67,218 research outputs found
Contact patterns among high school students
Face-to-face contacts between individuals contribute to shape social networks
and play an important role in determining how infectious diseases can spread
within a population. It is thus important to obtain accurate and reliable
descriptions of human contact patterns occurring in various day-to-day life
contexts. Recent technological advances and the development of wearable sensors
able to sense proximity patterns have made it possible to gather data giving
access to time-varying contact networks of individuals in specific
environments. Here we present and analyze two such data sets describing with
high temporal resolution the contact patterns of students in a high school. We
define contact matrices describing the contact patterns between students of
different classes and show the importance of the class structure. We take
advantage of the fact that the two data sets were collected in the same setting
during several days in two successive years to perform a longitudinal analysis
on two very different timescales. We show the high stability of the contact
patterns across days and across years: the statistical distributions of numbers
and durations of contacts are the same in different periods, and we observe a
very high similarity of the contact matrices measured in different days or
different years. The rate of change of the contacts of each individual from one
day to the next is also similar in different years. We discuss the interest of
the present analysis and data sets for various fields, including in social
sciences in order to better understand and model human behavior and
interactions in different contexts, and in epidemiology in order to inform
models describing the spread of infectious diseases and design targeted
containment strategies.Comment: Supplementary Information at
http://s3-eu-west-1.amazonaws.com/files.figshare.com/1677807/File_S1.pd
The naked singularity in the global structure of critical collapse spacetimes
We examine the global structure of scalar field critical collapse spacetimes
using a characteristic double-null code. It can integrate past the horizon
without any coordinate problems, due to the careful choice of constraint
equations used in the evolution. The limiting sequence of sub- and
supercritical spacetimes presents an apparent paradox in the expected Penrose
diagrams, which we address in this paper. We argue that the limiting spacetime
converges pointwise to a unique limit for all r>0, but not uniformly. The r=0
line is different in the two limits. We interpret that the two different
Penrose diagrams differ by a discontinuous gauge transformation. We conclude
that the limiting spacetime possesses a singular event, with a future removable
naked singularity.Comment: RevTeX 4; 6 pages, 7 figure
Collapse and dispersal in massless scalar field models
The phenomena of collapse and dispersal for a massless scalar field has drawn
considerable interest in recent years, mainly from a numerical perspective. We
give here a sufficient condition for the dispersal to take place for a scalar
field that initially begins with a collapse. It is shown that the change of the
gradient of the scalar field from a timelike to a spacelike vector must be
necessarily accompanied by the dispersal of the scalar field. This result holds
independently of any symmetries of the spacetime. We demonstrate the result
explicitly by means of an example, which is the scalar field solution given by
Roberts. The implications of the result are discussed.Comment: revised version, Accepted for publication in Int. Journ. of Mod.
Phys. D, 6 pages, 3 figure
Dynamic change-point detection using similarity networks
From a sequence of similarity networks, with edges representing certain
similarity measures between nodes, we are interested in detecting a
change-point which changes the statistical property of the networks. After the
change, a subset of anomalous nodes which compares dissimilarly with the normal
nodes. We study a simple sequential change detection procedure based on
node-wise average similarity measures, and study its theoretical property.
Simulation and real-data examples demonstrate such a simply stopping procedure
has reasonably good performance. We further discuss the faulty sensor isolation
(estimating anomalous nodes) using community detection.Comment: appeared in Asilomar Conference 201
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Coordinated neuronal ensembles in primary auditory cortical columns.
The synchronous activity of groups of neurons is increasingly thought to be important in cortical information processing and transmission. However, most studies of processing in the primary auditory cortex (AI) have viewed neurons as independent filters; little is known about how coordinated AI neuronal activity is expressed throughout cortical columns and how it might enhance the processing of auditory information. To address this, we recorded from populations of neurons in AI cortical columns of anesthetized rats and, using dimensionality reduction techniques, identified multiple coordinated neuronal ensembles (cNEs), which are groups of neurons with reliable synchronous activity. We show that cNEs reflect local network configurations with enhanced information encoding properties that cannot be accounted for by stimulus-driven synchronization alone. Furthermore, similar cNEs were identified in both spontaneous and evoked activity, indicating that columnar cNEs are stable functional constructs that may represent principal units of information processing in AI
Spatial encoding in primate hippocampus during free navigation.
The hippocampus comprises two neural signals-place cells and θ oscillations-that contribute to facets of spatial navigation. Although their complementary relationship has been well established in rodents, their respective contributions in the primate brain during free navigation remains unclear. Here, we recorded neural activity in the hippocampus of freely moving marmosets as they naturally explored a spatial environment to more explicitly investigate this issue. We report place cells in marmoset hippocampus during free navigation that exhibit remarkable parallels to analogous neurons in other mammalian species. Although θ oscillations were prevalent in the marmoset hippocampus, the patterns of activity were notably different than in other taxa. This local field potential oscillation occurred in short bouts (approximately .4 s)-rather than continuously-and was neither significantly modulated by locomotion nor consistently coupled to place-cell activity. These findings suggest that the relationship between place-cell activity and θ oscillations in primate hippocampus during free navigation differs substantially from rodents and paint an intriguing comparative picture regarding the neural basis of spatial navigation across mammals
Robust Detection of Dynamic Community Structure in Networks
We describe techniques for the robust detection of community structure in
some classes of time-dependent networks. Specifically, we consider the use of
statistical null models for facilitating the principled identification of
structural modules in semi-decomposable systems. Null models play an important
role both in the optimization of quality functions such as modularity and in
the subsequent assessment of the statistical validity of identified community
structure. We examine the sensitivity of such methods to model parameters and
show how comparisons to null models can help identify system scales. By
considering a large number of optimizations, we quantify the variance of
network diagnostics over optimizations (`optimization variance') and over
randomizations of network structure (`randomization variance'). Because the
modularity quality function typically has a large number of nearly-degenerate
local optima for networks constructed using real data, we develop a method to
construct representative partitions that uses a null model to correct for
statistical noise in sets of partitions. To illustrate our results, we employ
ensembles of time-dependent networks extracted from both nonlinear oscillators
and empirical neuroscience data.Comment: 18 pages, 11 figure
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Micromixing and microchannel design: Vortex shape and entropy
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.In very recent years microdevices, due to their potency in replacing large-scale conventional laboratory instrumentation, are becoming a fast and low cost technology for the treatment of several chemical and biological processes. In particular microfluidics has been massively investigated, aiming at improving the performance of chemical reactors. This is because of the fact that reaction is often an interface phenomenon where the greater the surface to volume ratio, the higher the reaction speed, and microscale mixing increases the interfacial area (in terms of mixing-induced-by-vortices generation). However, microfluidic systems suffer from the limitation that they are characterized mostly by very low Reynolds numbers, with the consequence that (i) they cannot take advantage from the turbulence mixing support, and (ii) viscosity hampers proper vortex detection. Therefore, the proper design of micro-channels (MCs) becomes essential. In this framework, several geometries have been proposed to induce mixing vortices in MCs. However a quantitative comparison between proposed geometries in terms of their passive mixing
potency can be done only after proper definition of vortex formation (topology, size) and mixing performance. The objective of this study is to test the ability of different fluid dynamic metrics in vortex
detection and mixing effectiveness in micromixers. This is done numerically solving different conditions for the flow in a classic passive mixer, a ring shaped MC. We speculate that MCs design could take advantage from fluidic metrics able to rank properly flow related mixing
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