A recent paper by Francesca Siclari, Benjamin Baird, Lampros Perogamvros, Giulio Bernardi1, Joshua J LaRocque, Brady Riedner, Melanie Boly, Bradley R Postle & Giulio Tononi; in Nature Neuroscience; (published online 10 April 2017; doi:10.1038/nn.4545) reported a very interesting set of findings on the neural correlates of dreaming. Since the 1950s we have known that if we awakened people during REM we would reliably get reports of dreams. About 70% of the time if we awakened people during the N2 light stage of sleep we would also get reports of dreams. Even if we awoke people during deep slow wave sleep states (N3) we could still get reports of dreams, though certainly not as reliably as when we awoke them from REM or N2. In short, even though REM, N2 and N3 were defined by dramatically different EEG signatures we could still get reports of dreams in each of these sleep states. Clearly, the standard EEG sleep montage was too gross an instrument to isolate those brain states most reliably associated with dream reports.
Siclari et al used high density EEG recordings to isolate neural correlates of dreaming regardless of standardly identified sleep state. The authors contrasted the presence and absence of dreaming in both NREM and REM sleep. When a posterior “hot zone” showed a decrease in low-frequency EEG activity—traditionally known as ‘EEG activation’—subjects reported upon awakening that they did have dream experiences. By contrast, when low-frequency EEG activity increased in the same area, subjects reported that they had no dreams. Thus those neural sites that were consistently activated whenever participants reported dreams and consistently inactivated whenever participants reported the absence of dreaming—regardless of standardly defined sleep states included sites within the "hot zone" including occipital cortex, precuneus and posterior cingulate gyrus. By monitoring neural activity in this posterior ‘hot zone’ the authors could predict when a subject would report dreaming.
Use of high density EEG is extremely technically challenging. That is why so few sleep studies use this technique. It is subject to all kinds of artifacts and so researchers have to use extraordinary precautions (e.g. specially built soundproof rooms etc) to control noise when using these huge EEG montages. The authors not only used high density EEG during sleep; they apparently were able to avoid artifactual contaminants even when repeatedly awakening subjects for dream reports! This is a significant accomplishment in itself.
The neural correlates for dream experience was localized to the so called hot zone for dream experience in the posterior cortical region in this study. That region included occipital cortex (visual center) and the precuneus and the posterior cingulate. This site seems reasonable to me to be a hot zone for dream experience. Precuneus activation, for example, has been associated with self-awareness and visual memory but some fMRI studies have reported that this site is down regulated or de-activated during REM, which seems to contradict claims that this would be the sole hot zone for dreaming. It has been known for some time that damage to the temporal-occipital-parietal (TPO) junction can result in cessation of dream recall. The hot zone described in this paper likely overlaps to some extent with the TPO.
When the authors looked at the hot zone during REM, cortical activation patterns extended far beyond the posterior cortical hot zone into the frontal lobes. Lesions to the frontal lobes, particularly medial aspects of the frontal lobes also results in cessation of dream recall. This activation of medial PFC results in dream experience and lesioning or down regulation of medial PFC is associated with absence of dream experience. Given that down regulation of this region is also associated with cessation of dream experience should it too be included in a hot zone for dream experience?
While the authors have hopefully finally put to rest the old equation of REM sleep with dreaming it should be noted that high density EEG cannot resolve with any reliability subcortical components of the brain so the participation of subcortical brain regions in the dreaming process must still rely on more standard imaging protocols of MRI and PET.