225 research outputs found
Synchronization of weakly perturbed Markov chain oscillators
Rate processes are simple and analytically tractable models for many
dynamical systems which switch stochastically between a discrete set of quasi
stationary states but they may also approximate continuous processes by coarse
grained, symbolic dynamics. In contrast to limit cycle oscillators which are
weakly perturbed by noise, the stochasticity in such systems may be strong and
more complicated system topologies than the circle can be considered. Here we
employ second order, time dependent perturbation theory to derive expressions
for the mean frequency and phase diffusion constant of discrete state
oscillators coupled or driven through weakly time dependent transition rates.
We also describe a method of global control to optimize the response of the
mean frequency in complex transition networks.Comment: 16 pages, 7 figure
Noise-Induced Synchronization of a Large Population of Globally Coupled Nonidentical Oscillators
We study a large population of globally coupled phase oscillators subject to
common white Gaussian noise and find analytically that the critical coupling
strength between oscillators for synchronization transition decreases with an
increase in the intensity of common noise. Thus, common noise promotes the
onset of synchronization. Our prediction is confirmed by numerical simulations
of the phase oscillators as well as of limit-cycle oscillators
Collective fluctuations in networks of noisy components
Collective dynamics result from interactions among noisy dynamical
components. Examples include heartbeats, circadian rhythms, and various pattern
formations. Because of noise in each component, collective dynamics inevitably
involve fluctuations, which may crucially affect functioning of the system.
However, the relation between the fluctuations in isolated individual
components and those in collective dynamics is unclear. Here we study a linear
dynamical system of networked components subjected to independent Gaussian
noise and analytically show that the connectivity of networks determines the
intensity of fluctuations in the collective dynamics. Remarkably, in general
directed networks including scale-free networks, the fluctuations decrease more
slowly with the system size than the standard law stated by the central limit
theorem. They even remain finite for a large system size when global
directionality of the network exists. Moreover, such nontrivial behavior
appears even in undirected networks when nonlinear dynamical systems are
considered. We demonstrate it with a coupled oscillator system.Comment: 5 figure
Dynamics-based centrality for general directed networks
Determining the relative importance of nodes in directed networks is
important in, for example, ranking websites, publications, and sports teams,
and for understanding signal flows in systems biology. A prevailing centrality
measure in this respect is the PageRank. In this work, we focus on another
class of centrality derived from the Laplacian of the network. We extend the
Laplacian-based centrality, which has mainly been applied to strongly connected
networks, to the case of general directed networks such that we can
quantitatively compare arbitrary nodes. Toward this end, we adopt the idea used
in the PageRank to introduce global connectivity between all the pairs of nodes
with a certain strength. Numerical simulations are carried out on some
networks. We also offer interpretations of the Laplacian-based centrality for
general directed networks in terms of various dynamical and structural
properties of networks. Importantly, the Laplacian-based centrality defined as
the stationary density of the continuous-time random walk with random jumps is
shown to be equivalent to the absorption probability of the random walk with
sinks at each node but without random jumps. Similarly, the proposed centrality
represents the importance of nodes in dynamics on the original network supplied
with sinks but not with random jumps.Comment: 7 figure
Strong Effects of Network Architecture in the Entrainment of Coupled Oscillator Systems
Entrainment of randomly coupled oscillator networks by periodic external
forcing applied to a subset of elements is numerically and analytically
investigated. For a large class of interaction functions, we find that the
entrainment window with a tongue shape becomes exponentially narrow for
networks with higher hierarchical organization. However, the entrainment is
significantly facilitated if the networks are directionally biased, i.e.,
closer to the feedforward networks. Furthermore, we show that the networks with
high entrainment ability can be constructed by evolutionary optimization
processes. The neural network structure of the master clock of the circadian
rhythm in mammals is discussed from the viewpoint of our results.Comment: 15 pages, 11 figures, RevTe
Estimated population access to acute stroke and telestroke centers in the US, 2019
This cross-sectional study assesses US population access to emergency departments with acute stroke capabilities and telestroke capacity in 2019
Acute Oral Toxicity Test of Nicotiana tabacum L. Bio-Oil Against Female Winstar Rats
Tobacco plants are notably known for its pesticidal properties, particularly due to its
nicotine content. In this study, Nicotiana tabacum L. bio-oil was obtained using pyrolysis
technique. The safety of the bio-oil to be used as bioinsecticide was analyzed through acute oral
toxicity test by administering 5000 mg bio-oil/kg body weight of female winstar rats that were
analogous to humans. It was concluded that the bio-oil was not toxic due to absence of mortality
and no significant change in the body weight and behavior of the rats
Formation of feedforward networks and frequency synchrony by spike-timing-dependent plasticity
Spike-timing-dependent plasticity (STDP) with asymmetric learning windows is
commonly found in the brain and useful for a variety of spike-based
computations such as input filtering and associative memory. A natural
consequence of STDP is establishment of causality in the sense that a neuron
learns to fire with a lag after specific presynaptic neurons have fired. The
effect of STDP on synchrony is elusive because spike synchrony implies unitary
spike events of different neurons rather than a causal delayed relationship
between neurons. We explore how synchrony can be facilitated by STDP in
oscillator networks with a pacemaker. We show that STDP with asymmetric
learning windows leads to self-organization of feedforward networks starting
from the pacemaker. As a result, STDP drastically facilitates frequency
synchrony. Even though differences in spike times are lessened as a result of
synaptic plasticity, the finite time lag remains so that perfect spike
synchrony is not realized. In contrast to traditional mechanisms of large-scale
synchrony based on mutual interaction of coupled neurons, the route to
synchrony discovered here is enslavement of downstream neurons by upstream
ones. Facilitation of such feedforward synchrony does not occur for STDP with
symmetric learning windows.Comment: 9 figure
Prevalence of unstable attractors in networks of pulse-coupled oscillators
We present and analyze the first example of a dynamical system that naturally
exhibits attracting periodic orbits that are \textit{unstable}. These unstable
attractors occur in networks of pulse-coupled oscillators where they prevail
for large networks and a wide range of parameters. They are enclosed by basins
of attraction of other attractors but are remote from their own basin volume
such that arbitrarily small noise leads to a switching among attractors.Comment: 5 pages, 3 figure
2018 Robotic Scene Segmentation Challenge
In 2015 we began a sub-challenge at the EndoVis workshop at MICCAI in Munich using endoscope images of ex-vivo tissue with automatically generated annotations from robot forward kinematics and instrument CAD models. However, the limited background variation and simple motion rendered the dataset uninformative in learning about which techniques would be suitable for segmentation in real surgery. In 2017, at the same workshop in Quebec we introduced the robotic instrument segmentation dataset with 10 teams participating in the challenge to perform binary, articulating parts and type segmentation of da Vinci instruments. This challenge included realistic instrument motion and more complex porcine tissue as background and was widely addressed with modifications on U-Nets and other popular CNN architectures. In 2018 we added to the complexity by introducing a set of anatomical objects and medical devices to the segmented classes. To avoid over-complicating the challenge, we continued with porcine data which is dramatically simpler than human tissue due to the lack of fatty tissue occluding many organs
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