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would be related to the activation of posterior (occipital
and temporal) cortices, whereas emotional features in
dreams would be related to the activation of amygdalar
complexes, orbitofrontal cortex, and anterior cingulate
cortex. The recruitment of mesiotemporal areas would
account for the memory content commonly observed in
dreams. The relative hypoactivation of the prefrontal
cortex might help to explain the alteration in logical rea-
soning, working memory, episodic memory, and executive
functions characterizing dream reports collected from
experimentally induced REM sleep awakenings. Activation
of the anterior cingulate cortex and surrounding mesial
prefrontal cortex has been described, in studies on waking
cognitive neuroscience, to be related to self-referential
cognition and to the monitoring of performance. Activa-
tion of these structures within REM sleep might represent
a role for REM sleep in the internal monitoring of aspects
of the self, especially those having emotional significance
given the activation of other related limbic and paralimbic
structures. 33
retrieval on the next day, suggesting that hippocampal
activity during NREM sleep is related to offline processing
of spatial memory. 40 Similarly, several brain areas activated
during the execution of a serial reaction time task during
wake (brainstem, thalamus, and occipital, parietal and pre-
motor areas) are significantly more active during REM
sleep in subjects previously trained on the task in compari-
son to nontrained subjects. 41 This enhancement of regional
brain activity during posttraining REM sleep is observed
only when the material to learn is presented in a structured
manner as compared to random presentation. 42 It is then
suggested to be associated with significant changes in brain
functional connectivity. 43 Collectively, these results support
the hypothesis that the memory of a motor sequence is
further processed during REM sleep in humans ( Fig.
18-4 ). (For more information on memory processing in
relation to sleep, see Chapter 29.)
BRAIN IMAGING AND NEURAL
CORRELATES OF SLEEP-WAKE CYCLE
REGULATION
The timing of sleep and wake episodes is thought to be
regulated by the interaction between the homeostatic sleep
pressure and an intrinsic circadian oscillation. 44 One study
compared regional relative brain glucose metabolism
between morning and evening wakefulness in healthy
humans in order to define the mechanisms that maintain
wakefulness across the day, in relation to the increasing
sleep drive that accumulates over the wake period (see
Chapter 37 for more information of sleep and wake
process). Brain scans, using 18 F-FDG PET, were con-
ducted during quiet wakefulness in the morning and in the
evening in 13 healthy adults (10 women, 3 men; mean age,
37 years). As expected, subjective ratings of alertness were
lower in the evening than in morning. Conversely, relative
regional glucose metabolism was significantly higher in the
evening than in the morning in a large cluster of midline
and brainstem structures. More specifically, changes were
localized in the pontine and midbrain reticular formation,
midbrain raphe, locus coeruleus, and posterior hypothala-
mus. Note that evening wakefulness is associated with
increased relative metabolism in brainstem and hypotha-
lamic arousal systems and decreased relative metabolism in
posterior cortical regions. These patterns might reflect
input from the circadian timing system(s) to promote
wakefulness, or they might reflect the effects of increasing
homeostatic sleep drive (see Section 5 of this volume).
The activity of suprachiasmatic nucleus, the master cir-
cadian clock, is influenced by external temporal markers
(zeitgebers), the most important of which is light. In addi-
tion to vision, light profoundly affects human physiology
and modulates sleep-wake cycles, body temperature, endo-
crine functions, alertness, and performance. 44 Animal and
human studies demonstrated that a nonvisual photorecep-
tion system mediates these effects, which include the
synchronization of the circadian system, suppression of
melatonin, regulation of sleep, and improvements in alert-
ness and cognition. 45-49 This photoreception system recruits
the retinal photoreceptors (rods and cones) and intrinsi-
cally photosensitive retinal ganglion cells expressing mela-
nopsin. 50 , 51 These retinal ganglion cells project to numerous
BRAIN IMAGING AND OTHER CHARACTERISTIC FEATURES
OF REM SLEEP
Rapid eye movements constitute a prominent feature of
REM sleep. Cerebral mechanisms underpinning the gen-
eration of spontaneous ocular movements differ between
REM sleep and wakefulness in humans. Regional cerebral
blood low changes in the lateral geniculate bodies and in
the striate cortex are significantly more correlated to ocular
movement density during REM sleep than during wakeful-
ness, 35 a pattern later confirmed using fMRI. 36 This pattern
of activity is reminiscent of ponto-geniculo-occipital
(PGO) waves, prominent phasic bioelectrical potentials
associated with eye movements, which occur in isolation
or in bursts just before and during REM sleep and are most
easily recorded in cats and rats in the mesopontine teg-
mentum, the lateral geniculate bodies, and the occipital
cortex. 37
Another important feature in REM sleep is the instabil-
ity in autonomic regulation and especially in cardiovascu-
lar regulation. During wake, the right insula is involved
in cardiovascular regulation 38 but during REM sleep,
the variability in heart rate is related to the activity in the
right amygdaloid complex. 39 The functional connectivity
between the amygdala and the insular cortex, two brain
areas involved in cardiovascular regulation, differ signifi-
cantly in REM sleep as compared to wake. 39 These results
suggest a functional reorganization of central cardiovascu-
lar regulation during REM sleep.
Experience-Dependent Modifications of
Regional Brain Function during NREM
and REM Sleep
Waking experience substantially influences regional brain
activity during subsequent sleep. For instance, the blood
low of the hippocampus and parahippocampal gyrus
during NREM sleep is increased in subjects who were
navigating in a virtual town during the previous waking
period, as compared to naive participants. 40 The level of
hippocampal activity expressed during SWS positively cor-
relates with the improvement of performance in route
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