Rev Neuroscience
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The diagnosis and management of patients with persistent vegetative (PVS) and minimally conscious (MCS) states entail powerful medical, ethical and legal debates. The recent description of the MCS highlights the crucial role of unexpected and well-documented recoveries of cognitive functions. ⋯ We present a review on this topic, emphasizing the clinical and neuroimaging assessment of these states, with some of our recent results in this area. We conclude that the development of rehabilitation techniques for patients with PVS and others suffering long-lasting effects of brain injury is a crucial challenge for actual and future generations of neuroscientists.
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Review Historical Article
Networks of conscious experience: computational neuroscience in understanding life, death, and consciousness.
We demonstrate brain locations appearing to correlate with consciousness, but not being directly responsible for it. Technology reveals that brain activity is associated with consciousness but is not equivalent to it. We examine how consciousness occurs at critical levels of complexity. ⋯ There is no threshold or rationale specified; rather, consciousness 'just happens'. Consciousness then involves an awareness of what we are sensing or experiencing and some ability to control or coordinate voluntary actions. These issues of life, death, and consciousness are discussed in the context of Mike, the headless chicken, who survived for 18 months, and in the context of consciousness with high degrees of intellectual and cognitive function in a congenitally anencephalic brain; additionally, in the reanimation work of Soviet scientists in the 1920-30s, and in auditory sentence processing in patients in comatose, vegetative, and minimally conscious states.
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There exists much controversy in providing an effective definition of human death, largely due to the lack of a rigorous separation and ordered formulation of three distinct elements: a universally accepted definition of death, the medical criterion (anatomical substrata) for determining that death has occurred, and the tests to prove that the criterion has been satisfied. The papers herein review medical standards, philosophical arguments, neurophysiological knowledge, behavioural and cognitive theory and the legal ramifications of the brain-oriented standards of death (whole brain, brainstem and higher brain). The papers examine the notion of connectivities and networks of conscious experience in order to formulate an effective definition of death, based on the basic physiopathological mechanisms of consciousness. ⋯ The thread of the arguments is the basis for a standard of human death that includes consciousness as the most important function of the body, because it provides the capacity for integrating the functions of the body. The notion of consciousness as the ultimate integrative function is more consistent with the biologically-based systems than the more philosophically-based notions of personhood. Both sides of the argument are presented herein.
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Placebo analgesia is one of the most robust and best-studied placebo effects. With the help of brain imaging tools, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), our understanding of the brain's role in placebo analgesia has been greatly expanded. Previous studies suggest that multiple mechanisms may underlie the phenomenon of placebo analgesia. ⋯ According to this framework, placebo treatment may exert an analgesic effect on at least three stages of pain processing, by 1) influencing pre-stimulus expectation of pain relief, 2) modifying pain perception, and 3) distorting post-stimulus pain rating. Importantly, change in one such stage may hasten change in another, and furthermore, contribution from any or all of the three stages may vary by circumstance, or between individuals. The literature suggests that multiple brain regions, including the anterior cingulate cortex, anterior insula, prefrontal cortex and periaqueductal grey, play a pivotal role in these processes.
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The adult brain retains the capacity to rewire mature neural circuits in response to environmental changes, brain damage or sensory and motor experiences. Two plastic processes, synaptic remodeling and neurogenesis, have been the subject of numerous studies due to their involvement in the maturation of the nervous system, their prevalence and re-activation in adulthood, and therapeutic relevance. However, most of the research looking for the mechanistic and molecular events underlying synaptogenic phenomena has been focused on the extensive synaptic reorganization occurring in the developing brain. ⋯ Several items of evidence support an interrelationship between NO and BDNF in the regulation of synaptic remodeling processes in adulthood: i) BDNF and its receptor TrkB are expressed by motoneurons and upregulated by axonal injury; ii) they promote axon arborization and synaptic formation, and modulate the structural dynamics of excitatory synapses; iii) NO and BDNF each control the production and activity of the other at the level of individual synapses; iv) the NO/cGMP pathway inhibits BDNF secretion; and finally, v) BDNF protects F-actin from depolymerization by NO, thus preventing the collapsing and retracting effects of NO on growth cones. Therefore, we propose a mechanism of action in which the NO/BDNF ratio regulates synapse dynamics after peripheral nerve lesion. This hypothesis also raises the possibility that variations in this NO/BDNF balance constitute a common hallmark leading to synapse loss in the progression of diverse neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's and Parkinson's diseases.