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The relationship between the level of consciousness and the level of brain activation was studied. A model for a gradual decrease in the level of consciousness was provided by the transition from waking to drowsiness. Measures of the level of consciousness, assessed by the subjects themselves, consisted of changes in the content of consciousness, i.e., the intensity and quality of involuntarily arising mental processes. In each subject, the level of consciousness was found to correspond to a pert EEG profile. The results obtained from these studies showed that the lowest level of activation corresponded to the state in which the subject noted inhibition of “inner speech,” a “lapse of thoughts,” and in this state the interhemisphere associations involving the major mechanism of transmission of cognitive information, i.e., the high-frequency EEG rhythm, was weakened.