2,058 research outputs found
Storage of phase-coded patterns via STDP in fully-connected and sparse network: a study of the network capacity
We study the storage and retrieval of phase-coded patterns as stable
dynamical attractors in recurrent neural networks, for both an analog and a
integrate-and-fire spiking model. The synaptic strength is determined by a
learning rule based on spike-time-dependent plasticity, with an asymmetric time
window depending on the relative timing between pre- and post-synaptic
activity. We store multiple patterns and study the network capacity.
For the analog model, we find that the network capacity scales linearly with
the network size, and that both capacity and the oscillation frequency of the
retrieval state depend on the asymmetry of the learning time window. In
addition to fully-connected networks, we study sparse networks, where each
neuron is connected only to a small number z << N of other neurons. Connections
can be short range, between neighboring neurons placed on a regular lattice, or
long range, between randomly chosen pairs of neurons. We find that a small
fraction of long range connections is able to amplify the capacity of the
network. This imply that a small-world-network topology is optimal, as a
compromise between the cost of long range connections and the capacity
increase.
Also in the spiking integrate and fire model the crucial result of storing
and retrieval of multiple phase-coded patterns is observed. The capacity of the
fully-connected spiking network is investigated, together with the relation
between oscillation frequency of retrieval state and window asymmetry
Neural Avalanches at the Critical Point between Replay and Non-Replay of Spatiotemporal Patterns
We model spontaneous cortical activity with a network of coupled spiking
units, in which multiple spatio-temporal patterns are stored as dynamical
attractors. We introduce an order parameter, which measures the overlap
(similarity) between the activity of the network and the stored patterns. We
find that, depending on the excitability of the network, different working
regimes are possible. For high excitability, the dynamical attractors are
stable, and a collective activity that replays one of the stored patterns
emerges spontaneously, while for low excitability, no replay is induced.
Between these two regimes, there is a critical region in which the dynamical
attractors are unstable, and intermittent short replays are induced by noise.
At the critical spiking threshold, the order parameter goes from zero to one,
and its fluctuations are maximized, as expected for a phase transition (and as
observed in recent experimental results in the brain). Notably, in this
critical region, the avalanche size and duration distributions follow power
laws. Critical exponents are consistent with a scaling relationship observed
recently in neural avalanches measurements. In conclusion, our simple model
suggests that avalanche power laws in cortical spontaneous activity may be the
effect of a network at the critical point between the replay and non-replay of
spatio-temporal patterns
Thermal Operators in Ising Percolation
We discuss a new cluster representation for the internal energy and the
specific heat of the d-dimensional Ising model, obtained by studying the
percolation mapping of an Ising model with an arbitrary set of
antiferromagnetic links. Such a representation relates the thermal operators to
the topological properties of the Fortuin-Kasteleyn clusters of Ising
percolation and is a powerful tool to get new exact relations on the
topological structure of FK clusters of the Ising model defined on an arbitrary
graph.Comment: 17 pages, 2 figures. Improved version. Major changes in the text and
in the notations. A missing term added in the specific heat representatio
Scaling and universality in glass transition
Kinetic facilitated models and the Mode Coupling Theory (MCT) model B are
within those systems known to exhibit a discontinuous dynamical transition with
a two step relaxation. We consider a general scaling approach, within mean
field theory, for such systems by considering the behavior of the density
correlator and the dynamical susceptibility -^2. Focusing
on the Fredrickson and Andersen (FA) facilitated spin model on the Bethe
lattice, we extend a cluster approach that was previously developed for
continuous glass transitions by Arenzon et al (Phys. Rev. E 90, 020301(R)
(2014)) to describe the decay to the plateau, and consider a damage spreading
mechanism to describe the departure from the plateau. We predict scaling laws,
which relate dynamical exponents to the static exponents of mean field
bootstrap percolation. The dynamical behavior and the scaling laws for both
density correlator and dynamical susceptibility coincide with those predicted
by MCT. These results explain the origin of scaling laws and the universal
behavior associated with the glass transition in mean field, which is
characterized by the divergence of the static length of the bootstrap
percolation model with an upper critical dimension d_c=8.Comment: 16 pages, 9 figure
Glassy behavior of the site frustrated percolation model
The dynamical properties of the site frustrated percolation model are
investigated and compared with those of glass forming liquids. When the density
of the particles on the lattice becomes high enough, the dynamics of the model
becomes very slow, due to geometrical constraints, and rearrangement on large
scales is needed to allow relaxation. The autocorrelation functions, the
specific volume for different cooling rates, and the mean square displacement
are evaluated, and are found to exhibit glassy behavior.Comment: 8 pages, RevTeX, 11 fig
Disordered jammed packings of frictionless spheres
At low volume fraction, disordered arrangements of frictionless spheres are
found in un--jammed states unable to support applied stresses, while at high
volume fraction they are found in jammed states with mechanical strength. Here
we show, focusing on the hard sphere zero pressure limit, that the transition
between un-jammed and jammed states does not occur at a single value of the
volume fraction, but in a whole volume fraction range. This result is obtained
via the direct numerical construction of disordered jammed states with a volume
fraction varying between two limits, and . We identify these
limits with the random loose packing volume fraction \rl and the random close
packing volume fraction \rc of frictionless spheres, respectively
Off equilibrium response function in the one dimensional random field Ising model
A thorough numerical investigation of the slow dynamics in the d=1 random
field Ising model in the limit of an infinite ferromagnetic coupling is
presented. Crossovers from the preasymptotic pure regime to the asymptotic
Sinai regime are investigated for the average domain size, the autocorrelation
function and staggered magnetization. By switching on an additional small
random field at the time tw the linear off equilibrium response function is
obtained, which displays as well the crossover from the nontrivial behavior of
the d=1 pure Ising model to the asymptotic behavior where it vanishes
identically.Comment: 12 pages, 10 figure
Crossover properties from random percolation to frustrated percolation
We investigate the crossover properties of the frustrated percolation model
on a two-dimensional square lattice, with asymmetric distribution of
ferromagnetic and antiferromagnetic interactions. We determine the critical
exponents nu, gamma and beta of the percolation transition of the model, for
various values of the density of antiferromagnetic interactions pi in the range
0<pi<0.5. Our data are consistent with the existence of a crossover from random
percolation behavior for pi=0, to frustrated percolation behavior,
characterized by the critical exponents of the ferromagnetic 1/2-state Potts
model, as soon as pi>0.Comment: 5 pages, 7 figs, RevTe
Relationship between cigarette smoking and body mass index in the Italian population
Objective: Tobacco smoke, as numerous studies have established, is due to some diseases related to the respiratory, cardio - circulatory and gastrointestinal systems. In addition, tobacco consumption is often associated with a decrease in weight, while former smokers often experience an increase in BMI. The purpose of this research is to test the behavior of body mass index in relation to a representative sample of the Italian population, covering the period 2004-2005. Design: Calculations were made using SPSS software. The data were stratified by age and gender, using odds ratio (OR) to determine the significance of smoking in relation to BMI. The OR was calculated both in a timely manner, both considering a confidence interval of 95%. Material and methods: In this research, we will analyze the variable BMIâ€, in relation to the variable tobacco smokeâ€, derived from the multipurpose ISTAT called Condizione di salute e ricorso ai servizi sanitari. The sample was stratified by age, 18 years and over, and gender. Results: The analysis shows that in 2005, 10% of the Italian population results in overweight, while 22% said they smoke; 23% of female smokers claiming to be underweight, while for the former smokers the value drops to 10%. For males, the difference in BMI between smokers and former smokers is minimal. The OR shows that weight gain among former smokers and smokers is actually due to custom/cessation from tobacco smoke. Conclusion: The structure of BMI in relation to smoking has confirmed the existence emerged in the literature, so that those who quit smoking reflects an increase in weight. However, the results highlighted by some foreign studies, which propose an increase in obesity among former smokers, is not confirmed in the Italian population.Keywords: body mass index; smoking; odds ratio
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