1,544 research outputs found
Monte Carlo study of the growth of striped domains
We analyze the dynamical scaling behavior in a two-dimensional spin model
with competing interactions after a quench to a striped phase. We measure the
growth exponents studying the scaling of the interfaces and the scaling of the
shrinking time of a ball of one phase plunged into the sea of another phase.
Our results confirm the predictions found in previous papers. The correlation
functions measured in the direction parallel and transversal to the stripes are
different as suggested by the existence of different interface energies between
the ground states of the model. Our simulations show anisotropic features for
the correlations both in the case of single-spin-flip and spin-exchange
dynamics.Comment: 15 pages, ReVTe
Natural clustering: the modularity approach
We show that modularity, a quantity introduced in the study of networked
systems, can be generalized and used in the clustering problem as an indicator
for the quality of the solution. The introduction of this measure arises very
naturally in the case of clustering algorithms that are rooted in Statistical
Mechanics and use the analogy with a physical system.Comment: 11 pages, 5 figure enlarged versio
Redundant variables and Granger causality
We discuss the use of multivariate Granger causality in presence of redundant
variables: the application of the standard analysis, in this case, leads to
under-estimation of causalities. Using the un-normalized version of the
causality index, we quantitatively develop the notions of redundancy and
synergy in the frame of causality and propose two approaches to group redundant
variables: (i) for a given target, the remaining variables are grouped so as to
maximize the total causality and (ii) the whole set of variables is partitioned
to maximize the sum of the causalities between subsets. We show the application
to a real neurological experiment, aiming to a deeper understanding of the
physiological basis of abnormal neuronal oscillations in the migraine brain.
The outcome by our approach reveals the change in the informational pattern due
to repetitive transcranial magnetic stimulations.Comment: 4 pages, 5 figures. Accepted for publication in Physical Review
Conserved Ising Model on the Human Connectome
Dynamical models implemented on the large scale architecture of the human
brain may shed light on how function arises from the underlying structure. This
is the case notably for simple abstract models, such as the Ising model. We
compare the spin correlations of the Ising model and the empirical functional
brain correlations, both at the single link level and at the modular level, and
show that their match increases at the modular level in anesthesia, in line
with recent results and theories. Moreover, we show that at the peak of the
specific heat (the \it{critical state}) the spin correlations are minimally
shaped by the underlying structural network, explaining how the best match
between structure and function is obtained at the onset of criticality, as
previously observed. These findings confirm that brain dynamics under
anesthesia shows a departure from criticality and could open the way to novel
perspectives when the conserved magnetization is interpreted in terms of an
homeostatic principle imposed to neural activity
Modelling Cortical Spreading Depression by a computational algorithm of distributed neural excitability: correlation with clinical features in single migraine with aura patients
Clinical correlates of mathematical modeling of cortical spreading depression: Single‐cases study
Introduction: Considerable connections between migraine with aura and cortical spreading depression (CSD), a depolarization wave originating in the visual cortex and traveling toward the frontal lobe, lead to the hypothesis that CSD is underlying migraine aura. The highly individual and complex characteristics of the brain cor‐ tex suggest that the geometry might impact the propagation of cortical spreading depression.
Methods: In a single‐case study, we simulated the CSD propagation for five migraine with aura patients, matching their symptoms during a migraine attack to the CSD wavefront propagation. This CSD wavefront was simulated on a patient‐specific tri‐ angulated cortical mesh obtained from individual MRI imaging and personalized dif‐ fusivity tensors derived locally from diffusion tensor imaging data.
Results: The CSD wave propagation was simulated on both hemispheres, despite in all but one patient the symptoms were attributable to one hemisphere. The CSD wave diffused with a large wavefront toward somatosensory and prefrontal regions, devoted to pain processing.
Discussion: This case‐control study suggests that the cortical geometry may con‐ tribute to the modality of CSD evolution and partly to clinical expression of aura symptoms. The simulated CSD is a large and diffuse phenomenon, possibly capa‐ ble to activate trigeminal nociceptors and to involve cortical areas devoted to pain processing
Expanding the Transfer Entropy to Identify Information Subgraphs in Complex Systems
We propose a formal expansion of the transfer entropy to put in evidence
irreducible sets of variables which provide information for the future state of
each assigned target. Multiplets characterized by a large contribution to the
expansion are associated to informational circuits present in the system, with
an informational character which can be associated to the sign of the
contribution. For the sake of computational complexity, we adopt the assumption
of Gaussianity and use the corresponding exact formula for the conditional
mutual information. We report the application of the proposed methodology on
two EEG data sets
Anisotropic dynamical scaling in a spin model with competing interactions
Results are presented for the kinetics of domain growth of a two-dimensional
Ising spin model with competing interactions quenched from a disordered to a
striped phase. The domain growth exponent are and for
single-spin-flip and spin-exchange dynamics, as found in previous simulations.
However the correlation functions measured in the direction parallel and
transversal to the stripes are different as suggested by the existence of
different interface energies between the ground states of the model. In the
case of single-spin-flip dynamics an anisotropic version of the
Ohta-Jasnow-Kawasaki theory for the pair scaling function can be used to fit
our data.Comment: 4 pages, REVTeX fil
Clustering data by inhomogeneous chaotic map lattices
A new approach to clustering, based on the physical properties of
inhomogeneous coupled chaotic maps, is presented. A chaotic map is assigned to
each data-point and short range couplings are introduced. The stationary regime
of the system corresponds to a macroscopic attractor independent of the initial
conditions. The mutual information between couples of maps serves to partition
the data set in clusters, without prior assumptions about the structure of the
underlying distribution of the data. Experiments on simulated and real data
sets show the effectiveness of the proposed algorithm.Comment: 8 pages, 6 figures. Revised version accepted for publication on
Physical Review Letter
Phase shifts of synchronized oscillators and the systolic/diastolic blood pressure relation
We study the phase-synchronization properties of systolic and diastolic
arterial pressure in healthy subjects. We find that delays in the oscillatory
components of the time series depend on the frequency bands that are
considered, in particular we find a change of sign in the phase shift going
from the Very Low Frequency band to the High Frequency band. This behavior
should reflect a collective behavior of a system of nonlinear interacting
elementary oscillators. We prove that some models describing such systems, e.g.
the Winfree and the Kuramoto models offer a clue to this phenomenon. For these
theoretical models there is a linear relationship between phase shifts and the
difference of natural frequencies of oscillators and a change of sign in the
phase shift naturally emerges.Comment: 8 figures, 9 page
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