Annals of the New York Academy of Sciences
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Expectations, positive or negative, are modulating factors influencing behavior. They are also thought to underlie placebo effects, potentially impacting perceptions and biological processes. We used sustained pain as a model to determine the neural mechanisms underlying placebo-induced analgesia and affective changes in healthy humans. ⋯ Both dopamine and opioid neurotransmission were related to expectations of analgesia and deviations from those initial expectations. When the activity of the nucleus accumbens was probed with fMRI using a monetary reward expectation paradigm, its activation was correlated with both dopamine, opioid responses to placebo in this region and the formation of placebo analgesia. These data confirm that specific neural circuits and neurotransmitter systems respond to the expectation of benefit during placebo administration, inducing measurable physiological changes.
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Ann. N. Y. Acad. Sci. · Mar 2009
ReviewExploring altered consciousness states by magnetic resonance imaging in brain injury.
Traumatic brain injury (TBI) occurs abruptly, involves multiple specialized teams, calls on the health-care system in its emergency dimension, and engages the well-being of the patient and his relatives for a lifetime period. Clinicians in charge of these patients are faced with issues of uppermost importance: medical issues such as predicting the long-term neurological outcome of the comatose patient; ethical issues because of the influence of intensive care on the long-term survival of patients in a vegetative and minimally conscious state; legal issues because of the law that has set the concept of proportionality of care as the legal rule; and social issues as the result of the very high cost of these pathologies. Today's larger availability of magnetic resonance imaging (MRI) in ventilated patients and the recent improvements in hardware and in imaging techniques that have made the last-developed imaging techniques such as diffusion tensor imaging and magnetic resonance spectroscopy available in brain-trauma patients, are changing the paradigm in neurointensive care regarding outcome prediction. ⋯ This major change opens new challenging ethical questions. This review focuses on the brain explorations that are required, such as MRI, magnetic resonance spectroscopy, and diffusion tensor imaging, to provide the clinician with a multimodal assessment of the brain state to predict outcome of coma. Such an assessment will become mandatory in the near future to answer the crucial question of proportionality of care in these patients.
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Ann. N. Y. Acad. Sci. · Mar 2009
Simultaneous electroencephalography and functional magnetic resonance imaging of general anesthesia.
It has been long appreciated that anesthetic drugs induce stereotyped changes in electroencephalogram (EEG), but the relationships between the EEG and underlying brain function remain poorly understood. Functional imaging methods including positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), have become important tools for studying how anesthetic drugs act in the human brain to induce the state of general anesthesia. To date, no investigation has combined functional MRI with EEG to study general anesthesia. ⋯ We discuss the several technical and safety problems that must be solved to undertake this type of multimodal functional imaging and show combined recordings from a human subject. Combined fMRI and EEG exploits simultaneously the high spatial resolution of fMRI and the high temporal resolution of EEG. In addition, combined fMRI and EEG offers a direct way to relate established EEG patterns induced by general anesthesia to changes in neural activity in specific brain regions as measured by changes in fMRI blood oxygen level dependent (BOLD) signals.
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Ann. N. Y. Acad. Sci. · Mar 2009
ReviewCentral thalamic deep-brain stimulation in the severely injured brain: rationale and proposed mechanisms of action.
This review outlines the scientific rationale supporting the potential use of deep-brain electrical stimulation (DBS) in the central thalamus as a method to improve behavioral responsiveness following severe brain injury. Neurons within the central thalamus are selectively vulnerable to disconnection and dysfunction following severe brain injuries because of their unique geometry of cerebral connections. Because the central thalamus plays a key role in forebrain arousal regulation, impaired function of these cells has a broad impact. ⋯ Here important differences in conceptual framework, consideration of diagnostic categories for patient selection, and anticipated mechanisms of effect that distinguish earlier approaches and current studies are reviewed. As opposed to targeting chronically unresponsive patients, current efforts focus on identification of conscious patients with significant preservation of large-scale integrative cerebral networks. The potential mechanisms and limitations of this evolving strategy are discussed, including the need to develop frameworks to calibrate patient selection to potential clinical benefits, range of potential effect size, and other present unknowns.
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Although the precise mechanisms for control of consciousness are not fully understood, emerging data show that conscious information processing depends on the activation of certain networks in the brain and that the impairment of consciousness is related to abnormal activity in these systems. Epilepsy can lead to transient impairment of consciousness, providing a window into the mechanisms necessary for normal consciousness. ⋯ We discuss a "network inhibition hypothesis" in which focal temporal lobe seizure activity disrupts normal cortical-subcortical interactions, leading to depressed neocortical function and impaired consciousness. This review of the major prior theories of impaired consciousness in epilepsy allows us to put more recent data into context and to reach a better understanding of the mechanisms important for normal consciousness.