Recent technical developments have made it possible to measure cortical responses evoked by sensory stimulation in recordings taken from the human scalp. These potentials are not detectable in the usual electroencephalogram (EEG) because they are so small in relation to the "spontaneous" brain rhythms. They may, however, be recorded by averaging methods.1 Using such methods in our laboratory, we have been studying evoked potentials of the somatosensory cortex with the aim of determining whether psychiatric illness is associated with measurable abnormalities in cortical reactivity.2
In our work we have been particularly interested in measuring the cycle of cortical reactivity, which can be derived by com-paring the responses to paired stimuli separated by varying time intervals. The procedure for determining the reactivity cycle is similar to the classical neurophysiological method for measuring the excitability cycle of nerve, except that suprathreshold
SHAGASS C, SCHWARTZ M. Cerebral Cortical Reactivity in Psychotic Depressions. Arch Gen Psychiatry. 1962;6(3):235–242. doi:10.1001/archpsyc.1962.01710210051006
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