BEHAVIORAL AND BRAIN SCIENCES (2000) 23, 793–1121Printed in the United States of America Dreaming and REM sleep are controlled by different brain mechanisms Mark SolmsAcademic Department of Neurosurgery, St. Bartholomew's and Royal London School of Medicine, Royal London Hospital, London E1 1BB, United Kingdom Abstract: The paradigmatic assumption that REM sleep is the physiological equivalent of dreaming is in need of fundamental revision.A mounting body of evidence suggests that dreaming and REM sleep are dissociable states, and that dreaming is controlled by forebrainmechanisms. Recent neuropsychological, radiological, and pharmacological findings suggest that the cholinergic brain stem mechanismsthat control the REM state can only generate the psychological phenomena of dreaming through the mediation of a second, probablydopaminergic, forebrain mechanism. The latter mechanism (and thus dreaming itself) can also be activated by a variety of nonREM trig-gers. Dreaming can be manipulated by dopamine agonists and antagonists with no concomitant change in REM frequency, duration,and density. Dreaming can also be induced by focal forebrain stimulation and by complex partial (forebrain) seizures during nonREMsleep, when the involvement of brainstem REM mechanisms is precluded. Likewise, dreaming is obliterated by focal lesions along a spe-cific (probably dopaminergic) forebrain pathway, and these lesions do not have any appreciable effects on REM frequency, duration, anddensity. These findings suggest that the forebrain mechanism in question is the final common path to dreaming and that the brainstemoscillator that controls the REM state is just one of the many arousal triggers that can activate this forebrain mechanism. The "REM-on"mechanism (like its various NREM equivalents) therefore stands outside the dream process itself, which is mediated by an independent,forebrain "dream-on" mechanism.