1,720,989 research outputs found
The statistics of repeating patterns of cortical activity can be reproduced by a model network of stochastic binary neurons
No full textCalcium imaging of the spontaneous activity in cortical slices has revealed repeating spatiotemporal patterns of transitions between so-called down states and up states (Ikegaya et al., 2004). Here we fit a model network of stochastic binary neurons to data from these experiments, and in doing so reproduce the distributions of such patterns. We use two versions of this model: (1) an unconnected network in which neurons are activated as independent Poisson processes; and (2) a network with an interaction matrix, estimated from the data, representing effective interactions between the neurons. The unconnected model (model 1) is sufficient to account for the statistics of repeating patterns in 11 of the 15 datasets studied. Model 2, with interactions between neurons, is required to account for pattern statistics of the remaining four. Three of these four datasets are the ones that contain the largest number of transitions, suggesting that long datasets are in general necessary to render interactions statistically visible. We then study the topology of the matrix of interactions estimated for these four datasets. For three of the four datasets, we find sparse matrices with long-tailed degree distributions and an overrepresentation of certain network motifs. The remaining dataset exhibits a strongly interconnected, spatially localized subgroup of neurons. In all cases, we find that interactions between neurons facilitate the generation of long patterns that do not repeat exactly
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Role of delays in shaping spatiotemporal dynamics of neuronal activity in large networks.
No full textWe study the effect of delays on the dynamics of large networks of neurons. We show that delays give rise to a wealth of bifurcations and to a rich phase diagram, which includes oscillatory bumps, traveling waves, lurching waves, standing waves arising via a period-doubling bifurcation, aperiodic regimes, and regimes of multistability. We study the existence and the stability of the various dynamical patterns analytically and numerically in a simplified rate model as a function of the interaction parameters. The results derived in that framework allow us to understand the origin of the diversity of dynamical states observed in large networks of spiking neurons
On the Distribution of Firing Rates in Networks of Cortical Neurons
No full textThe distribution ofin vivoaverage firing rates within local cortical networks has been reported to be highly skewed and long tailed. The distribution of average single-cell inputs, conversely, is expected to be Gaussian by the central limit theorem. This raises the issue of how a skewed distribution of firing rates might result from a symmetric distribution of inputs. We argue that skewed rate distributions are a signature of the nonlinearity of thein vivo f–Icurve. Duringin vivoconditions, ongoing synaptic activity produces significant fluctuations in the membrane potential of neurons, resulting in an expansive nonlinearity of thef–Icurve for low and moderate inputs. Here, we investigate the effects of single-cell and network parameters on the shape of thef–Icurve and, by extension, on the distribution of firing rates in randomly connected networks.</jats:p
Variations on the Author
Full text link“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
Full text linkWe 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
Estudi de les equacions de Wilson i Cowan, un model de xarxa neuronal
No full textTreballs Finals de Grau de Matemàtiques, Facultat de Matemàtiques, Universitat de Barcelona, Any: 2013, Director: Àlex HaroThe theoretical knowledge about dynamical systems has important applications to the study of
those biological models that are described by means of differential equations. This work is a study of some basic aspects about the theory of dynamical systems and its application to the analysis of a statistical neural model: the Wilson-Cowan model. First of all, we have explored some important results about bifurcations of equilibria in systems of ordinary differential equations that depend on parameters.
Later on, we have presented the Wilson-Cowan model and its mathematical formulation. The model
describes, by means of a system of two ordinary differential equations, the temporal evolution of the mean activity in two neuronal populations of a network, one of them being excitatory and the other, inhibitory. The equations include constants that depend on intrinsic properties of the network and two real parameters that represent the external stimuli received by the network. We have applied some of the learnt theoretical results to the analysis of the model and its dynamical regimes. The results show that, for some chosen values in the constant parameters of the model, the system exhibits rich dynamics, with possible state changes between oscillatory and stationary regimes when the external stimuli vary in certain regions in the parameter space. These phenomena may represent basic mechanisms underlying more complex processes as sleep oscillatory rhythms or the establishment of memories
Estudi de les equacions de Wilson i Cowan, un model de xarxa neuronal
No full textTreballs Finals de Grau de Matemàtiques, Facultat de Matemàtiques, Universitat de Barcelona, Any: 2013, Director: Àlex HaroThe theoretical knowledge about dynamical systems has important applications to the study of
those biological models that are described by means of differential equations. This work is a study of some basic aspects about the theory of dynamical systems and its application to the analysis of a statistical neural model: the Wilson-Cowan model. First of all, we have explored some important results about bifurcations of equilibria in systems of ordinary differential equations that depend on parameters.
Later on, we have presented the Wilson-Cowan model and its mathematical formulation. The model
describes, by means of a system of two ordinary differential equations, the temporal evolution of the mean activity in two neuronal populations of a network, one of them being excitatory and the other, inhibitory. The equations include constants that depend on intrinsic properties of the network and two real parameters that represent the external stimuli received by the network. We have applied some of the learnt theoretical results to the analysis of the model and its dynamical regimes. The results show that, for some chosen values in the constant parameters of the model, the system exhibits rich dynamics, with possible state changes between oscillatory and stationary regimes when the external stimuli vary in certain regions in the parameter space. These phenomena may represent basic mechanisms underlying more complex processes as sleep oscillatory rhythms or the establishment of memories
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
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