AVERAGED EVOKED SOMATOSENSORY RESPONSES FROM EXPOSED CORTEX OF MAN.

Abstract

RELATIVELY few observations on sensory evoked responses are documented for the cerebral cortex of man.1-3A major obstacle to acquisition of human records has been the small size of the evoked activities observed at the cortical surface. They are smaller than similar responses seen in animals and therefore more easily obscured by spontaneous rhythms of the electroencephalogram (EEG). Recently digital computers have made it possible to uncover such submerged electrical evocations. The computer is programmed to add the electrical activities—both evoked and spontaneous —that follow each stimulus of a repetitive sequence. The sum of the evoked potentials, which are nonrandom events, increases directly as the number of responses added, while the sum of random EEG waves increase as the square root of the number of samples obtained. Thus the two activities grow at a disproportionate rate, and the evoked response is exposed above the spontaneous background rhythms. The technique