606 research outputs found
Dynamic walking features and improved walking performance in multiple sclerosis patients treated with fampridine (4-aminopyridine)
Background: Impaired walking capacity is a frequent confinement in Multiple Sclerosis (MS). Patients are affected by limitations in coordination, walking speed and the distance they may cover. Also abnormal dynamic walking patterns have been reported, involving continuous deceleration over time. Fampridine (4-aminopyridine), a potassium channel blocker, may improve walking in MS. The objective of the current study was to comprehensively examine dynamic walking characteristics and improved walking capacity in MS patients treated with fampridine. Methods: A sample of N = 35 MS patients (EDSS median: 4) underwent an electronic walking examination prior to (Time 1), and during treatment with fampridine (Time 2). Patients walked back and forth a distance of 25 ft for a maximum period of 6 min (6-minute 25-foot-walk). Besides the total distance covered, average speed on the 25-foot distance and on turns was determined separately for each test minute, at Time 1 and Time 2. Results: Prior to fampridine administration, 27/35 patients (77 %) were able to complete the entire 6 min of walking, while following the administration, 34/35 patients (97 %) managed to walk for 6 min. In this context, walking distance considerably increased and treatment was associated with faster walking and turning across all six test minutes (range of effect sizes: partial eta squared = .34-.72). Importantly, previously reported deceleration across test minutes was consistently observable at Time 1 and Time 2. Discussion: Fampridine administration is associated with improved walking speed and endurance. Regardless of a treatment effect of fampridine, the previously identified, abnormal dynamic walking feature, i.e. the linear decline in walking speed, may represent a robust feature. Conclusions: The dynamic walking feature might hence be considered as a candidate for a new outcome measure in clinical studies involving interventions other than symptomatic treatment, such as immune-modulating medication. Trial registration: DRKS00009228 (German Clinical Trials Register). Date obtained: 25.08.2015
Cluster approximation solution of a two species annihilation model
A two species reaction-diffusion model, in which particles diffuse on a
one-dimensional lattice and annihilate when meeting each other, has been
investigated. Mean field equations for general choice of reaction rates have
been solved exactly. Cluster mean field approximation of the model is also
studied. It is shown that, the general form of large time behavior of one- and
two-point functions of the number operators, are determined by the diffusion
rates of the two type of species, and is independent of annihilation rates.Comment: 9 pages, 7 figure
Autonomous models solvable through the full interval method
The most general exclusion single species one dimensional reaction-diffusion
models with nearest-neighbor interactions which are both autonomous and can be
solved exactly through full interval method are introduced. Using a generating
function method, the general solution for, , the probability that
consecutive sites be full, is obtained. Some other correlation functions of
number operators at nonadjacent sites are also explicitly obtained. It is shown
that for a special choice of initial conditions some correlation functions of
number operators called full intervals remain uncorrelated
Exactly solvable models through the empty interval method
The most general one dimensional reaction-diffusion model with
nearest-neighbor interactions, which is exactly-solvable through the empty
interval method, has been introduced. Assuming translationally-invariant
initial conditions, the probability that consecutive sites are empty
(), has been exactly obtained. In the thermodynamic limit, the large-time
behavior of the system has also been investigated. Releasing the translational
invariance of the initial conditions, the evolution equation for the
probability that consecutive sites, starting from the site , are empty
() is obtained. In the thermodynamic limit, the large time behavior of
the system is also considered. Finally, the continuum limit of the model is
considered, and the empty-interval probability function is obtained.Comment: 12 pages, LaTeX2
Kovacs effect and fluctuation-dissipation relations in 1D kinetically constrained models
Strong and fragile glass relaxation behaviours are obtained simply changing
the constraints of the kinetically constrained Ising chain from symmetric to
purely asymmetric. We study the out-of-equilibrium dynamics of those two models
focusing on the Kovacs effect and the fluctuation--dissipation relations. The
Kovacs or memory effect, commonly observed in structural glasses, is present
for both constraints but enhanced with the asymmetric ones. Most surprisingly,
the related fluctuation-dissipation (FD) relations satisfy the FD theorem in
both cases. This result strongly differs from the simple quenching procedure
where the asymmetric model presents strong deviations from the FD theorem.Comment: 13 pages and 7 figures. To be published in J. Phys.
Unified Solution of the Expected Maximum of a Random Walk and the Discrete Flux to a Spherical Trap
Two random-walk related problems which have been studied independently in the
past, the expected maximum of a random walker in one dimension and the flux to
a spherical trap of particles undergoing discrete jumps in three dimensions,
are shown to be closely related to each other and are studied using a unified
approach as a solution to a Wiener-Hopf problem. For the flux problem, this
work shows that a constant c = 0.29795219 which appeared in the context of the
boundary extrapolation length, and was previously found only numerically, can
be derived explicitly. The same constant enters in higher-order corrections to
the expected-maximum asymptotics. As a byproduct, we also prove a new universal
result in the context of the flux problem which is an analogue of the Sparre
Andersen theorem proved in the context of the random walker's maximum.Comment: Two figs. Accepted for publication, Journal of Statistical Physic
Coherent State path-integral simulation of many particle systems
The coherent state path integral formulation of certain many particle systems
allows for their non perturbative study by the techniques of lattice field
theory. In this paper we exploit this strategy by simulating the explicit
example of the diffusion controlled reaction . Our results are
consistent with some renormalization group-based predictions thus clarifying
the continuum limit of the action of the problem.Comment: 20 pages, 4 figures. Minor corrections. Acknowledgement and reference
correcte
Exactly solvable reaction diffusion models on a Cayley tree
The most general reaction-diffusion model on a Cayley tree with
nearest-neighbor interactions is introduced, which can be solved exactly
through the empty-interval method. The stationary solutions of such models, as
well as their dynamics, are discussed. Concerning the dynamics, the spectrum of
the evolution Hamiltonian is found and shown to be discrete, hence there is a
finite relaxation time in the evolution of the system towards its stationary
state.Comment: 9 pages, 2 figure
Models solvable through the empty-interval method
The most general one dimensional reaction-diffusion model with
nearest-neighbor interactions solvable through the empty interval method, and
without any restriction on the particle-generation from two adjacent empty
sites is studied. It is shown that turning on the reactions which generate
particles from two adjacent empty sites, results in a gap in the spectrum of
the evolution operator (or equivalently a finite relaxation time).Comment: 8 page
Reaction Kinetics of Clustered Impurities
We study the density of clustered immobile reactants in the
diffusion-controlled single species annihilation. An initial state in which
these impurities occupy a subspace of codimension d' leads to a substantial
enhancement of their survival probability. The Smoluchowski rate theory
suggests that the codimensionality plays a crucial role in determining the long
time behavior. The system undergoes a transition at d'=2. For d'<2, a finite
fraction of the impurities survive: ni(t) ~ ni(infinity)+const x log(t)/t^{1/2}
for d=2 and ni(t) ~ ni(infinity)+const/t^{1/2} for d>2. Above this critical
codimension, d'>=2, the subspace decays indefinitely. At the critical
codimension, inverse logarithmic decay occurs, ni(t) ~ log(t)^{-a(d,d')}. Above
the critical codimension, the decay is algebraic ni(t) ~ t^{-a(d,d')}. In
general, the exponents governing the long time behavior depend on the dimension
as well as the codimension.Comment: 10 pages, late
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