4,796 research outputs found
Mechanisms of Zero-Lag Synchronization in Cortical Motifs
Zero-lag synchronization between distant cortical areas has been observed in
a diversity of experimental data sets and between many different regions of the
brain. Several computational mechanisms have been proposed to account for such
isochronous synchronization in the presence of long conduction delays: Of
these, the phenomenon of "dynamical relaying" - a mechanism that relies on a
specific network motif - has proven to be the most robust with respect to
parameter mismatch and system noise. Surprisingly, despite a contrary belief in
the community, the common driving motif is an unreliable means of establishing
zero-lag synchrony. Although dynamical relaying has been validated in empirical
and computational studies, the deeper dynamical mechanisms and comparison to
dynamics on other motifs is lacking. By systematically comparing
synchronization on a variety of small motifs, we establish that the presence of
a single reciprocally connected pair - a "resonance pair" - plays a crucial
role in disambiguating those motifs that foster zero-lag synchrony in the
presence of conduction delays (such as dynamical relaying) from those that do
not (such as the common driving triad). Remarkably, minor structural changes to
the common driving motif that incorporate a reciprocal pair recover robust
zero-lag synchrony. The findings are observed in computational models of
spiking neurons, populations of spiking neurons and neural mass models, and
arise whether the oscillatory systems are periodic, chaotic, noise-free or
driven by stochastic inputs. The influence of the resonance pair is also robust
to parameter mismatch and asymmetrical time delays amongst the elements of the
motif. We call this manner of facilitating zero-lag synchrony resonance-induced
synchronization, outline the conditions for its occurrence, and propose that it
may be a general mechanism to promote zero-lag synchrony in the brain.Comment: 41 pages, 12 figures, and 11 supplementary figure
The propofol infusion syndrome: more puzzling evidence on a complex and poorly characterized disorder
The propofol infusion syndrome is a potentially devastating cardiovascular and metabolic derangement that has been described in both pediatric and adult patients sedated with propofol. Despite a large number of case reports that have appeared in the literature since 1992, the precise clinical features and pathophysiology of this disorder remain uncertain. Historically, the syndrome has been characterized by the occurrence of lactic acidosis, rhabdomyolysis, and circulatory collapse after several days of high-dose propofol infusion. The affected patients were typically young and critically ill, and the reported mortality was high. More recently, a number of atypical cases have been reported with favorable outcomes. These occurred after short-term or lower-dose infusions in noncritically ill patients in whom generally only a subset of the classical syndrome features was observed. It remains unclear whether these reports reflect true propofol infusion syndrome detected at an earlier and more salvageable stage, or mere associations with the use of sedative agents in general. Without better information on the true incidence of the propofol infusion syndrome, clinical guidelines on the safe use of this drug remain unsupported by good evidence
Practical Network Coding in Sensor Networks: Quo Vadis?
Abstract. Network coding is a novel concept for improving network ca-pacity. This additional capacity may be used to increase throughput or reliability. Also in wireless networks, network coding has been proposed as a method for improving communication. We present our experience from two studies of applying network coding in realistic wireless sen-sor networks scenarios. As we show, network coding is not as useful in practical deployments as earlier theoretical work suggested. We discuss limitations and future opportunities for network coding in sensor net-works. 1 Network Coding in Wireless Sensor Networks Network Coding was introduced by Ahlswede et al. [1], proving that it can in-crease multicast capacity. Since then, it has been investigated in several different networked scenarios which demand different traffic characteristics. Most previous research has focused on theoretical aspects of applying network coding to sensor networks. There are, however, also more practical examples of applying networ
Path-Integral Quantization of the (2,2) String
A complete treatment of the (2,2) NSR string in flat (2+2) dimensional
space-time is given, from the formal path integral over N=2 super Riemann
surfaces to the computational recipe for amplitudes at any loop or gauge
instanton number. We perform in detail the superconformal gauge fixing, discuss
the spectral flow, and analyze the supermoduli space with emphasis on the gauge
moduli. Background gauge field configurations in all instanton sectors are
constructed. We develop chiral bosonization on punctured higher-genus surfaces
in the presence of gauge moduli and instantons. The BRST cohomology is
recapitulated, with a new space-time interpretation for picture-changing. We
point out two ways of combining left- and right-movers, which lead to different
three-point functions.Comment: 36 pages, LaTeX; published version (typos & eq.(7.12) corrected
Scalar Deformations of Schwarzschild Holes and Their Stability
We construct two solutions of the minimally coupled Einstein-scalar field
equations, representing regular deformations of Schwarzschild black holes by a
self-interacting, static, scalar field. One solution features an exponentially
decaying scalar field and a triple-well interaction potential; the other one is
completely analytic and sprouts Coulomb-like scalar hair. Both evade the
no-hair theorem by having partially negative potential, in conflict with the
dominant energy condition. The linear perturbation theory around such
backgrounds is developed in general, and yields stability criteria in terms of
effective potentials for an analog Schr\"odinger problem. We can test for more
than half of the perturbation modes, and our solutions prove to be stable
against those.Comment: 24 pp, 16 figs, Latex; version published in Int. J. Mod. Phys.
Consciousness without Physical Basis. A Metaphysical Meditation on the Immortality of the Soul
Can we conceive of a mind without body? Does, for example, the idea of the soul's immortality make sense? Certain versions of materialism deny such questions; I shall try to prove that these versions of materialism cannot be right. They fail because they cannot account for the mental vocabulary from the language of brains in the vat. Envatted expressions such as "I think", "I believe", etc., do not have to be reinterpreted when we translate them to our language; they are semantically stable. By contrast, physical expressions from the vat language are semantically instable; due to Putnam's externalism they cannot be transported to our language without change. This contrast opens the way to a new understanding of what the immortality of the soul might be like: A brain in a vat (and its mental life) might survive what the brain calls "my physical body's death"
Long term Arecibo monitoring of the water megamaser in MG J0414+0534
We monitored the 22 GHz maser line in the lensed quasar MG J0414+0534 at
z=2.64 with the 300-m Arecibo telescope for almost two years to detect possible
additional maser components and to measure a potential velocity drift of the
lines. The main maser line profile is complex and can be resolved into a number
of broad features with line widths of 30-160 km/s. A new maser component was
tentatively detected in October 2008 at a velocity of +470 km/s. After
correcting for the estimated lens magnification, we find that the H2O isotropic
luminosity of the maser in MG J0414+0534 is about 26,000 solar luminosities,
making this source the most luminous ever discovered. Both the main line peak
and continuum flux densities are surprisingly stable throughout the period of
the observations. An upper limit on the velocity drift of the main peak of the
line has been estimated from our observations and is of the order of 2 km/s per
year. We discuss the results of the monitoring in terms of the possible nature
of the maser emission, associated with an accretion disk or a radio jet. This
is the first time that such a study is performed in a water maser source at
high redshift, potentially allowing us to study the parsec-scale environment
around a powerful radio source at cosmological distances.Comment: 5 pages, 2 figures, to appear in the Proceedings of the IAU Symposium
287, 2012, "Cosmic masers: from OH to H0
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