1,720,966 research outputs found
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
Altered spread of waves of activities at large scale is influenced by cortical thickness organization in temporal lobe epilepsy: a magnetic resonance imaging-high-density electroencephalography study
No full textTemporal lobe epilepsy is a brain network disorder characterized by alterations at both the structural and the functional levels. It remains unclear how structure and function are related and whether this has any clinical relevance. In the present work, we adopted a novel methodological approach investigating how network structural features influence the large-scale dynamics. The functional network was defined by the spatio-temporal spreading of aperiodic bursts of activations (neuronal avalanches), as observed utilizing high-density electroencephalography in patients with temporal lobe epilepsy. The structural network was modelled as the region-based thickness covariance. Loosely speaking, we quantified the similarity of the cortical thickness of any two brain regions, both across groups and at the individual level, the latter utilizing a novel approach to define the subject-wise structural covariance network. In order to compare the structural and functional networks (at the nodal level), we studied the correlation between the probability that a wave of activity would propagate from a source to a target region and the similarity of the source region thickness as compared with other target brain regions. Building on the recent evidence that large-waves of activities pathologically spread through the epileptogenic network in temporal lobe epilepsy, also during resting state, we hypothesize that the structural cortical organization might influence such altered spatio-temporal dynamics. We observed a stable cluster of structure-function correlation in the bilateral limbic areas across subjects, highlighting group-specific features for left, right and bilateral temporal epilepsy. The involvement of contralateral areas was observed in unilateral temporal lobe epilepsy. We showed that in temporal lobe epilepsy, alterations of structural and functional networks pair in the regions where seizures propagate and are linked to disease severity. In this study, we leveraged on a well-defined model of neurological disease and pushed forward personalization approaches potentially useful in clinical practice. Finally, the methods developed here could be exploited to investigate the relationship between structure-function networks at subject level in other neurological conditions.Duma et al. used innovative methods like subject-wise structural covariance network and neuronal avalanches to study structural and functional brain organization in temporal lobe epilepsy. A coupling was observed between the propagation of aperiodic burst of activity and cortical thickness organization focused in brain regions related to seizure initiation and propagation.Graphical Abstrac
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
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
Local cohomology, master equation and renormalization of higher-derivative and nonlocal quantum gravity
No full textIn this thesis we have studied the properties of a higher-derivative toy model of quantum gravity and we have used the results to analyze the main features of nonlocal theories of quantum gravity. The renormalizability of higher-derivative quantum gravity have been revisited by using the Batalin-Vilkovisky formalism. The very hearth of this formalism is the master equation which plays a foundamental role in the analysis of a gauge system. In particular, it is very useful to prove the renormalizability of a gauge theory. We have considered two main approaches to prove the renormalizability of higher-derivative quantum gravity, both performed by induction in the number of loops.
The “cohomological” approach preserves the validity of the master equation at each inductive step. We assume that the theory is renormalized up to n-loops and then prove that we can cancel out all the divergences at (n + 1)-loops. It turns out that the (n+1)-loops counterterms must satisfy a certain cohomological condition. The cohomological method only works if we can write the solution of this condition
as a sum of a gauge invariant functional of the gauge fields plus a trivial term. This is also known as Kluberg-Stern-Zuber conjecture. In the
original work on the renormalizability of higher-derivative quantum gravity, Stelle assumes the validity of the conjecture. So far, it has been proved for Yang-Mills theory and Einstein gravity. We have extended the proof to higher-derivative quantum gravity. Our results naturally extend to nonlocal theories of quantum gravity, which turn out to be super-renormalizable.
In general, it is not obvious that the Kluberg-Stern-Zuber condition holds. In such a situation, the cohomological approach would fail. A more powerful method which avoid this constraint is a so-called “quadratic” approach. In this case, the inductive procedure absorbs the divergences automatically at each loop. On the other hand, it postpones the solution of the master equation to the end of the inductive procedure, after the theory is renormalized to all orders. We have worked out the exact solution of the master equation at the renormalized level. This result also proves that higher-derivative quantum gravity is renormalizable, and generates the same number of renormalization constants as the cohomological approach. In particular,the structure of the BRST transformations is preserved by renormalization.
This last property, extended also to nonlocal gravity, enabled us to conclude that general covariance is the most general gauge symmetry of a renormalizable higher-derivative and nonlocal quantum theory of gravity.
Besides the investigation of higher-derivative quantum gravity, we have also considered aspects of nonlocal theories. The interest in studying nonlocal theories is twofold. On the one hand, it brings to light a sector of quantum field theory that is still vastly unexplored and worth of investigation. On the other hand, it is a candidate for a unitary and super-renormalizable quantum theory of gravitation. It is worth to note that these theories are also predictive to a certain extent. Indeed, they have a rather constrained behaviour in the UV limit and provide predictive transition amplitudes at high energies
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Spontaneous neuronal avalanches as a correlate of access consciousness
No full textInternational audienceDecades of research have advanced our understanding of the biophysical mechanisms underlying consciousness. However, an overarching framework bridging between models of consciousness and the large-scale organization of spontaneous brain activity is still missing. Based on the observation that spontaneous brain activity dynamically switches between epochs of segregation and large-scale integration of information, we hypothesize a brain-state dependence of conscious access, whereby the presence of either segregated or integrated states marks distinct modes of information processing. We first review influential works on the neuronal correlates of consciousness, spontaneous resting-state brain activity and dynamical system theory. Then, we propose a test experiment to validate our hypothesis that conscious access occurs in aperiodic cycles, alternating windows where new incoming information is collected but not experienced, to punctuated short-lived integration events, where conscious access to previously collected content occurs. In particular, we suggest that the integration events correspond to neuronal avalanches, which are collective bursts of neuronal activity ubiquitously observed in electrophysiological recordings. If confirmed, the proposed framework would link the physics of spontaneous cortical dynamics, to the concept of ignition within the global neuronal workspace theory, whereby conscious access manifest itself as a burst of neuronal activity
Die Renormierungsgruppe Flussanalyse für eine Kosmologischer Sektor von Spin-Schaum Modells
No full textSpin Foam Models (SFM) provide a non-perturbative and background independent path integral formulation of Quantum Gravity. The models are built on a lattice which represents spacetime and serves as a tool to control the d.o.f. of geometry. In fact, a given discretization can be thought as a scale at which we look at spacetime, while its refinement resembles a shift towards UV regimes. In the light of this interpretation, a SFM state sum is understood as an effective theory for the available degrees of freedom provided by the lattice. Then, the Wilsonian renormalization group approach stands out as an ideal tool to organize and describe the flow of the theory along a scale of complexity of the base lattice.
While many promising results have been achieved in SFM, the dynamics of these models is still hard to solve and most calculations are performed on extremely coarse discretizations. In order to get access to finer lattices we reduce the path integral state sum to certain symmetric configurations of geometry. This allows a numerical evaluation of some geometric observables on coarser and finer discretizations. Their comparison defines the renormalization group flow of the model in the parameter space. Notably, we find a fixed point with one attractive and two repulsive directions in the three-dimensional parameter space of the asymptotic Euclidean EPRL-FK Spin Foam Model. In such point, the expectation value of the observables do not depend on the lattice complexity. The existence of a fixed point opens the way to study another open problem of SFM, i.e. the continuum (infinite refinement) limit.Spin-Schaum Modells ermöglichen eine nicht-perturbative Pfadintegral Formulierung der Quantengravitation unabhängig von Hintergrund. Diese Modelle werden auf einem Gitter konstruiert, welches die Raumzeit repräsentiert und als Hilfsmittel dient um die Freiheitsgrade der Geometrie zu kontrollieren. Tatsächlich kann eine bestimmte Diskretisierung als Größenordnung auf welcher wir die Raumzeit betrachten, verstanden werden. Eine Verfeinerung bedeutet eine Verschiebung in den ultraviolett Bereich. In Anbetracht dieser Interpretation kann eine Spin-Schaum Zustandssumme als eine effektive Theorie für die durch das Gitter vorgegeben verfügbaren Freiheitsgrade verstanden werden. Daraus ergibt sich der Ansatz der Wilsonschen Renormierungsgruppe als ideales Hilfsmittel um den Fluss der Theorie in die Richtung einer Größenordnung an Komplexität des Grund-Gitters zu organisieren und zu beschreiben.
Während viele vielversprechende Ergebnisse mit Spin-Schaum Modellen erreicht wurden, ist die Dynamik dieser Modelle immer noch schwer zu lösen und die meisten Berechnungen werden auf extrem groben Diskretisierungen durchgeführt. Um eine feinere Gitterauflösung zu erreichen, reduzieren wir die Zustandssumme des Pfadintegrals auf gewisse symmetrische Konfigurationen der Geometrie. Dies erlaubt eine numerische Auswertung von einigen geometrischen Observablen auf gröberen und feineren Diskretisierungen. Der Vergleich von diesen definiert den Fluss der Renormierungsgruppe des Modells im Parameterraum. Bemerkenswerterweise finden wir einen Fixpunkt mit einer attraktiven und zwei repulsiven Richtungen im dreidimensionalen Parameterraum eines asymptotisch euklidischen EPRL-FK Spin-Schaum Modell. In einem solchen Punkt hängt der Erwartungswert der Observablen nicht von der Gitterkomplexität ab. Die Existenz eines solchen Fixpunktes öffnet einen neuen Weg um anderen offene Probleme des Spin-Schaum Modells wie das kontinuierliche Limit (unendliche Verfeinerung) zu studieren
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