172 research outputs found
Modulational instability in nonlocal Kerr-type media with random parameters
Modulational instability of continuous waves in nonlocal focusing and
defocusing Kerr media with stochastically varying diffraction (dispersion) and
nonlinearity coefficients is studied both analytically and numerically. It is
shown that nonlocality with the sign-definite Fourier images of the medium
response functions suppresses considerably the growth rate peak and bandwidth
of instability caused by stochasticity. Contrary, nonlocality can enhance
modulational instability growth for a response function with negative-sign
bands.Comment: 6 pages, 12 figures, revTeX, to appear in Phys. Rev.
Aperiodicity in one-way Markov cycles and repeat times of large earthquakes in faults
A common use of Markov Chains is the simulation of the seismic cycle in a
fault, i.e. as a renewal model for the repetition of its characteristic
earthquakes. This representation is consistent with Reid's elastic rebound
theory. Here it is proved that in {\it any} one-way Markov cycle, the
aperiodicity of the corresponding distribution of cycle lengths is always lower
than one. This fact concurs with observations of large earthquakes in faults
all over the world
The effects of thyrotropin-releasing hormone and scopolamine in Alzheimer's disease and normal volunteers
Thyrotropin-releasing hormone (TRH), a neuromodulator and possibly a neurotransmitter in the central nervous system, was shown in a prior study of young normal volunteers to attenuate the memory impairment induced by the anticholinergic drug scopolamine. In the present study, the cognitive, behavioral and physiologic effects of high dose TRH (0.5 mg/kg), both alone and following administration of scopolamine, were examined in 10 Alzheimer's disease (AD) patients (mean age±SD=63.5 years) and 12 older normal volunteers (mean age=64.9±8.8 years). On the day AD subjects received TRH alone, modest but statistically significant improvement from baseline performance was documented on some tests of learning and memory, especially in those with mild dementia severity. In comparing cognitive test performance between the scopolamine alone and scopolamine+TRH conditions, only two test scores were significantly higher in the latter condition. In the group of older volunteers, TRH did not attenuate scopolamine-induced cognitive impairment, contrary to prior findings in a group of younger controls. In fact, older subjects performed worse after receiving scopolamine followed by TRH than after receiving scopolamine alone. In addition, no change from baseline cognitive performance was detected after subjects received TRH alone. These findings raise several questions and speculations on possible age-related changes in the cholinergic system, as well as on the mechanism of the interaction of TRH with the cholinergic system.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68371/2/10.1177_026988119200600404.pd
Scale free networks of earthquakes and aftershocks
We propose a new metric to quantify the correlation between any two
earthquakes. The metric consists of a product involving the time interval and
spatial distance between two events, as well as the magnitude of the first one.
According to this metric, events typically are strongly correlated to only one
or a few preceding ones. Thus a classification of events as foreshocks, main
shocks or aftershocks emerges automatically without imposing predefined
space-time windows. To construct a network, each earthquake receives an
incoming link from its most correlated predecessor. The number of aftershocks
for any event, identified by its outgoing links, is found to be scale free with
exponent . The original Omori law with emerges as a
robust feature of seismicity, holding up to years even for aftershock sequences
initiated by intermediate magnitude events. The measured fat-tailed
distribution of distances between earthquakes and their aftershocks suggests
that aftershock collection with fixed space windows is not appropriate.Comment: 7 pages and 7 figures. Submitte
The global picture of self-similar and not self-similar decay in Burgers Turbulence
This paper continue earlier investigations on the decay of Burgers turbulence
in one dimension from Gaussian random initial conditions of the power-law
spectral type . Depending on the power , different
characteristic regions are distinguished. The main focus of this paper is to
delineate the regions in wave-number and time in which self-similarity
can (and cannot) be observed, taking into account small- and large-
cutoffs. The evolution of the spectrum can be inferred using physical arguments
describing the competition between the initial spectrum and the new frequencies
generated by the dynamics. For large wavenumbers, we always have
region, associated to the shocks. When is less than one, the large-scale
part of the spectrum is preserved in time and the global evolution is
self-similar, so that scaling arguments perfectly predict the behavior in time
of the energy and of the integral scale. If is larger than two, the
spectrum tends for long times to a universal scaling form independent of the
initial conditions, with universal behavior at small wavenumbers. In the
interval the leading behaviour is self-similar, independent of and
with universal behavior at small wavenumber. When , the spectrum
has three scaling regions : first, a region at very small \ms1 with
a time-independent constant, second, a region at intermediate
wavenumbers, finally, the usual region. In the remaining interval,
the small- cutoff dominates, and also plays no role. We find also
(numerically) the subleading term in the evolution of the spectrum
in the interval . High-resolution numerical simulations have been
performed confirming both scaling predictions and analytical asymptotic theory.Comment: 14 pages, 19 figure
Modulational instability and nonlocality management in coupled NLS system
The modulational instability of two interacting waves in a nonlocal Kerr-type
medium is considered analytically and numerically. For a generic choice of wave
amplitudes, we give a complete description of stable/unstable regimes for zero
group-velocity mismatch. It is shown that nonlocality suppresses considerably
the growth rate and bandwidth of instability. For nonzero group-velocity
mismatch we perform a geometrical analysis of a nonlocality management which
can provide stability of waves otherwise unstable in a local medium.Comment: 15 pages, 12 figures, to be published in Physica Script
Point-occurrence self-similarity in crackling-noise systems and in other complex systems
It has been recently found that a number of systems displaying crackling
noise also show a remarkable behavior regarding the temporal occurrence of
successive events versus their size: a scaling law for the probability
distributions of waiting times as a function of a minimum size is fulfilled,
signaling the existence on those systems of self-similarity in time-size. This
property is also present in some non-crackling systems. Here, the uncommon
character of the scaling law is illustrated with simple marked renewal
processes, built by definition with no correlations. Whereas processes with a
finite mean waiting time do not fulfill a scaling law in general and tend
towards a Poisson process in the limit of very high sizes, processes without a
finite mean tend to another class of distributions, characterized by double
power-law waiting-time densities. This is somehow reminiscent of the
generalized central limit theorem. A model with short-range correlations is not
able to escape from the attraction of those limit distributions. A discussion
on open problems in the modeling of these properties is provided.Comment: Submitted to J. Stat. Mech. for the proceedings of UPON 2008 (Lyon),
topic: crackling nois
A pilot placebo-controlled study of chronic m-CPP administration in Alzheimer's disease
Meta000000-Chlorophenylpiperazine (m-CPP), a serotonin agonist and metabolite of the anti-depressant trazodone, was administered chronically to eight moderate to severely affected Alzheimer patients to determine whether it would produce improvement in behavioral symptoms complicating this illness. In doses up to 80 mg/day for 16 days, m-CPP was well tolerated and resulted in small but significant increases in anergy and depression-related symptoms compared with placebo. The effects of chronic m-CPP in this study contrast with the reported beneficial effects of the parent compound trazodone and selective 5-HT reuptake inhibitors in treating behavioral symptoms in Alzheimer patients.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29223/1/0000278.pd
- …