From Dreaming and the brain: Toward a cognitive neuroscience of conscious states by J. Allan Hobson, Edward F. Pace-Schott, and Robert Stickgold in Behavioral and Brain Sciences (2000), 23: 793-842

3.1.3. Selective deactivation of the dorsolateral prefrontal cortex in REM sleep.
Relevant to the cognitive deficits in self-reflective awareness, orientation, and memory during dreaming was the H215O PET finding of significant deactivation, in REM, of a vast area of dorsolateral prefrontal cortex (Braun et al. 1997; Maquet et al. 1996). A similar decrease in cerebral blood flow to frontal areas during REM has been noted by Madsen et al. (1991a) using single photon emission computed tomography (SPECT) and by Lovblad et al. (1999) using fMRI. Dorsolateral prefrontal deactivation during REM, however, was not replicated by an FDG PET study (Nofzinger et al. 1997) and this discrepancy, therefore, remains to be clarified by other FDG as well as H215O studies. (A potential cause of this discrepancy arising from differences between FDG and H215O methods is discussed further in sect. 3.3.5.2.) Nevertheless, it seems likely that considerable portions of executive and association cortex active in waking may be far less active in REM, leading Braun et al. (1997) to speculate that