Thien Thanh Dang Vu

Sleep Promotes the Neural Reorganization of Remote Emotional Memory

Sleep promotes memory consolidation, a process by which fresh and labile memories are reorganized into stable memories. Emotional memories are usually better remembered than neutral ones, even at long retention delays. In this study, we assessed the influence of sleep during the night after encoding onto the neural correlates of recollection of emotional memories 6 months later. After incidental encoding of emotional and neutral pictures, one-half of the subjects were allowed to sleep, whereas the others were totally sleep deprived, on the first postencoding night. During subsequent retest, functional magnetic resonance imaging sessions taking place 3 d and 6 months later, subjects made recognition memory judgments about the previously studied and new pictures. Between these retest sessions, all participants slept as usual at home. At 6 month retest, recollection was associated with significantly larger responses in subjects allowed to sleep than in sleep-deprived subjects, in the ventral medial prefrontal cortex (vMPFC) and the precuneus, two areas involved in memory retrieval, as well as in the extended amygdala and the occipital cortex, two regions the response of which was modulated by emotion at encoding. Moreover, the functional connectivity was enhanced between the vMPFC and the precuneus, as well as between the extended amygdala, the vMPFC, and the occipital cortex in the sleep group relative to the sleep-deprived group. These results suggest that sleep during the first postencoding night profoundly influences the long-term systems-level consolidation of emotional memory and modifies the functional segregation and integration associated with recollection in the long term.

A prominent role for amygdaloid complexes in the Variability in Heart Rate (VHR) during Rapid Eye Movement (REM) sleep relative to wakefulness.

Rapid eye movement sleep (REMS) is associated with intense neuronal activity, rapid eye movements, muscular atonia and dreaming. Another important feature in REMS is the instability in autonomic, especially in cardiovascular regulation. The neural mechanisms underpinning the variability in heart rate (VHR) during REMS are not known in detail, especially in humans. During wakefulness, the right insula has frequently been reported as involved in cardiovascular regulation but this might not be the case during REMS. We aimed at characterizing the neural correlates of VHR during REMS as compared to wakefulness and to slow wave sleep (SWS), the other main component of human sleep, in normal young adults, based on the statistical analysis of a set of H(2)(15)O positron emission tomography (PET) sleep data acquired during SWS, REMS and wakefulness. The results showed that VHR correlated more tightly during REMS than during wakefulness with the rCBF in the right amygdaloid complex. Moreover, we assessed whether functional relationships between amygdala and any brain area changed depending the state of vigilance. Only the activity within in the insula was found to covary with the amygdala, significantly more tightly during wakefulness than during REMS in relation to the VHR. The functional connectivity between the amygdala and the insular cortex, two brain areas involved in cardiovascular regulation, differs significantly in REMS as compared to wakefulness. This suggests a functional reorganization of central cardiovascular regulation during REMS.

Spontaneous neural activity during human non-rapid eye movement sleep.

Recent neuroimaging studies characterized the neural correlates of slow waves and spindles during human non-rapid eye movement (NREM) sleep. They showed that significant activity was consistently associated with slow (> 140 μV) and delta waves (75-140 μV) during NREM sleep in several cortical areas including inferior frontal, medial prefrontal, precuneus, and posterior cingulate cortices. Unexpectedly, slow waves were also associated with transient responses in the pontine tegmentum and in the cerebellum. On the other hand, spindles were associated with a transient activity in the thalami, paralimbic areas (anterior cingulate and insular cortices), and superior temporal gyri. Moreover, slow spindles (11-13 Hz) were associated with increased activity in the superior frontal gyrus. In contrast, fast spindles (13-15 Hz) recruited a set of cortical regions involved in sensorimotor processing, as well as the mesial frontal cortex and hippocampus. These findings indicate that human NREM sleep is an active state during which brain activity is temporally organized by spontaneous oscillations (spindles and slow oscillation) in a regionally specific manner. The functional significance of these NREM sleep oscillations is currently interpreted in terms of synaptic homeostasis and memory consolidation.

Functional neuroimaging in sleep, sleep deprivation, and sleep disorders.

Publisher Summary This chapter presents the neuroimaging studies conducted during normal sleep. The studies using positron emission tomography (PET), single photon emission computed tomography (SPECT) or functional magnetic resonance imaging (fMRI) is reviewed in the chapter. They show that global and regional patterns of brain activity during sleep are outstandingly different from wakefulness. These studies also demonstrated the persistence of brain responses to external stimuli during sleep as well as plastic changes in brain activity related to previous waking experience. The chapter discusses that brain functional imaging in patients affected by sleep disorders may address different kinds of issues. The first topic is the characterization of the cerebral aftermath of sleep disruption due to intrinsic sleep disorders or to extrinsic environmental or medical causes. The second, more ambitious aim would be to characterize better the primary physiopathological mechanisms of sleep disorders, or at least their cerebral correlates. This attempt is hampered by several factors described in the chapter.

Sleep and Sleep States: PET Activation Patterns

During the past few decades, the development of functional neuroimaging techniques has provided an important tool for noninvasive investigation in humans, allowing us to detect variations in cerebral blood flow or metabolism, notably throughout the sleep–wake cycle. Up to now, positron emission tomography (PET) has been the most commonly used technique during normal human sleep. PET studies showed that the brain activity during sleep is segregated at the regional level within specific cortical and subcortical areas in relation to the sleep stage, sleep physiological events, and previous waking activity. These results lend support to physiological theories of sleep based on animal data but also introduce original concepts about the neurobiological mechanisms of sleep, dreams, and memory in humans.

Sleep : Implications for Theories of Dreaming and Consciousness

Sleep is composed of two main stages, Rapid Eye Movement (REM) sleep and non-REM sleep, each of them generated by specific cerebral networks. REM sleep is the behavioral state that produces the greatest recall and intensity of dreaming. Moreover, correlations between functional patterns of brain activity during REM sleep and specific properties of dream content have been emphasized, in an attempt to characterize the neural basis of dreaming. Fading of consciousness occurs in non-REM sleep, in parallel with a breakdown in cortical connectivity, but does not prevent external sensory information to be processed in the sleeping brain.

An in computo investigation of the Landau-Kleffner syndrome

We describe a computational model of the thalamus and the cortex able to reproduce some essential epileptiform features commonly observed in the Landau-Kleffner syndrome. Investigation with this realistic model leads us to the formulation of a cellular mechanism that could be responsible for the epileptic discharges occuring with this severe syndrome. Understanding this mechanism is of prime importance for developing new therapeutical strategies.