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The Neurobiology of Dreaming
Edward F. Pace-Schott
Dreaming is a universal human mental state characterized by
hallucinatory imagery congruent with a confabulated, tempo-
rally ordered, storylike experience. As in waking conscious-
ness, such experiences in both rapid eye movement (REM) and
non-REM (NREM) sleep are associated with activation of fore-
brain structures by ascending arousal systems of the brain-
stem, hypothalamus, and basal forebrain. Differences between
forebrain activation patterns in waking and REM sleep suggest
bases for their phenomenological differences. In REM com-
pared to waking, there is relatively more activation of the
brain's limbic system and relatively less activity of cortical
areas involved in higher-level cognition. REM sleep dreaming
may activate anterior and midline portions of the brain's
“default mode,” a network of structures that supports self-
related cognition when the brain is unoccupied by external
stimuli. A variety of neurochemical systems can influence
dreaming, including the neuromodulators acetylcholine,
dopamine, serotonin, and norepinephrine. Dream phenom-
ena are strikingly similar to neuropsychiatric symptom com-
plexes such as complex hallucinosis and spontaneous
confabulation as well as delirium and misidentification syn-
dromes. A descriptive model of dream generation links sleep
imaging studies with known regional specialization in waking
neurocognitive functions.
recall of mental experiences from non-REM (NREM)
sleep. 3 Nonetheless, REM reports are more frequent,
longer, more bizarre, more visual, more motoric, and
more emotional than are NREM reports (see review in
Hobson et al. 4 ). In an extensive review, Nielsen estimates
an NREM mental-experience recall rate of 42.5% con-
trasting with 81.8% from REM and suggests that brain
activation processes occurring outside polysomnographi-
cally scored REM (“covert REM”) may account for NREM
dreaming. 5
Dreaming is a universal human experience occurring
during sleep in which fictive events follow one another in
an organized, storylike manner and into which are woven
hallucinatory, primarily visual, images that are largely con-
gruent with an ongoing confabulated plot. Most often, this
wholly imaginary experience is uncritically accepted in the
same manner as are veridical waking percepts and events.
The neuroscientific significance of dreams becomes appar-
ent by considering just two of the many remarkable aspects
of this universally human mental state. The first, dream-
ing's “single-mindedness and isolation,” was eloquently
described by Rechtschaffen, 1 and refers to the dreamer's
absorption in the dream world and plot without awareness
of an alternate reality in waking excepting rare “lucidity”
(the awareness that one is dreaming). From this condition
of limited insight, upon awakening, one abruptly regains
sufficient insight to conceptualize alternate states of mind
and decide which of these one is currently experiencing.
The second is the occurrence in dreams of entirely de novo
imagery, plots, personages, and even motor skills (e.g.,
lying), emotion (e.g., religious feelings) and memory (e.g.,
deja vu). Therefore, dreaming is an imprecise experiential
simulacrum of waking resulting from neurobiological pro-
cesses that must differ from those that generate waking
consciousness. Because no published functional neuroim-
aging study has yet awoken subjects during dreaming to
link subjectively experienced features of a dream report
with immediately preceding brain activity, the wealth of
information on the cognitive neuroscience of waking and
the neurophysiology of sleep must be related to the less
experimentally accessible dream state.
Recent Electrophysiological Findings
REM sleep shows much more gamma frequency (30 to
80 Hz), fast brain waves (“oscillations” or “rhythms”) than
does NREM sleep as measured by scalp electroencepha-
lography (EEG), 6 , 7 intracranial EEG (iEEG) 8 , 9 and mag-
netoencephalography (MEG). 6 In waking, these fast
oscillations are associated with attention to stimuli and
other forms of active or effortful cognition. 10 During REM
dreaming, fast oscillations have been hypothetically associ-
ated with cognitive and perceptual processing, 11 memory
processes 12 , 13 and the temporal binding of dream imagery. 14
In contrast to REM sleep, human slow-wave sleep shows
very little gamma activity. It is instead associated with
slower oscillations produced by recurrent interactions
between the thalamus and cortex (intrinsic “corticothala-
mocortical” rhythms) such as sleep spindles and delta
waves, and with the cortical slow (<1 Hz) oscillation that
groups in time the other corticothalamocortical oscilla-
tions. 15 , 16 The slow oscillation consists of periods of neu-
ronal quiescence (hyperpolarized or “down” states) that
alternate with shorter periods of rapid neuronal firing
(depolarized or “up” states). 17 Slow intrinsic oscillations
may interfere with ongoing mental activity and lead to a
lower frequency of dreams in NREM sleep. 4
Early speculation that rapid eye movement (REM) sleep
was the exclusive physiological substrate of dreaming 2 was
soon followed by awakening studies showing substantial
Declines in phase synchrony (“coherence”) of EEG
rhythms between different brain regions occurring
during sleep relative to waking may reflect functional
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