudes of specific frequency components, harmonically related to each other and to the frequency of the stimulus. In a few instances in which a series of 125 individual evoked potentials was studied, it was found that the average computed from the last 50 potentials contained late components of considerably lower amplitudes than the average computed from the first 50 potentials. The administration of barbiturates caused a selective decrease in the amplitude of the late components and a redistribution of energy in the frequency spectrum. The essential characteristics of these late components: their wide distribution over the frontal, central and vertex regions, their occurrence over long periods after the stimulus, the decrease in their amplitude with repeated stimulation, indicating the possibility of “habituation”, and their enhancement at specific frequencies of stimulation, suggest that they may be related to the activities of the nonspecific systems of the brain.
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