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Cortical mechanisms of loss of consciousness: insight from TMS/EEG studies
No full textIn a recent series of experiments we recorded the electroencephalogram (EEG) response to a direct cortical stimulation in humans during wakefulness, NREM sleep, REM sleep and anesthesia by means of a combination of transcranial magnetic stimulation (TMS) and high-density EEG (hd-EEG). TMS/hd-EEG measurements showed that, while during wakefulness and REM sleep the brain is able to sustain long-range specific patterns of activation, during NREM sleep and Midazolam-induced anesthesia, when consciousness fades, this ability is lot: the thalamocortical system, despite being active and reactive, either breaks down in causally independent modules (producing a local slow wave), or it bursts into an explosive and non-specific response (producing a global EEG slow wave). We hypothesize that, like spontaneous sleep slow waves, the slow waves triggered by TMS during sleep and anaesthesia are due to bistability between upand down-states in thalamocortical circuits. In this condition, the inescapable occurrence of a silent, down state after an initial activation impairs the ability of thalamocortical circuits to sustain long-range, differentiated patterns of activation, a theoretical requisite for consciousness. According to animal experiments and computer simulations, thalamocortical bistability may result from increased K-currents, from alterations of the balance between excitation and inhibition and from partial cortical de-afferentation. We hypothesize that these factor may play an important role in determining loss, and recovery, of consciousness also in brain-injured subjects. If this is the case, some types of brain lesions may impair information transmission, above and beyond the associated anatomical disconnection, by inducing bistability in portions of the thalamocortical system that are otherwise health
Sleep spindle deficit in schizophrenia: contextualization of recent findings
No full textSleep spindles are wax and waning brain oscillations at a frequency range of 11–16 Hz, lasting 0.5–2 s, that define non-rapid eye movement sleep stage 2. Over the past few years, several independent studies pointed to a decrease of sleep spindles in schizophrenia. The aim of this review is to contextualize these findings within the growing literature on these oscillations across other neuro-psychiatric disorders. Indeed, spindles reflect the coordinated activity of thalamocortical networks, and their abnormality can be observed in a variety of conditions that disrupt local or global thalamocortical connectivity. Although the broad methodological variability across studies limits the possibility of drawing firm conclusions, impaired spindling activity has been observed in several neurodevelopmental and neurodegenerative disorders. Despite such lack of specificity, schizophrenia remains the only condition with a typical late adolescence to young adulthood onset in which impaired spindling has been consistently reported. Further research is necessary to clearly define the pathogenetic mechanisms that lead to this deficit and the validity of its widespread use as a clinical biomarker
The sleep slow oscillation as a traveling wave
No full textDuring much of sleep, virtually all cortical neurons undergo a slow oscillation (<1 Hz) in membrane potential, cycling from a hyperpolarized state of silence to a depolarized state of intense firing. This slow oscillation is the fundamental cellular phenomenon that organizes other sleep rhythms such as spindles and slow waves. Using high-density electroencephalogram recordings in humans, we show here that each cycle of the slow oscillation is a traveling wave. Each wave originates at a definite site and travels over the scalp at an estimated speed of 1.2-7.0 m/sec. Waves originate more frequently in prefrontal-orbitofrontal regions and propagate in an anteroposterior direction. Their rate of occurrence increases progressively reaching almost once per second as sleep deepens. The pattern of origin and propagation of sleep slow oscillations is reproducible across nights and subjects and provides a blueprint of cortical excitability and connectivity. The orderly propagation of correlated activity along connected pathways may play a role in spike timing-dependent synaptic plasticity during sleep
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Non-fluent aphasia and neural reorganization after speech therapy: insights from human sleep electrophysiology and functional magnetic resonance imaging
No full textStroke is associated with long-term functional deficits. Behavioral interventions are often effective in promoting functional recovery and plastic changes. Recent studies in normal subjects have shown that sleep, and particularly slow wave activity (SWA), is tied to local brain plasticity and may be used as a sensitive marker of local cortical reorganization after stroke. In a pilot study, we assessed the local changes induced by a single exposure to a therapeutic session of IMITATE (Intensive Mouth Imitation and Talking for Aphasia Therapeutic Effects), a behavioral therapy used for recovery in patients with post-stroke aphasia. In addition, we measured brain activity changes with functional magnetic resonance imaging (fMRI) in a language observation task before, during and after the full IMITATE rehabilitative program. Speech production improved both after a single exposure and the full therapy program as measured by the Western Aphasia Battery (WAB) Repetition subscale. We found that IMITATE induced reorganization in functionally-connected, speech-relevant areas in the left hemisphere. These preliminary results suggest that sleep hd-EEGs, and the topographical analysis of SWA parameters, are well suited to investigate brain plastic changes underpinning functional recovery in neurological disorders
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Local experience-dependent changes in the wake EEG after prolonged wakefulness
No full textSTUDY OBJECTIVES: Prolonged wakefulness leads to a progressive increase in sleep pressure, reflected in a global increase in slow wave activity (SWA, 0.5-4.5 Hz) in the sleep electroencephalogram (EEG). A global increase in wake theta activity (5-9 Hz) also occurs. Recently, it was shown that prolonged wakefulness in rodents leads to signs of "local sleep" in an otherwise awake brain, accompanied by a slow/theta wave (2-6 Hz) in the local EEG that occurs at different times in different cortical areas. Compelling evidence in animals and humans also indicates that sleep is locally regulated by the amount of experience-dependent plasticity. Here, we asked whether the extended practice of tasks that involve specific brain circuits results in increased occurrence of local intermittent theta waves in the human EEG, above and beyond the global EEG changes previously described.
DESIGN: Participants recorded with high-density EEG completed 2 experiments during which they stayed awake ≥ 24 h practicing a language task (audiobook listening [AB]) or a visuomotor task (driving simulator [DS]).
SETTING: Sleep laboratory.
PATIENTS OR PARTICIPANTS: 16 healthy participants (7 females).
INTERVENTIONS: Two extended wake periods.
MEASUREMENTS AND RESULTS: Both conditions resulted in global increases in resting wake EEG theta power at the end of 24 h of wake, accompanied by increased sleepiness. Moreover, wake theta power as well as the occurrence and amplitude of theta waves showed regional, task-dependent changes, increasing more over left frontal derivations in AB, and over posterior parietal regions in DS. These local changes in wake theta power correlated with similar local changes in sleep low frequencies including SWA.
CONCLUSIONS: Extended experience-dependent plasticity of specific circuits results in a local increase of the wake theta EEG power in those regions, followed by more intense sleep, as reflected by SWA, over the same areas
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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