118 research outputs found

    Large-scale behaviour of local and entanglement entropy of the free Fermi gas at any temperature

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    The leading asymptotic large-scale behaviour of the spatially bipartite entanglement entropy (EE) of the free Fermi gas infinitely extended in multidimensional Euclidean space at zero absolute temperature, T=0, is by now well understood. Here, we present and discuss the first rigorous results for the corresponding EE of thermal equilibrium states at T>0. The leading large-scale term of this thermal EE turns out to be twice the first-order finite-size correction to the infinite-volume thermal entropy (density). Not surprisingly, this correction is just the thermal entropy on the interface of the bipartition. However, it is given by a rather complicated integral derived from a semiclassical trace formula for a certain operator on the underlying one-particle Hilbert space. But in the zero-temperature limit the leading large-scale term of the thermal EE considerably simplifies and displays a ln(1/T)-singularity which one may identify with the known logarithmic enhancement at T=0 of the so-called area-law scaling.Comment: The paper has been slightly rewritten to improve its readability. In particular, we now restrict ourselves to von Neumann entropies. The title and the abstract have been changed. The references have been updated. 10 page

    Anesthetic-induced unresponsiveness: Electroencephalographic correlates and subjective experiences

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    Anesthetic drugs can induce reversible alterations in responsiveness, connectedness and consciousness. The measures based on electroencephalogram (EEG) have marked potential for monitoring the anesthetized state because of their relatively easy use in the operating room. In this study, 79 healthy young men participated in an awake experiment, and 47 participants continued to an anesthesia experiment where they received either dexmedetomidine or propofol as target-controlled infusion with stepwise increments until the loss of responsiveness. The participants were roused during the constant drug infusion and interviewed. The drug dose was increased to 1.5-fold to achieve a deeper unresponsive state. After regaining responsiveness, the participants were interviewed. EEG was measured throughout the experiment and the N400 event-related potential component and functional and directed connectivity were studied. Prefrontal-frontal connectivity in the alpha frequency band discriminated the states that differed with respect to responsiveness or drug concentration. The net direction of connectivity was frontal-to-prefrontal during unresponsiveness and reversed back to prefrontal-to-frontal upon return of responsiveness. The understanding of the meaning of spoken language, as measured with the N400 effect, was lost along with responsiveness but, in the dexmedetomidine group, the N400 component was preserved suggesting partial preservation of the processing of words during anesthetic-induced unresponsiveness. However, the N400 effect could not be detected in all the awake participants and the choice of analysis method had marked impact on its detection rate at the individual-level. Subjective experiences were common during unresponsiveness induced by dexmedetomidine and propofol but the experiences most often suggested disconnectedness from the environment. In conclusion, the doses of dexmedetomidine or propofol minimally sufficient to induce unresponsiveness do not render the participants unconscious and dexmedetomidine does not completely abolish the processing of semantic stimuli. The local anterior EEG connectivity in the alpha frequency band may have potential in monitoring the depth of dexmedetomidine- and propofol-induced anesthesia.Anesteettien aiheuttama vastauskyvyttömyys: aivosähkökäyräpohjaiset korrelaatit ja subjektiiviset kokemukset Anestesialääkkeillä voidaan saada aikaan palautuvia muutoksia vastauskykyisyydessä, kytkeytyneisyydessä ja tajunnassa. Aivosähkökäyrään (EEG) pohjautuvat menetelmät tarjoavat lupaavia mahdollisuuksia mitata anestesian vaikutusta aivoissa, sillä niitä on suhteellisen helppo käyttää leikkaussalissa. Tässä tutkimuksessa 79 tervettä nuorta miestä osallistui valvekokeeseen ja 47 heistä jatkoi anestesiakokeeseen. Anestesiakokeessa koehenkilöille annettiin joko deksmedetomidiinia tai propofolia tavoiteohjattuna infuusiona nousevia annosportaita käyttäen, kunnes he menettivät vastauskykynsä. Koehenkilöt herätettiin tasaisen lääkeinfuusion aikana ja haastateltiin. Koko kokeen ajan mitattiin EEG:tä, josta tutkittiin N400-herätevastetta sekä toiminnallista ja suunnattua konnektiivisuutta. Prefrontaali-frontaalivälillä mitattu konnektiivisuus alfa-taajuuskaistassa erotteli toisistaan tilat, jotka erosivat vastauskykyisyyden tai lääkepitoisuuden suhteen. Konnektiivisuuden vallitseva suunta oli frontaalialueilta prefrontaalialueille vastauskyvyttömyyden aikana, mutta se kääntyi takaisin prefrontaalisesta frontaaliseen kulkevaksi koehenkilöiden vastauskyvyn palatessa. N400-efektillä mitattu puhutun kielen ymmärtäminen katosi vastauskyvyn menettämisen myötä. Deksmedetomidiiniryhmässä N400-komponentti säilyi, mikä viittaa siihen, että anesteettien aiheuttaman vastauskyvyttömyyden aikana sanojen prosessointi voi säilyä osittain. Yksilötasolla N400-efektiä ei kuitenkaan havaittu edes kaikilla hereillä olevilla henkilöillä, ja analyysimenetelmän valinnalla oli suuri vaikutus herätevasteen havaitsemiseen. Subjektiiviset kokemukset olivat yleisiä deksmedetomidiinin ja propofolin aiheuttaman vastauskyvyttömyyden aikana, mutta kokemukset olivat usein ympäristöstä irtikytkeytyneitä. Yhteenvetona voidaan todeta, että deksmedetomidiini- ja propofoliannokset, jotka juuri ja juuri riittävät aikaansaamaan vastauskyvyttömyyden, eivät aiheuta tajuttomuutta. Deksmedetomidiini ei myöskään täysin estä merkityssisällöllisten ärsykkeiden käsittelyä. Frontaalialueen sisällä EEG:llä mitattu konnektiivisuus alfataajuuskaistassa saattaa olla tulevaisuudessa hyödyllinen menetelmä deksmedetomidiini- ja propofolianestesian syvyyden mittaamiseksi

    Energy landscape analysis of neuroimaging data

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    Computational neuroscience models have been used for understanding neural dynamics in the brain and how they may be altered when physiological or other conditions change. We review and develop a data-driven approach to neuroimaging data called the energy landscape analysis. The methods are rooted in statistical physics theory, in particular the Ising model, also known as the (pairwise) maximum entropy model and Boltzmann machine. The methods have been applied to fitting electrophysiological data in neuroscience for a decade, but their use in neuroimaging data is still in its infancy. We first review the methods and discuss some algorithms and technical aspects. Then, we apply the methods to functional magnetic resonance imaging data recorded from healthy individuals to inspect the relationship between the accuracy of fitting, the size of the brain system to be analyzed, and the data length.Comment: 22 pages, 4 figures, 1 tabl

    Maximizing the divergence from a hierarchical model of quantum states

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    We study many-party correlations quantified in terms of the Umegaki relative entropy (divergence) from a Gibbs family known as a hierarchical model. We derive these quantities from the maximum-entropy principle which was used earlier to define the closely related irreducible correlation. We point out differences between quantum states and probability vectors which exist in hierarchical models, in the divergence from a hierarchical model and in local maximizers of this divergence. The differences are, respectively, missing factorization, discontinuity and reduction of uncertainty. We discuss global maximizers of the mutual information of separable qubit states.Comment: 18 pages, 1 figure, v2: improved exposition, v3: less typo

    Dynamic Functional Connectivity Reveals Temporal Differences in Wake and Stage-2 Sleep

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    The transition from wakefulness to sleep is marked by changes in neurophysiology, suggesting that changes in consciousness might be accompanied by changes in functional network organization. Brain activity of 21 healthy participants was measured via simultaneous EEG-fMRI as participants transitioned from wakefulness into sleep. All fMRI volumes were ICA-decomposed, yielding 42 neurophysiological sources. Independent component time courses were used to estimate mean functional connectivity (FC) and dynamic FC using a sliding window technique. Windowed matrices were submitted to k-means clustering (k = 7, L2-norm). Mean FC in Wake and Stage-2 Sleep (S2S) were similar. Dynamic analysis revealed differences in temporal features of FC. Participants transitioned more between connectivity states (CSs) and spent less time across all CSs in Wake than in S2S. Four of the seven CSs differed in their frequencies. The current analysis suggests conventional FC analyses obscure features in FC that are observable on a finer temporal scale

    Disruption of transfer entropy and inter-hemispheric brain functional connectivity in patients with disorder of consciousness

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    Severe traumatic brain injury can lead to disorders of consciousness (DOC) characterized by deficit in conscious awareness and cognitive impairment including coma, vegetative state, minimally consciousness, and lock-in syndrome. Of crucial importance is to find objective markers that can account for the large-scale disturbances of brain function to help the diagnosis and prognosis of DOC patients and eventually the prediction of the coma outcome. Following recent studies suggesting that the functional organization of brain networks can be altered in comatose patients, this work analyzes brain functional connectivity (FC) networks obtained from resting-state functional magnetic resonance imaging (rs-fMRI). Two approaches are used to estimate the FC: the Partial Correlation (PC) and the Transfer Entropy (TE). Both the PC and the TE show significant statistical differences between the group of patients and control subjects; in brief, the inter-hemispheric PC and the intra-hemispheric TE account for such differences. Overall, these results suggest two possible rs-fMRI markers useful to design new strategies for the management and neuropsychological rehabilitation of DOC patients.Comment: 25 pages; 4 figures; 3 tables; 1 supplementary figure; 4 supplementary tables; accepted for publication in Frontiers in Neuroinformatic

    Consciousness, Anesthesia and Brain Resting State Networks

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    Consciousness is a great mystery to science. Despite several attempts, none of the current theories have managed to explain how and why it exists. Theories struggle with fundamental philosophical questions, such as the hard problem, that contests how something mental, like consciousness, can be explained by physical phenomena such as neural activity. Modern neuroscientific study of consciousness puts aside this and a few other yet unreachable questions. It focuses on finding the neural correlates of consciousness (NCC) comprising the physical phenomena, which correlate with certain aspects of consciousness. In the NCC studies, consciousness is usually taken to have two aspects: the states of consciousness, encompassing awake, dreaming, and unconscious states, and contents of consciousness, such as an experienced perceptual stimulus. Experimental anesthesia and functional brain imaging are essential tools for the search of the NCC. Anesthesia offers a reliable and reversible method to alter the subject's state of consciousness. The brain function during these altered states of consciousness can be measured with functional imaging methods, such as functional magnetic resonance imaging (fMRI). It measures neuronal activity via a blood oxygen level dependent (BOLD) signal. Functional connectivity analysis of the BOLD-signal can be used to explore the organization of spatially distinct brain areas into functional networks, which are associated with specific cognitive functions such as attention control and emotional regulation. Recently, several studies have shown that changes in functional connectivity between and within these networks are also associated with altered states of consciousness induced by anesthetic drugs. This review will cover the essential questions and methodology of current exploration of neural correlates of states of consciousness, focusing on the resting state networks and the use of fMRI and experimental anesthesia as research tools. Effects of different anesthetics on these networks are also compared. Different anesthetics used in experimental anesthesia have quite distinct pharmacological mechanisms of action, even though the induced brain functional connectivity patterns resemble each other. Propofol-induced unconsciousness is mediated either from corticocortical or thalamocortical disconnection. The nonspecific thalamic nodes, related to arousal and distribution of information, may have an important role in propofol-induced unconsciousness. Dexmedetomidine has similar patterns in connectivity changes as propofol, but the connectivity between deeper brain regions and thalamus remains less affected, possibly explaining the easier arousal from dexmedetomidine-induced unconsciousness. Ketamine increases the overall functional connectivity but disrupts the connectivity in the higher-order networks of the brain inducing “dissociative anesthesia”. The role of thalamic functional connectivity during ketamine anesthesia has not been studied and would be an interesting subject for future studies.Tietoisuuden syntymekanismi on yksi suurimmista ratkaisemattomista mysteereistä. Filosofiset kysymykset, kuten tietoisuuden niin kutsuttu vaikea ongelma ja selityksellinen kuilu, ovat edelleen ajankohtaisia ja ne kyseenalaistavat esimerkiksi pystytäänkö tietoisuuden syntymekanismeja koskaan selittämään tieteellisin keinoin. Nykyinen neurotieteellinen tutkimus pyrkii selvittämään tietoisuuden hermostollisia vastineita, eli kuvaamaan tiettyyn tietoisuuden tilaan tai sisältöön liittyviä aivotoiminnan mekanismeja. Näitä vastineita tutkittaessa käytetään jakoa tietoisuuden tiloihin, joita ovat hereillä olo, unennäkö ja tajuttomuus tai tietoisuuden komponentteihin joita ovat esimerkiksi koettu auditorinen tai visuaalinen ärsyke. Tässä katsausartikkelissa käydään läpi nykyisen tietoisuuden tutkimuksen ja sen tilojen neurologisten vastineiden etsimisen perusteet, keskittyen funktionaaliseen magneettikuvaukseen, lepotilaverkostoihin ja anestesiaan. Artikkelissa myös vertaillaan aikaisempien sellaisten tutkimusten tuloksia, jotka käsittelevät eri anesteettien vaikutuksista lepotilaverkostoihin. Kokeellista anestesiaa ja toiminnallista aivokuvantamista käytetään työkaluina tietoisuuden tiloihin liittyvien aivotoiminnan mekanismien tutkimisessa. Anestesian avulla voidaan luotettavasti ja palautuvasti aiheuttaa muutoksia koehenkilöiden tajunnan tilaan. Samalla tutkimuskysymyksiin etsitään vastauksia kuvantamalla tiettyihin tajunnan tiloihin tai niiden muutoksiin liittyviä aivotoiminnan mekanismeja. Kuvantamismenetelmistä toiminnallisella magneettikuvauksella pysytään mittaamaa hermokudoksen aktiivisuutta veren happipitoisuudesta riippuvalla signaalilla. Vertaamalla aktivoitumista tietyltä alueelta verraten sitä koko aivojen samanaikaiseen aktivoitumiseen, saadaan näkyviin toiminnallisia verkostoja, joilla on omia tehtäviä esimerkiksi kognitiivisessa prosessoinnissa ja tarkkaavaisuuden keskittämisessä. Myös tietoisuuden tasot korreloivat näiden verkostojen sisäisten ja keskinäisten yhteyksien sekä oikea-aikaisen aktivoitumisen kanssa. Kokeellista anestesiaa käytettäessä on huomioitava eri anesteettien erilaiset vaikutusmekanismit, vaikkakin aivokuvantamisella aktivoitumismuutokset muistuttavat toisiaan. Vertailtavissa anesteeteissa esiintyy yhtäläisyyksiä kortikaalisten verkostojen yhteyksien heikkenemiseen ja tietoisuuden häviämiseen liittyen. Propofolin keskeisenä mekanismina on joko yhteyksien katkaiseminen aivokuoren verkostoissa tai talamuksen ja aivokuoren välillä. Deksmedetomidiinin fMRI:llä mitatut vaikutukset muistuttavat paljon propofolia, mutta yhteys talamuksen ja syvempien aivoalueiden välillä säilyy, joka todennäköisesti mahdollistaa nopeamman heräämisen. Ketamiini lisää toiminnallista konnektiivisuutta verkostoissa, kuitenkin samalla hajottaen järjestäytymistä korkeamman asteen verkostoissa. Propofolista poiketen ketamiinin vaikutus toimintaverkostoon on vähäinen. Talamuksen merkitys yhteyksien muutoksissa yleisesti jää vielä epävarmaksi: ovatko muutokset syytä vai seurausta aivokuoren yhteyksien hajoamisesta

    Brain Map of Intrinsic Functional Flexibility in Anesthetized Monkeys and Awake Humans

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    Emerging neuroimaging studies emphasize the dynamic organization of spontaneous brain activity in both human and non-human primates, even under anesthesia. In a recent study, we were able to characterize the heterogeneous architecture of intrinsic functional flexibility in the awake, resting human brain using time-resolved analysis and a probabilistic model. However, it is unknown whether this organizational principle is preserved in the anesthetized monkey brain, and how anesthesia affects dynamic and static measurements of spontaneous brain activity. To investigate these issues, we collected resting-state functional magnetic resonance imaging (fMRI) datasets from 178 awake humans and 11 anesthetized monkeys (all healthy). Our recently established method, a complexity measurement (i.e., Shannon entropy) of dynamic functional connectivity patterns of each brain region, was used to map the intrinsic functional flexibility across the cerebral cortex. To further explore the potential effects of anesthesia, we performed time series analysis and correlation analysis between dynamic and static measurements within awake human and anesthetized monkey brains, respectively. We observed a heterogeneous profile of intrinsic functional flexibility in the anesthetized monkey brain, which showed some similarities to that of awake humans (r = 0.30, p = 0.007). However, we found that brain activity in anesthetized monkeys generally shifted toward random fluctuations. Moreover, there is a negative correlation between nodal entropy for the distribution of dynamic functional connectivity patterns and static functional connectivity strength in anesthetized monkeys, but not in awake humans. Our findings indicate that the heterogeneous architecture of intrinsic functional flexibility across cortex probably reflects an evolutionarily conserved aspect of functional brain organization, which persists across levels of cognitive processing (states of consciousness). The coupling between nodal entropy for the distribution of dynamic functional connectivity patterns and static functional connectivity strength may serve as a potential signature of anesthesia. This study not only offers fresh insight into the evolution of brain functional architecture, but also advances our understanding of the dynamics of spontaneous brain activity

    Long-Range Temporal Correlations of Patients in Minimally Conscious State Modulated by Spinal Cord Stimulation

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    Spinal cord stimulation (SCS) has been shown to improve the consciousness levels of patients with disorder of consciousness (DOC). However, the underlying mechanisms of SCS remain poorly understood. This study recorded resting-state electroencephalograms (EEG) from 16 patients with minimally conscious state (MCS), before and after SCS, and investigated the mechanisms of SCS on the neuronal dynamics in MCS patients. Detrended fluctuation analysis (DFA), combined with surrogate data method, was employed to measure the long-range temporal correlations (LRTCs) of the EEG signals. A surrogate data method was utilized to acquire the genuine DFA exponents (GDFAE) reflecting the genuine LRTCs of brain activity. We analyzed the GDFAE in four brain regions (frontal, central, posterior, and occipital) at five EEG frequency bands [delta (1–4 Hz), theta (4–8 Hz), alpha (8–13 Hz), beta (13–30 Hz), and gamma (30–45 Hz)]. The GDFAE values ranged from 0.5 to 1, and showed temporal and spatial variation between the pre-SCS and the post-SCS states. We found that the channels with GDFAE spread wider after SCS. This phenomenon may indicate that more cortical areas were engaged in the information integration after SCS. In addition, the GDFAE values increased significantly in the frontal area at delta, theta, and alpha bands after SCS. At the theta band, a significant increase in GDFAE was observed in the occipital area. No significant change was found at beta or gamma bands in any brain region. These findings show that the enhanced LRTCs after SCS occurred primarily at low-frequency bands in the frontal and occipital regions. As the LRTCs reflect the long-range temporal integration of EEG signals, our results indicate that information integration became more “complex” after SCS. We concluded that the brain activities at low-frequency oscillations, particularly in the frontal and occipital regions, were improved by SCS

    Physostigmine and Methylphenidate Induce Distinct Arousal States During Isoflurane General Anesthesia in Rats

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    BACKGROUND: Although emergence from general anesthesia is clinically treated as a passive process driven by the pharmacokinetics of drug clearance, agents that hasten recovery from general anesthesia may be useful for treating delayed emergence, emergence delirium, and postoperative cognitive dysfunction. Activation of central monoaminergic neurotransmission with methylphenidate has been shown to induce reanimation (active emergence) from general anesthesia. Cholinergic neurons in the brainstem and basal forebrain are also known to promote arousal. The objective of this study was to test the hypothesis that physostigmine, a centrally acting cholinesterase inhibitor, induces reanimation from isoflurane anesthesia in adult rats. METHODS: The dose-dependent effects of physostigmine on time to emergence from a standardized isoflurane general anesthetic were tested. It was then determined whether physostigmine restores righting during continuous isoflurane anesthesia. In a separate group of rats with implanted extradural electrodes, physostigmine was administered during continuous inhalation of 1.0% isoflurane, and the electroencephalogram changes were recorded. Finally, 2.0% isoflurane was used to induce burst suppression, and the effects of physostigmine and methylphenidate on burst suppression probability (BSP) were tested. RESULTS: Physostigmine delayed time to emergence from isoflurane anesthesia at doses ≥0.2 mg/kg (n = 9). During continuous isoflurane anesthesia (0.9% ± 0.1%), physostigmine did not restore righting (n = 9). Blocking the peripheral side effects of physostigmine with the coadministration of glycopyrrolate (a muscarinic antagonist that does not cross the blood-brain barrier) produced similar results (n = 9 each). However, during inhalation of 1.0% isoflurane, physostigmine shifted peak electroencephalogram power from δ ( < 4 Hz) to θ (4-8 Hz) in 6 of 6 rats. During continuous 2.0% isoflurane anesthesia, physostigmine induced large, statistically significant decreases in BSP in 6 of 6 rats, whereas methylphenidate did not. CONCLUSIONS: Unlike methylphenidate, physostigmine does not accelerate time to emergence from isoflurane anesthesia and does not restore righting during continuous isoflurane anesthesia. However, physostigmine consistently decreases BSP during deep isoflurane anesthesia, whereas methylphenidate does not. These findings suggest that activation of cholinergic neurotransmission during isoflurane anesthesia produces arousal states that are distinct from those induced by monoaminergic activation.National Institutes of Health (U.S.) (Grant TR01-GM104948)National Institutes of Health (U.S.) (Grant DP1-OD003646)National Institutes of Health (U.S.) (Grant K08-GM094394
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