Neurobiology of Dreams

Dreaming is a subjective experience created during sleep and accessible for recollection after awakening. Experimental studies, developed after the discovery of rapid eye movement (REM) sleep, indicate that, contrary to popular opinion, dreaming occurs, albeit with some quantitative and qualitative differences, during all stages and cycles of sleep. By monitoring sleep stages using electropolygraphic techniques, a successful dream recall is obtained after about 80% of awakenings from REM sleep and about 50% after NREM sleep in healthy subjects. Investigation on patients with acute cortical lesions has shown that the cessation of dreaming is caused by damage to either posterior brain areas, mostly unilateral, located in or near the temporo-parieto-occipital (TPO) junction, or anterior brain areas, mostly bilateral, in the ventromedial prefrontal cortex (vmPFC). Neuroimaging studies have both confirmed these findings and disclosed the role played in dream generation by some subcortical structures, such as the hippocampus and the amygdala. Recent advances in electrophysiological [e.g., surface high-density electroencephalographic (hd-EEG) and intracranial recordings], transcranial stimulation and neuroimaging techniques are providing relevant insights into the neural correlates of dreaming in healthy individuals and patients with brain damage, neurodegenerative diseases, sleep disorders, or parasomnias. These approaches are shedding light on the neural bases of (a) inter- and intraindividual differences in dream recall, (b) temporal localization of specific dream properties (e.g., lucidity), (c) how episodic and semantic memories are processed and incorporated into dream content, and (d) some dreamlike experiences (such as daydreaming and mind wandering) that occur while awake. These findings, which complement those from neuropsychological studies on brain-damaged patients, point to an overlap between functional and structural cerebral substrates of waking and REM sleep mental imagery.