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This chapter reviews the techniques currently applied to study brain function during sleep deprivation (SD) as opposed to the consequence of SD. It provides a bird's eye view of functional imaging studies performed on healthy young adult volunteers to date and comment on how this research has evolved the conceptualization of how SD modulates behavior. The first functional imaging studies involving SD utilized positron emission tomography (PET). Based on the initial findings, cognitive domain and task difficulty was proposed as determinants of the neural response to SD. It was postulated that changes in dopamine signaling in the SD state contributed to the change in functional connectivity, an idea reprised when discussing risky decision making in SD. The interaction of SD and circadian effects, including the effects of chronotype, could be a further target of functional neuroimaging studies, including the effect of countermeasures such as naps and stimulants.
Radiotracer imaging methods such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET) are well suited to provide information about the functional, metabolic, and molecular status of tissues and organs. Brain SPECT has a well-established role for a number of clinical indications. Cerebral perfusion studies are used in the evaluation of dementias, epilepsy, cerebrovascular disease, trauma, brain death, and to assist with neuropsychiatric evaluation. Brain function is evaluated at baseline, before and after pharmacotherapy or psychotherapy, and following a number of activation tasks to examine a large number of psychiatric conditions. The integration of SPECT and CT in a single imaging device facilitates anatomical localization of the radiopharmaceutical to differentiate physiological uptake from that associated with disease. SPECT and SPECT/CT is continuing to evolve with the introduction of new technologies that have the potential to improve performance beyond that possible with Anger's pioneering approach.
Greater insights into the mechanisms and consequences of sleep and sleep disorders have been achieved through advances in brain imaging methods that describe various aspects of neural function. These are collectively referred to as functional neuroimaging. These include techniques such as PET, fMRI, single-photon emission computed tomography (SPECT), transcranial sonography, magnetoencephalography (MEG), low-resolution brain electromagnetic tomography (LORETA), and combined methods such as combined EEG and fMRI. Extensive applications of brain imaging have been made to help clarify the changes in regional brain function that result from perturbations in either homeostatic or circadian processes, and also have clarified the relationship between these brain changes and the behavioral consequences of these disruptions. The earliest applications of neuroimaging to the study of sleep disorders were those of functional neuroimaging methods to study the global brain states of waking, NREM, and REM sleep. Brain imaging studies have been utilized in narcolepsy and the hypersomnias.
This chapter looks at how different memory systems are influenced by sleep. It describes the currently most-widely accepted model of consolidation of hippocampus-dependent memory. The chapter also looks at human functional magnetic resonance imaging (fMRI) studies which provide evidence that, in fact, memories are re-activated, re-organized, and re-processed during sleep. Reactivation occurs during post-learning sleep, and it seems to be an important component of memory consolidation. In general, it has been found that it occurs in those brain regions most strongly related to the specific learning task. Re-activation could therefore support synaptic consolidation of memory traces. However, recent studies also provide more and more evidence for systems memory consolidation. Looking for signs of re-processing during sleep is the most difficult to do, because based on imaging data alone it is hard to distinguish from re-organization, and there are only few behavioral tasks that are designed to examine such changes.
This chapter discusses imaging studies in insomnia and in association with insomnia complaints in people not diagnosed with insomnia. This review includes studies applying structural and functional MRI, magnetic resonance spectroscopy (MRS), high-density electroencephalography, and transcranial magnetic stimulation (TMS). The studies reviewed have reported almost exclusively on regions of the temporal lobe, frontal lobe, and parietal lobe. These cortical regions are of interest because of their key involvement in the cognitive domains that are most affected in insomnia and after sleep deprivation. For each lobe, the chapter systematically addresses differences between insomniacs and controls and correlations of insomnia symptom severity with brain changes in both insomniacs and people not diagnosed with insomnia. Subsequently, the findings are summarized and interpreted with respect to functional relevance, pitfalls, and conclusions on cause, risk factor, or consequence. Neuroimaging has a high promise to reveal insights into the causes and consequences of insomnia.
This chapter reports results on spontaneous brain activity during wakeful rest. It focuses on findings obtained with electroencephalography (EEG) and functional magnetic resonance imaging (fMRI), either acquired separately or simultaneously. The chapter discusses two approaches to analyze resting state activity with fMRI, namely reverse subtraction (activity correlated with task deactivation) and independent component analysis (ICA)-based region analysis. A third way to explore resting state blood oxygen level-dependent (BOLD) activity is to add information obtained with independent recording modalities, in particular EEG. When EEG and fMRI are recorded simultaneously, fMRI activity patterns associated with EEG-defined brain states can be analyzed. Relaxed wakefulness in the EEG is characterized by alpha and beta band oscillations. A thorough analysis of EEG and fMRI patterns during wakeful rest yields a complex relationship where certain EEG patterns can be associated with different BOLD maps and vice versa.
This up-to-date, superbly illustrated book is a practical guide to the effective use of neuroimaging in the patient with sleep disorders. There are detailed reviews of new neuroimaging techniques – including CT, MRI, advanced MR techniques, SPECT and PET – as well as image analysis methods, their roles and pitfalls. Neuroimaging of normal sleep and wake states is covered plus the role of neuroimaging in conjunction with tests of memory and how sleep influences memory consolidation. Each chapter carefully presents and analyzes the key findings in patients with sleep disorders indicating the clinical and imaging features of the various sleep disorders from clinical presentation to neuroimaging, aiding in establishing an accurate diagnosis. Written by neuroimaging experts from around the world, Neuroimaging of Sleep and Sleep Disorders is an invaluable resource for both researchers and clinicians including sleep specialists, neurologists, radiologists, psychiatrists, psychologists.
The organization of regional brain function during human rapid eye movement sleep (REMS) can be characterized at the macroscopic systems level by functional neuroimaging techniques. Several aspects of REMS have been investigated. During REMS, forebrain activation pattern is characterized by a hyperactivity in posterior cortical areas and regions of the limbic and paralimbic system, contrasting with a relative quiescence of the polymodal associative cortices of the lateral frontal and parietal cortices. This activity pattern has been related to the main characteristic of dreams. The activity associated with rapid eye movements has been identified in the thalamus and primary visual cortex, suggesting the existence of ponto-geniculo-occipital (PGO) waves in humans. The variability of heart rate during REMS is associated with the activity in the extended amygdala, suggesting a specific organization of autonomic regulation during REMS. The distribution of regional brain activity during REMS was shown to depend on experience acquired during previous wakefulness. Training on a serial reaction time task induces an increase in activity in the brain stem, thalamus, occipital, and premotor areas during subsequent REMS. These data suggest that REMS is implicated in offline memory processing. With the advent of multimodal functional imaging (electroencephalography/functional magnetic resonance imaging (EEG/fMRI), transcranial magnetic stimulation/ electroencephalography (TMS/EEG), and multichannel electroencephalography (MEEG)), a finer grain characterization of human REMS will lead to a better understanding of this intriguing state of vigilance.
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