711 research outputs found
Processing of information in synchroneously firing chains in networks of neurons
The Abeles model of cortical activity assumes that in absence of stimulation neural activity in zero order can be described by a Poisson process. Here the model is extended to describe information processing by synfire chains within a network of activity uncorrelated to the synfire chain. A quantitative derivation of the transfer function from this concept is given
Superconductivity-Related Insulating Behavior
We present the results of an experimental study of superconducting,
disordered, thin-films of amorphous Indium Oxide. These films can be driven
from the superconducting phase to a reentrant insulating state by the
application of a perpendicular magnetic field (). We find that the high-
insulator exhibits activated transport with a characteristic temperature,
. has a maximum value () that is close to the
superconducting transition temperature () at = 0, suggesting a
possible relation between the conduction mechanisms in the superconducting and
insulating phases. and display opposite dependences on the
disorder strength.Comment: Tex file and 5 figures; Revised version; To appear in Phys. Rev.
Lett. (2004
Nonlocal mechanism for cluster synchronization in neural circuits
The interplay between the topology of cortical circuits and synchronized
activity modes in distinct cortical areas is a key enigma in neuroscience. We
present a new nonlocal mechanism governing the periodic activity mode: the
greatest common divisor (GCD) of network loops. For a stimulus to one node, the
network splits into GCD-clusters in which cluster neurons are in zero-lag
synchronization. For complex external stimuli, the number of clusters can be
any common divisor. The synchronized mode and the transients to synchronization
pinpoint the type of external stimuli. The findings, supported by an
information mixing argument and simulations of Hodgkin Huxley population
dynamic networks with unidirectional connectivity and synaptic noise, call for
reexamining sources of correlated activity in cortex and shorter information
processing time scales.Comment: 8 pges, 6 figure
Theory of Interaction of Memory Patterns in Layered Associative Networks
A synfire chain is a network that can generate repeated spike patterns with
millisecond precision. Although synfire chains with only one activity
propagation mode have been intensively analyzed with several neuron models,
those with several stable propagation modes have not been thoroughly
investigated. By using the leaky integrate-and-fire neuron model, we
constructed a layered associative network embedded with memory patterns. We
analyzed the network dynamics with the Fokker-Planck equation. First, we
addressed the stability of one memory pattern as a propagating spike volley. We
showed that memory patterns propagate as pulse packets. Second, we investigated
the activity when we activated two different memory patterns. Simultaneous
activation of two memory patterns with the same strength led the propagating
pattern to a mixed state. In contrast, when the activations had different
strengths, the pulse packet converged to a two-peak state. Finally, we studied
the effect of the preceding pulse packet on the following pulse packet. The
following pulse packet was modified from its original activated memory pattern,
and it converged to a two-peak state, mixed state or non-spike state depending
on the time interval
Charge-Vortex Duality in Double-Layered Josephson Junction Arrays
A system of two parallel Josephson junction arrays coupled by interlayer
capacitances is considered in the situation where one layer is in the
vortex-dominated and the other in the charge-dominated regime. This system
shows a symmetry (duality) of the relevant degrees of freedom, i.e. the
vortices in one layer and the charges in the other. In contrast to single-layer
arrays both contribute to the kinetic energy. The charges feel the magnetic
field created by vortices, and, vice versa, the vortices feel a gauge field
created by charges. For long-range interaction of the charges the system
exhibits two Berezinskii-Kosterlitz-Thouless transitions, one for vortices and
another one for charges. The interlayer capacitance suppresses both transition
temperatures. The charge-unbinding transition is suppressed already for
relatively weak coupling, while the vortex-unbinding transition is more robust.
The shift of the transition temperature for vortices is calculated in the
quasi-classical approximation for arbitrary relations between the capacitances
(both weak and strong coupling).Comment: 12 pages, Revtex 3.
Coulomb blockade and quantum tunnelling in the low-conductivity phase of granular metals
We study the effects of Coulomb interaction and inter-grain quantum
tunnelling in an array of metallic grains using the phase-functional approach
for temperatures well below the charging energy of individual
grains yet large compared to the level spacing in the grains. When the
inter-grain tunnelling conductance , the conductivity in
dimensions decreases logarithmically with temperature
(), while for ,
the conductivity shows simple activated behaviour ().
We show, for bare tunnelling conductance , that the parameter
determines the competition between
charging and tunnelling effects. At low enough temperatures in the regime
, a charge is shared among a finite
number of grains, and we find a soft
activation behaviour of the conductivity, , where is the effective
coordination number of a grain.Comment: 11 pages REVTeX, 3 Figures. Appendix added, replaced with published
versio
Evidence for a novel functional role of cannabinoid CB2 receptors in the thalamus of neuropathic rats
Cannabinoid CB1 receptors have analgesic effects in models of neuropathic pain, but can also produce psychoactive side-effects. A supraspinal location of CB2 receptors has recently been described. CB2 agonists are also antinociceptive, although the functional role of supraspinal CB2 receptors in the control of nociception is unknown. Herein, we provide evidence that CB2 receptors in the thalamus play a functional role in the modulation of responses of neurons in the ventral posterior nucleus (VPL) of the thalamus in neuropathic, but not sham-operated, rats. Spontaneous and mechanically evoked activity of VPL neurons was recorded with a multichannel electrode array in anaesthetized spinal nerve-ligated (SNL) rats and compared to sham-operated rats. Intra-VPL administration of the CB2 agonist JWH-133 (30 ng in 500 nL) significantly reduced spontaneous (P < 0.05), non-noxious (P < 0.001) and noxious (P < 0.01) mechanically evoked responses of VPL neurons in SNL rats, but not in sham-operated rats. Inhibitory effects of JWH-133 on spontaneous (P < 0.01) and noxious-evoked (P < 0.001) responses of neurons were blocked by the CB2 antagonist SR144528. Local administration of SR144528 alone did not alter spontaneous or evoked responses of VPL neurons, but increased burst activity of VPL neurons in SNL rats. There were, however, no differences in levels of the endocannabinoids anandamide and 2AG in the thalamus of SNL and sham-operated rats. These data suggest that supraspinal CB2 receptors in the thalamus may contribute to the modulation of neuropathic pain responses
Stable Propagation of a Burst Through a One-Dimensional Homogeneous Excitatory Chain Model of Songbird Nucleus HVC
We demonstrate numerically that a brief burst consisting of two to six spikes
can propagate in a stable manner through a one-dimensional homogeneous
feedforward chain of non-bursting neurons with excitatory synaptic connections.
Our results are obtained for two kinds of neuronal models, leaky
integrate-and-fire (LIF) neurons and Hodgkin-Huxley (HH) neurons with five
conductances. Over a range of parameters such as the maximum synaptic
conductance, both kinds of chains are found to have multiple attractors of
propagating bursts, with each attractor being distinguished by the number of
spikes and total duration of the propagating burst. These results make
plausible the hypothesis that sparse precisely-timed sequential bursts observed
in projection neurons of nucleus HVC of a singing zebra finch are intrinsic and
causally related.Comment: 13 pages, 6 figure
Quantum effects in a superconducting glass model
We study disordered Josephson junctions arrays with long-range interaction
and charging effects. The model consists of two orthogonal sets of positionally
disordered parallel filaments (or wires) Josephson coupled at each crossing
and in the presence of a homogeneous and transverse magnetic field. The large
charging energy (resulting from small self-capacitance of the ultrathin wires)
introduces important quantum fluctuations of the superconducting phase within
each filament. Positional disorder and magnetic field frustration induce
spin-glass like ground state, characterized by not having long-range order of
the phases. The stability of this phase is destroyed for sufficiently large
charging energy. We have evaluated the temperature vs charging energy phase
diagram by extending the methods developed in the theory of infinite-range spin
glasses, in the limit of large magnetic field. The phase diagram in the
different temperature regimes is evaluated by using variety of methods, to wit:
semiclassical WKB and variational methods, Rayleigh-Schr\"{o}dinger
perturbation theory and pseudospin effective Hamiltonians. Possible
experimental consequences of these results are briefly discussed.Comment: 17 pages REVTEX. Two Postscript figures can be obtained from the
authors. To appear in PR
Charging Effects and Quantum Crossover in Granular Superconductors
The effects of the charging energy in the superconducting transition of
granular materials or Josephson junction arrays is investigated using a
pseudospin one model. Within a mean-field renormalization-group approach, we
obtain the phase diagram as a function of temperature and charging energy. In
contrast to early treatments, we find no sign of a reentrant transition in
agreement with more recent studies. A crossover line is identified in the
non-superconducting side of the phase diagram and along which we expect to
observe anomalies in the transport and thermodynamic properties. We also study
a charge ordering phase, which can appear for large nearest neighbor Coulomb
interaction, and show that it leads to first-order transitions at low
temperatures. We argue that, in the presence of charge ordering, a non
monotonic behavior with decreasing temperature is possible with a maximum in
the resistance just before entering the superconducting phase.Comment: 15 pages plus 4 fig. appended, Revtex, INPE/LAS-00
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