339 research outputs found
Filament tension and phase-locked drift of meandering scroll waves
Rotating scroll waves are self-organising patterns which are found in many
oscillating or excitable systems. Here we show that quasi-periodic (meandering)
scroll waves, which include the rotors that organise cardiac arrhythmias,
exhibit filament tension when averaged over the meander cycle. With strong
filament curvature or medium thickness gradients, however, scroll wave dynamics
are governed by phase-locked drift instead of filament tension. Our results are
validated in computational models of cycloidal meander and a cardiac tissue
model with linear core.Comment: accepted for publication in Physical Review Letters (December 2017
Noise Effects on the Complex Patterns of Abnormal Heartbeats
Patients at high risk for sudden death often exhibit complex heart rhythms in
which abnormal heartbeats are interspersed with normal heartbeats. We analyze
such a complex rhythm in a single patient over a 12-hour period and show that
the rhythm can be described by a theoretical model consisting of two
interacting oscillators with stochastic elements. By varying the magnitude of
the noise, we show that for an intermediate level of noise, the model gives
best agreement with key statistical features of the dynamics.Comment: 4 pages, 4 figures, RevTe
Mechanisms of Surviving Burial: Dune Grass Interspecific Differences Drive Resource Allocation After Sand Deposition
Sand dunes are important geomorphic formations of coastal ecosystems that are critical in protecting human populations that live in coastal areas. Dune formation is driven by ecomorphodynamic interactions between vegetation and sediment deposition. While there has been extensive research on responses of dune grasses to sand burial, there is a knowledge gap in understanding mechanisms of acclimation between similar, coexistent, dune-building grasses such as Ammophila breviligulata (C3), Spartina patens (C4), and Uniola paniculata (C4). Our goal was to determine how physiological mechanisms of acclimation to sand burial vary between species. We hypothesize that (1) in the presence of burial, resource allocation will be predicated on photosynthetic pathway and that we will be able to characterize the C3 species as a root allocator and the C4 species as leaf allocators. We also hypothesize that (2) despite similarities between these species in habitat, growth form, and life history, leaf, root, and whole plant traits will vary between species when burial is not present. Furthermore, when burial is present, the existing variability in physiological strategy will drive species-specific mechanisms of survival. In a greenhouse experiment, we exposed three dune grass species to different burial treatments: 0 cm (control) and a one-time 25-cm burial to mimic sediment deposition during a storm. At the conclusion of our study, we collected a suite of physiological and morphological functional traits. Results showed that Ammophila decreased allocation to aboveground biomass to maintain root biomass, preserving photosynthesis by allocating nitrogen (N) into light-exposed leaves. Conversely, Uniola and Spartina decreased allocation to belowground production to increase elongation and maintain aboveground biomass. Interestingly, we found that species were functionally distinct when burial was absent; however, all species became more similar when treated with burial. In the presence of burial, species utilized functional traits of rapid growth strategy, although mechanisms of change were interspecifically variable
Chain Restaurant Calorie Posting Laws, Obesity, and Consumer Welfare
The 2010 Patient Protection and Affordable Care Act (ACA) introduced a mandate requiring chain restaurants to post calorie counts on menus and menu boards. This paper investigates whether and why calorie posting laws work. To do so, we develop a model of calories consumed that highlights two potential channels through which mandates influence choice and outlines an empirical strategy to disentangle these alternatives. We test the predictions of our model using data from the Behavioral Risk Factor Surveillance System to compare changes in body mass index (BMI), obesity, and consumer well-being in locations that implemented calorie-posting laws between 2008 and 2011 to those in neighboring locations without such laws. We find that calorie mandates lead to a small but statistically significant reduction in average BMI of 0.2 kg/m2 (1.5 pounds) and reductions in self-reported measures of life satisfaction. Quantile regressions provide evidencethat reductions in BMI and life satisfaction are concentrated among those with healthy weight. Viewed in its totality, the pattern of results is consistent with an economic model in which calorie labels influence consumers both by providing information and by imposing a welfare-reducing moral cost on unhealthy eating
Complex Patterns in Reaction-Diffusion Systems: A Tale of Two Front Instabilities
Two front instabilities in a reaction-diffusion system are shown to lead to
the formation of complex patterns. The first is an instability to transverse
modulations that drives the formation of labyrinthine patterns. The second is a
Nonequilibrium Ising-Bloch (NIB) bifurcation that renders a stationary planar
front unstable and gives rise to a pair of counterpropagating fronts. Near the
NIB bifurcation the relation of the front velocity to curvature is highly
nonlinear and transitions between counterpropagating fronts become feasible.
Nonuniformly curved fronts may undergo local front transitions that nucleate
spiral-vortex pairs. These nucleation events provide the ingredient needed to
initiate spot splitting and spiral turbulence. Similar spatio-temporal
processes have been observed recently in the ferrocyanide-iodate-sulfite
reaction.Comment: Text: 14 pages compressed Postscript (90kb) Figures: 9 pages
compressed Postscript (368kb
Complex patterns of spontaneous initiations and terminations of reentrant circulation in a loop of cardiac tissue
A two-component model is developed that consists of a discrete loop of
cardiac cells that circulates action potentials together with a cardiac pacing
mechanism. Physiological properties of cells such as restitutions of
refractoriness and of conduction velocity are given via experimentally measured
functions. The dynamics of circulating pulses and their interactions with the
pacer are regulated by two threshold relations. Patterns of spontaneous
initiations and terminations of reentry (SITR) generated by this system are
studied through numerical simulations and analytical observations. These
patterns can be regular or irregular; causes of irregularities are identified
as the threshold bistability of reentrant circulation (T-bistability) and in
some cases, also phase-resetting interactions with the pacer.Comment: 27 pages, 10 figures, 61 references; A version of this paper (same
results) is to appear in the Journal of Theoretical Biology; arXiv V2 adds
helpful commments to facilitate reading and corrects minor errors in
presentatio
Points, Walls and Loops in Resonant Oscillatory Media
In an experiment of oscillatory media, domains and walls are formed under the
parametric resonance with a frequency double the natural one. In this bi-stable
system, %phase jumps by crossing walls. a nonequilibrium transition from
Ising wall to Bloch wall consistent with prediction is confirmed
experimentally. The Bloch wall moves in the direction determined by its
chirality with a constant speed. As a new type of moving structure in
two-dimension, a traveling loop consisting of two walls and Neel points is
observed.Comment: 9 pages (revtex format) and 6 figures (PostScript
From Labyrinthine Patterns to Spiral Turbulence
A new mechanism for spiral vortex nucleation in nongradient reaction
diffusion systems is proposed. It involves two key ingredients: An Ising-Bloch
type front bifurcation and an instability of a planar front to transverse
perturbations. Vortex nucleation by this mechanism plays an important role in
inducing a transition from labyrinthine patterns to spiral turbulence. PACS
numbers: 05.45.+b, 82.20.MjComment: 4 pages uuencoded compressed postscrip
The Dynamics of Sustained Reentry in a Loop Model with Discrete Gap Junction Resistance
Dynamics of reentry are studied in a one dimensional loop of model cardiac
cells with discrete intercellular gap junction resistance (). Each cell is
represented by a continuous cable with ionic current given by a modified
Beeler-Reuter formulation. For below a limiting value, propagation is found
to change from period-1 to quasi-periodic () at a critical loop length
() that decreases with . Quasi-periodic reentry exists from
to a minimum length () that is also shortening with .
The decrease of is not a simple scaling, but the bifurcation can
still be predicted from the slope of the restitution curve giving the duration
of the action potential as a function of the diastolic interval. However, the
shape of the restitution curve changes with .Comment: 6 pages, 7 figure
Instability and Spatiotemporal Dynamics of Alternans in Paced Cardiac Tissue
We derive an equation that governs the spatiotemporal dynamics of small
amplitude alternans in paced cardiac tissue. We show that a pattern-forming
linear instability leads to the spontaneous formation of stationary or
traveling waves whose nodes divide the tissue into regions with opposite phase
of oscillation of action potential duration. This instability is important
because it creates dynamically an heterogeneous electrical substrate for
inducing fibrillation if the tissue size exceeds a fraction of the pattern
wavelength. We compute this wavelength analytically as a function of three
basic length scales characterizing dispersion and inter-cellular electrical
coupling.Comment: 4 pages, 3 figures, submitted to PR
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