20,725 research outputs found
Myocardial Architecture and Patient Variability in Clinical Patterns of Atrial Fibrillation
Atrial fibrillation (AF) increases the risk of stroke by a factor of four to
five and is the most common abnormal heart rhythm. The progression of AF with
age, from short self-terminating episodes to persistence, varies between
individuals and is poorly understood. An inability to understand and predict
variation in AF progression has resulted in less patient-specific therapy.
Likewise, it has been a challenge to relate the microstructural features of
heart muscle tissue (myocardial architecture) with the emergent temporal
clinical patterns of AF. We use a simple model of activation wavefront
propagation on an anisotropic structure, mimicking heart muscle tissue, to show
how variation in AF behaviour arises naturally from microstructural differences
between individuals. We show that the stochastic nature of progressive
transversal uncoupling of muscle strands (e.g., due to fibrosis or gap
junctional remodelling), as occurs with age, results in variability in AF
episode onset time, frequency, duration, burden and progression between
individuals. This is consistent with clinical observations. The uncoupling of
muscle strands can cause critical architectural patterns in the myocardium.
These critical patterns anchor micro-re-entrant wavefronts and thereby trigger
AF. It is the number of local critical patterns of uncoupling as opposed to
global uncoupling that determines AF progression. This insight may eventually
lead to patient specific therapy when it becomes possible to observe the
cellular structure of a patient's heart.Comment: 5 pages, 4 figures. For supplementary materials please contact Kishan
A. Manani at [email protected]
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Subtype-specific plasticity of inhibitory circuits in motor cortex during motor learning.
Motor skill learning induces long-lasting reorganization of dendritic spines, principal sites of excitatory synapses, in the motor cortex. However, mechanisms that regulate these excitatory synaptic changes remain poorly understood. Here, using in vivo two-photon imaging in awake mice, we found that learning-induced spine reorganization of layer (L) 2/3 excitatory neurons occurs in the distal branches of their apical dendrites in L1 but not in the perisomatic dendrites. This compartment-specific spine reorganization coincided with subtype-specific plasticity of local inhibitory circuits. Somatostatin-expressing inhibitory neurons (SOM-INs), which mainly inhibit distal dendrites of excitatory neurons, showed a decrease in axonal boutons immediately after the training began, whereas parvalbumin-expressing inhibitory neurons (PV-INs), which mainly inhibit perisomatic regions of excitatory neurons, exhibited a gradual increase in axonal boutons during training. Optogenetic enhancement and suppression of SOM-IN activity during training destabilized and hyperstabilized spines, respectively, and both manipulations impaired the learning of stereotyped movements. Our results identify SOM inhibition of distal dendrites as a key regulator of learning-related changes in excitatory synapses and the acquisition of motor skills
Astrophysicists on Twitter: An in-depth analysis of tweeting and scientific publication behavior
This paper analyzes the tweeting behavior of 37 astrophysicists on Twitter
and compares their tweeting behavior with their publication behavior and
citation impact to show whether they tweet research-related topics or not.
Astrophysicists on Twitter are selected to compare their tweets with their
publications from Web of Science. Different user groups are identified based on
tweeting and publication frequency. A moderate negative correlation (p=-0.390*)
is found between the number of publications and tweets per day, while retweet
and citation rates do not correlate. The similarity between tweets and
abstracts is very low (cos=0.081). User groups show different tweeting behavior
such as retweeting and including hashtags, usernames and URLs. The study is
limited in terms of the small set of astrophysicists. Results are not
necessarily representative of the entire astrophysicist community on Twitter
and they most certainly do not apply to scientists in general. Future research
should apply the methods to a larger set of researchers and other scientific
disciplines. To a certain extent, this study helps to understand how
researchers use Twitter. The results hint at the fact that impact on Twitter
can neither be equated with nor replace traditional research impact metrics.
However, tweets and other so-called altmetrics might be able to reflect other
impact of scientists such as public outreach and science communication. To the
best of our knowledge, this is the first in-depth study comparing researchers'
tweeting activity and behavior with scientific publication output in terms of
quantity, content and impact.Comment: 14 pages, 5 figures, 7 table
The Interplay of Magnetic Fields, Fragmentation and Ionization Feedback in High-Mass Star Formation
Massive stars disproportionately influence their surroundings. How they form
has only started to become clear recently through radiation gas dynamical
simulations. However, until now, no simulation has simultaneously included both
magnetic fields and ionizing radiation. Here we present the results from the
first radiation-magnetohydrodynamical (RMHD) simulation including ionization
feedback, comparing an RMHD model of a 1000 M_sol rotating cloud to earlier
radiation gas dynamical models with the same initial density and velocity
distributions. We find that despite starting with a strongly supercritical mass
to flux ratio, the magnetic field has three effects. First, the field offers
locally support against gravitational collapse in the accretion flow,
substantially reducing the amount of secondary fragmentation in comparison to
the gas dynamical case. Second, the field drains angular momentum from the
collapsing gas, further increasing the amount of material available for
accretion by the central, massive, protostar, and thus increasing its final
mass by about 50% from the purely gas dynamical case. Third, the field is wound
up by the rotation of the flow, driving a tower flow. However, this flow never
achieves the strength seen in low-mass star formation simulations for two
reasons: gravitational fragmentation disrupts the circular flow in the central
regions where the protostars form, and the expanding H II regions tend to
further disrupt the field geometry. Therefore, outflows driven by ionization
heating look likely to be more dynamically important in regions of massive star
formation.Comment: ApJ in pres
Symmetric box-splines on root lattices
AbstractRoot lattices are efficient sampling lattices for reconstructing isotropic signals in arbitrary dimensions, due to their highly symmetric structure. One root lattice, the Cartesian grid, is almost exclusively used since it matches the coordinate grid; but it is less efficient than other root lattices. Box-splines, on the other hand, generalize tensor-product B-splines by allowing non-Cartesian directions. They provide, in any number of dimensions, higher-order reconstructions of fields, often of higher efficiency than tensored B-splines. But on non-Cartesian lattices, such as the BCC (Body-Centered Cubic) or the FCC (Face-Centered Cubic) lattice, only some box-splines and then only up to dimension three have been investigated.This paper derives and completely characterizes efficient symmetric box-spline reconstruction filters on all irreducible root lattices that exist in any number of dimensions n≥2 (n≥3 for Dn and Dn∗ lattices). In all cases, box-splines are constructed by convolution using the lattice directions, generalizing the known constructions in two and three variables. For each box-spline, we document the basic properties for computational use: the polynomial degree, the continuity, the linear independence of shifts on the lattice and optimal quasi-interpolants for fast approximation of fields
A Practical Box Spline Compendium
Box splines provide smooth spline spaces as shifts of a single generating
function on a lattice and so generalize tensor-product splines. Their elegant
theory is laid out in classical papers and a summarizing book. This compendium
aims to succinctly but exhaustively survey symmetric low-degree box splines
with special focus on two and three variables. Tables contrast the lattices,
supports, analytic and reconstruction properties, and list available
implementations and code.Comment: 15 pages, 10 figures, 8 table
Avalanche Behavior in an Absorbing State Oslo Model
Self-organized criticality can be translated into the language of absorbing
state phase transitions. Most models for which this analogy is established have
been investigated for their absorbing state characteristics. In this article,
we transform the self-organized critical Oslo model into an absorbing state
Oslo model and analyze the avalanche behavior. We find that the resulting gap
exponent, D, is consistent with its value in the self-organized critical model.
For the avalanche size exponent, \tau, an analysis of the effect of the
external drive and the boundary conditions is required.Comment: 4 pages, 2 figures, REVTeX 4, submitted to PRE Brief Reports; added
reference and some extra information in V
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