Sleep in disorders of consciousness.

in Sleep Medicine Reviews (2009)

From a behavioral as well as neurobiological point of view, sleep and consciousness are intimately connected. A better understanding of sleep cycles and sleep architecture of patients suffering from ... [more ▼]

From a behavioral as well as neurobiological point of view, sleep and consciousness are intimately connected. A better understanding of sleep cycles and sleep architecture of patients suffering from disorders of consciousness (DOC) might therefore improve the clinical care for these patients as well as our understanding of the neural correlations of consciousness. Defining sleep in severely brain-injured patients is however problematic as both their electrophysiological and sleep patterns differ in many ways from healthy individuals. This paper discusses the concepts involved in the study of sleep of patients suffering from DOC and critically assesses the applicability of standard sleep criteria in these patients. The available literature on comatose and vegetative states as well as that on locked-in and related states following traumatic or non-traumatic severe brain injury will be reviewed. A wide spectrum of sleep disturbances ranging from almost normal patterns to severe loss and architecture disorganization are reported in cases of DOC and some patterns correlate with diagnosis and prognosis. At the present time the interactions of sleep and consciousness in brain-injured patients are a little studied subject but, the authors suggest, a potentially very interesting field of research. [less ▲]

Sleep in the unresponsive wakefulness syndrome and minimally conscious state

in Journal of Neurotrauma (2013), 30(5), 339-346

The goal of our study was to investigate different aspects of sleep, namely the sleep-wake cycle and sleep stages, in the vegetative state/unresponsive wakefulness syndrome (VS/UWS) and minimally ... [more ▼]

The goal of our study was to investigate different aspects of sleep, namely the sleep-wake cycle and sleep stages, in the vegetative state/unresponsive wakefulness syndrome (VS/UWS) and minimally conscious state (MCS). 24h polysomnography was performed in 20 patients in a UWS (n=10) or in a MCS (n=10) due to brain injury. The data were first tested for the presence of a sleep-wake cycle and the observed sleep patterns were compared to standard scoring criteria. Sleep spindles, slow waves sleep and rapid eye movement sleep were quantified and their clinical value was investigated. According to our results, an electrophysiological sleep-wake cycle was identified in 5 MCS and 3 VS/UWS patients. Sleep stages did not always match the standard scoring criteria which therefore needed to be adapted. Sleep spindles were more present in patients who clinically improved within 6 months. Slow wave sleep was present in 8 MCS and 3 VS/UWS patients but never in the ischemic etiology. Rapid eye movement sleep, and therefore dreaming which is a form of consciousness, was present in all MCS and 3 VS/UWS patients. In conclusion, the presence of alternating periods of eyes-open/eyes-closed cycles does not necessarily imply preserved electrophysiological sleep architecture in the UWS and MCS, contrary to previous definition. The investigation of sleep is a little studied yet simple and informative way to evaluate the integrity of residual brain function in patients with disorders of consciousness with possible clinical diagnostic and prognostic implications. [less ▲]

Sleep modulates the neural substrates of both spatial and contextual memory consolidation

in PLoS ONE (2008), 3(8), 2949

Sleep slow wave changes during the middle years of life

in European Journal of Neuroscience (2011), 33(4), 758-66

Slow waves (SW; < 4 Hz and > 75 muV) during non-rapid eye movement (NREM) sleep in humans are characterized by hyperpolarization [surface electroencephalogram (EEG) SW negative phase], during which ... [more ▼]

Slow waves (SW; < 4 Hz and > 75 muV) during non-rapid eye movement (NREM) sleep in humans are characterized by hyperpolarization [surface electroencephalogram (EEG) SW negative phase], during which cortical neurons are silent, and depolarization (surface EEG positive phase), during which the cortical neurons fire intensively. We assessed the effects of age, sex and topography on the dynamics of SW characteristics in a large population (n = 87) of healthy young (23.3 +/- 2.4 years) and middle-aged (51.9 +/- 4.6 years) volunteers. Older subjects showed lower SW density and amplitude than young subjects. Age-related lower SW density in men was especially marked in prefrontal/frontal brain areas, where they originate more frequently. Older subjects also showed longer SW positive and negative phase durations. These last results indicate that, in young subjects, cortical neurons would synchronously enter the SW hyperpolarization and depolarization phases, whereas this process would take longer in older subjects, leading to lower slope and longer SW positive and negative phases. Importantly, after controlling for SW amplitude, middle-aged subjects still showed lower slope than young subjects in prefrontal, frontal, parietal and occipital derivations. Age-related effects on SW density, frequency and positive phase duration were more prominent at the beginning of the night, when homeostatic sleep pressure is at its highest. Age-related SW changes may be associated with changes in synaptic density and white matter integrity and may underlie greater sleep fragmentation and difficulty in recuperating and maintaining sleep under challenges in older subjects. [less ▲]

Sleep Spindles Predict Neural and Behavioral Changes in Motor Sequence Consolidation

in Human Brain Mapping (2012), Epub ahead of print

Sleep transforms the cerebral trace of declarative memories

in Proceedings of the National Academy of Sciences of the United States of America (2007), 104(47), 18778-18783

After encoding, memory traces are initially fragile and have to be reinforced to become permanent. The initial steps of this process occur at a cellular level within minutes or hours. Besides this rapid ... [more ▼]

After encoding, memory traces are initially fragile and have to be reinforced to become permanent. The initial steps of this process occur at a cellular level within minutes or hours. Besides this rapid synaptic consolidation, systems consolidation occurs within a time frame of days to years. For declarative memory, the latter is presumed to rely on an interaction between different brain regions, in particular the hippocampus and the medial prefrontal cortex (mPFC). Specifically, sleep has been proposed to provide a setting that supports such systems consolidation processes, leading to a transfer and perhaps transformation of memories. Using functional MRI, we show that postlearning sleep enhances hippocampal responses during recall of word pairs 48 h after learning, indicating intrahippocampal memory processing during sleep. At the same time, sleep induces a memory-related functional connectivity between the hippocampus and the mPFC. Six months after learning, memories activated the mPFC more strongly when they were encoded before sleep, showing that sleep leads to long-lasting changes in the representation of memories on a systems level. [less ▲]

Sleep, memory and the hippocampus

in Clinical Neurobiology of the Hippocampus (2012)

Sleep-dependent consolidation of declarative memories is triggered by hippocampal activation at encoding

in Proceedings of the Annual Meeting of th Belgian Association for Psychological Science (2010)

Sleep-related hippocampo-cortical interplay during emotional memory recollection.

in PLoS Biology (2007), 5(11), 282

Emotional events are usually better remembered than neutral ones. This effect is mediated in part by a modulation of the hippocampus by the amygdala. Sleep plays a role in the consolidation of declarative ... [more ▼]

Emotional events are usually better remembered than neutral ones. This effect is mediated in part by a modulation of the hippocampus by the amygdala. Sleep plays a role in the consolidation of declarative memory. We examined the impact of sleep and lack of sleep on the consolidation of emotional (negative and positive) memories at the macroscopic systems level. Using functional MRI (fMRI), we compared the neural correlates of successful recollection by humans of emotional and neutral stimuli, 72 h after encoding, with or without total sleep deprivation during the first post-encoding night. In contrast to recollection of neutral and positive stimuli, which was deteriorated by sleep deprivation, similar recollection levels were achieved for negative stimuli in both groups. Successful recollection of emotional stimuli elicited larger responses in the hippocampus and various cortical areas, including the medial prefrontal cortex, in the sleep group than in the sleep deprived group. This effect was consistent across subjects for negative items but depended linearly on individual memory performance for positive items. In addition, the hippocampus and medial prefrontal cortex were functionally more connected during recollection of either negative or positive than neutral items, and more so in sleeping than in sleep-deprived subjects. In the sleep-deprived group, recollection of negative items elicited larger responses in the amygdala and an occipital area than in the sleep group. In contrast, no such difference in brain responses between groups was associated with recollection of positive stimuli. The results suggest that the emotional significance of memories influences their sleep-dependent systems-level consolidation. The recruitment of hippocampo-neocortical networks during recollection is enhanced after sleep and is hindered by sleep deprivation. After sleep deprivation, recollection of negative, potentially dangerous, memories recruits an alternate amygdalo-cortical network, which would keep track of emotional information despite sleep deprivation. [less ▲]

Sleeping Site Selection and Presleep Behavior in Wild Pigtailed Macaques

in American Journal of Primatology (2011), 73

Several factors are likely to control sleeping site selection and presleep behavior in nonhuman primates, including predation risk and location of food resources. We examined the effects of these factors ... [more ▼]

Several factors are likely to control sleeping site selection and presleep behavior in nonhuman primates, including predation risk and location of food resources. We examined the effects of these factors on the sleeping behavior of northern pigtailed macaques (Macaca leonina). While following a troop living in the surroundings of the Visitor Center of Khao Yai National Park (Thailand), we recorded the physical characteristics and location of each sleeping site, tree, the individuals’ place in the tree, posture, and behavior. We collected data for 154 nights between April 2009 and November 2010. The monkeys preferred tall sleeping trees (20.97SD 4.9 m) and high sleeping places (15.87SD 4.3 m), which may be an antipredator strategy. The choice of sleeping trees close to the last (146.77SD 167.9 m) or to the first (150.47SD 113.0 m) feeding tree of the day may save energy and decrease predation risk when monkeys are searching for food. Similarly, the choice of sleeping sites close to human settlements eases the access to human food during periods of fruit scarcity. Finally, the temporal pattern of use of sleeping sites, with a preference for four of the sleeping sites but few reuses during consecutive nights, may be a tradeoff between the need to have several sleeping sites (decreasing detection by predators and travel costs to feeding sites), and the need to sleep in well-known sites (guaranteeing a faster escape in case of predator attack). [less ▲]