147,210 research outputs found
Neural networks with dynamical synapses: from mixed-mode oscillations and spindles to chaos
Understanding of short-term synaptic depression (STSD) and other forms of
synaptic plasticity is a topical problem in neuroscience. Here we study the
role of STSD in the formation of complex patterns of brain rhythms. We use a
cortical circuit model of neural networks composed of irregular spiking
excitatory and inhibitory neurons having type 1 and 2 excitability and
stochastic dynamics. In the model, neurons form a sparsely connected network
and their spontaneous activity is driven by random spikes representing synaptic
noise. Using simulations and analytical calculations, we found that if the STSD
is absent, the neural network shows either asynchronous behavior or regular
network oscillations depending on the noise level. In networks with STSD,
changing parameters of synaptic plasticity and the noise level, we observed
transitions to complex patters of collective activity: mixed-mode and spindle
oscillations, bursts of collective activity, and chaotic behaviour.
Interestingly, these patterns are stable in a certain range of the parameters
and separated by critical boundaries. Thus, the parameters of synaptic
plasticity can play a role of control parameters or switchers between different
network states. However, changes of the parameters caused by a disease may lead
to dramatic impairment of ongoing neural activity. We analyze the chaotic
neural activity by use of the 0-1 test for chaos (Gottwald, G. & Melbourne, I.,
2004) and show that it has a collective nature.Comment: 7 pages, Proceedings of 12th Granada Seminar, September 17-21, 201
Critical and resonance phenomena in neural networks
Brain rhythms contribute to every aspect of brain function. Here, we study
critical and resonance phenomena that precede the emergence of brain rhythms.
Using an analytical approach and simulations of a cortical circuit model of
neural networks with stochastic neurons in the presence of noise, we show that
spontaneous appearance of network oscillations occurs as a dynamical
(non-equilibrium) phase transition at a critical point determined by the noise
level, network structure, the balance between excitatory and inhibitory
neurons, and other parameters. We find that the relaxation time of neural
activity to a steady state, response to periodic stimuli at the frequency of
the oscillations, amplitude of damped oscillations, and stochastic fluctuations
of neural activity are dramatically increased when approaching the critical
point of the transition.Comment: 8 pages, Proceedings of 12th Granada Seminar, September 17-21, 201
Voyager cartography
The Jovian and Saturnian satellites are being mapped at several scales from Voyager 1 and 2 data. The maps are especially formatted color mosaics, controlled photomosaics, and airbrush maps. At 1:5,000,000 scale, mapping of Io, Europa, and Ganymede is complete. At 1:15,000,000 scale, mapping of Io and Europa is complete, and mapping of Ganymede is approximately complete. A controlled mosaic of Rhea has been compiled as a Digital Image Model (DIM) in the same format as is being used for Mars. The mosaic is being formatted for publication as a two-sheet set (Lambert Azimuthal Equal Area, Mercator, and Polar Stereographic projections). Magnetic tape copies of the DIM have been distributed to regional Planetary Image Facilities and other interested users. The DIM has a scale of 1/16 degree/pixel, corresponding to approximately 833 m/pixel on Rhea. Details of the status of the various map series are reported quarterly to Planetary Geology Principal Investigators
Diagnostics of accelerating plasma Semiannual progress report, 1 Mar. - 31 Aug. 1968
Plasma diagnostics in electromagnetically driven shock tubes using laser scattering methods as compared to spectroscopic technique
Dyons in N=4 Supersymmetric Theories and Three-Pronged Strings
We construct and explore BPS states that preserve 1/4 of supersymmetry in N=4
Yang-Mills theories. Such states are also realized as three-pronged strings
ending on D3-branes. We correct the electric part of the BPS equation and
relate its solutions to the unbroken abelian gauge group generators. Generic
1/4-BPS solitons are not spherically symmetric, but consist of two or more
dyonic components held apart by a delicate balance between static
electromagnetic force and scalar Higgs force. The instability previously found
in three-pronged string configurations is due to excessive repulsion by one of
these static forces. We also present an alternate construction of these 1/4-BPS
states from quantum excitations around a magnetic monopole, and build up the
supermultiplet for arbitrary (quantized) electric charge. The degeneracy and
the highest spin of the supermultiplet increase linearly with a relative
electric charge. We conclude with comments.Comment: 33 pages, two figures, LaTex, a footnote added, the figure caption of
Fig.2 expanded, one more referenc
Critical dynamics of the k-core pruning process
We present the theory of the k-core pruning process (progressive removal of
nodes with degree less than k) in uncorrelated random networks. We derive exact
equations describing this process and the evolution of the network structure,
and solve them numerically and, in the critical regime of the process,
analytically. We show that the pruning process exhibits three different
behaviors depending on whether the mean degree of the initial network is
above, equal to, or below the threshold _c corresponding to the emergence of
the giant k-core. We find that above the threshold the network relaxes
exponentially to the k-core. The system manifests the phenomenon known as
"critical slowing down", as the relaxation time diverges when tends to
_c. At the threshold, the dynamics become critical characterized by a
power-law relaxation (1/t^2). Below the threshold, a long-lasting transient
process (a "plateau" stage) occurs. This transient process ends with a collapse
in which the entire network disappears completely. The duration of the process
diverges when tends to _c. We show that the critical dynamics of the
pruning are determined by branching processes of spreading damage. Clusters of
nodes of degree exactly k are the evolving substrate for these branching
processes. Our theory completely describes this branching cascade of damage in
uncorrelated networks by providing the time dependent distribution function of
branching. These theoretical results are supported by our simulations of the
-core pruning in Erdos-Renyi graphs.Comment: 12 pages, 10 figure
Ensemble of Vortex Loops in the Abelian-Projected SU(3)-Gluodynamics
Grand canonical ensemble of small vortex loops emerging in the London limit
of the effective Abelian-projected theory of the SU(3)-gluodynamics is
investigated in the dilute gas approximation. An essential difference of this
system from the SU(2)-case is the presence of two interacting gases of vortex
loops. Two alternative representations for the partition function of such a
grand canonical ensemble are derived, and one of them, which is a
representation in terms of the integrals over vortex loops, is employed for the
evaluation of the correlators of both kinds of loops in the low-energy limit.Comment: 10 pages, LaTeX2e, no figures, minor corrections, to appear in Mod.
Phys. Lett.
Critical behavior of charmonia across the phase transition: A QCD sum rule approach
We investigate medium-induced change of mass and width of J/psi and eta_c
across the phase transition in hot gluonic matter using QCD sum rules. In the
QCD sum rule approach, the medium effect on heavy quarkonia is induced by the
change of both scalar and twist-2 gluon condensates, whose temperature
dependences are extracted from the lattice calculations of energy density and
pressure. Although the stability of the operator product expansion side seems
to break down at T > 1.06Tc for the vector channel and T>1.04Tc for the
pseudoscalar channel, we find a sudden change of the spectral property across
the critical temperature Tc, which originates from an equally rapid change of
the scalar gluon condensate characterized by e-3p. By parameterizing the ground
state of the spectral density by the Breit-Wigner form, we find that for both
J/psi and eta_c, the masses suddenly decrease maximally by a few hundreds of
MeV and the widths broaden to ~100 MeV slightly above Tc. Implications for
recent and future heavy ion experiments are discussed.
We also carry out a similar analysis for charmonia in nuclear matter, which
could serve as a testing ground for observing the precursor phenomena of the
QCD phase transition. We finally discuss the possibility of observing the mass
shift at nuclear matter at the FAIR project at GSI.Comment: 18 pages, 21 figures, 2 figures are added and discussion on effect of
dynamical quarks is extended. version to appear in Phys.Rev.
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