Annals of the New York Academy of Sciences
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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.
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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
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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The rapid advancement of neuroimaging methodology and its growing availability has transformed neuroscience research. The answers to many questions that we ask about how the brain is organized depend on the quality of data that we are able to obtain about the locations, dynamics, fluctuations, magnitudes, and types of brain activity and structural changes. In this review an attempt is made to take a snapshot of the cutting edge of a small component of the very rapidly evolving field of neuroimaging. ⋯ Then, several outstanding papers, published in the past year or so, are described, providing an example of the directions in which each area is progressing. The areas covered include functional magnetic resonance imaging (fMRI), voxel-based morphometry (VBM), diffusion tensor imaging (DTI), electroencephalography (EEG), magnetoencephalography (MEG), optical imaging, and positron emission tomography (PET). More detail is included on fMRI; its subsections include fMRI interpretation, new fMRI contrasts, MRI technology, MRI paradigms and processing, and endogenous oscillations in fMRI.
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Ann. N. Y. Acad. Sci. · Mar 2009
ReviewEthical issues in the treatment of severe brain injury: the impact of new technologies.
Technological developments in functional neuroimaging have important ethical implications for the care of brain-injured patients. Patterns of fMRI and PET responses to stimuli may help clarify if a patient is utterly unaware, and thereby enhance a physician's confidence in reaching an accurate diagnosis of vegetative state or minimally conscious state. The analysis of similar responses may enhance a physician's confidence in pronouncing an accurate prognosis for functional recovery and help avoid committing the fallacy of the self-fulfilling prophesy. ⋯ Advance care planning can help inform surrogate decision making, but is available less commonly among young, previously healthy brain-injured patients. Functional neuroimaging technologies also impact on ethical issues of treatment, rehabilitation, and palliation. Families of brain-injured patients should be compassionately counseled that, despite provocative and highly publicized case reports, these technologies, while promising, are currently investigational and have not been sufficiently validated yet to be available for routine clinical use.