4,229 research outputs found
Audiovisual temporal correspondence modulates human multisensory superior temporal sulcus plus primary sensory cortices
The brain should integrate related but not unrelated information from different senses. Temporal patterning of inputs to different modalities may provide critical information about whether those inputs are related or not. We studied effects of temporal correspondence between auditory and visual streams on human brain activity with functional magnetic resonance imaging ( fMRI). Streams of visual flashes with irregularly jittered, arrhythmic timing could appear on right or left, with or without a stream of auditory tones that coincided perfectly when present ( highly unlikely by chance), were noncoincident with vision ( different erratic, arrhythmic pattern with same temporal statistics), or an auditory stream appeared alone. fMRI revealed blood oxygenation level-dependent ( BOLD) increases in multisensory superior temporal sulcus (mSTS), contralateral to a visual stream when coincident with an auditory stream, and BOLD decreases for noncoincidence relative to unisensory baselines. Contralateral primary visual cortex and auditory cortex were also affected by audiovisual temporal correspondence or noncorrespondence, as confirmed in individuals. Connectivity analyses indicated enhanced influence from mSTS on primary sensory areas, rather than vice versa, during audiovisual correspondence. Temporal correspondence between auditory and visual streams affects a network of both multisensory ( mSTS) and sensory-specific areas in humans, including even primary visual and auditory cortex, with stronger responses for corresponding and thus related audiovisual inputs
Renormalization for Discrete Optimization
The renormalization group has proven to be a very powerful tool in physics
for treating systems with many length scales. Here we show how it can be
adapted to provide a new class of algorithms for discrete optimization. The
heart of our method uses renormalization and recursion, and these processes are
embedded in a genetic algorithm. The system is self-consistently optimized on
all scales, leading to a high probability of finding the ground state
configuration. To demonstrate the generality of such an approach, we perform
tests on traveling salesman and spin glass problems. The results show that our
``genetic renormalization algorithm'' is extremely powerful.Comment: 4 pages, no figur
Critical points and quenched disorder: From Harris criterion to rare regions and smearing
We consider the influence of quenched spatial disorder on phase transitions
in classical and quantum systems. We show that rare strong disorder
fluctuations can have dramatic effects on critical points. In classical systems
with sufficiently correlated disorder or in quantum systems with overdamped
dynamics they can completely destroy the sharp phase transition by smearing.
This is caused by effects similar to but stronger than Griffiths phenomena:
True static order can develop on a rare region while the bulk system is still
in the disordered phase. We discuss the thermodynamic behavior in the vicinity
of such a smeared transition using optimal fluctuation theory, and we present
numerical results for a two-dimensional model system.Comment: 10 pages, 5 eps figures, contribution to the Festschrift for Michael
Schreiber's 50th birthday, final version as publishe
Antiferromagnetic Heisenberg chains with bond alternation and quenched disorder
We consider S=1/2 antiferromagnetic Heisenberg chains with alternating bonds
and quenched disorder, which represents a theoretical model of the compound
CuCl_{2x}Br_{2(1-x)}(\gamma-{pic})_2. Using a numerical implementation of the
strong disorder renormalization group method we study the low-energy properties
of the system as a function of the concentration, x, and the type of
correlations in the disorder. For perfect correlation of disorder the system is
in the random dimer (Griffiths) phase having a concentration dependent
dynamical exponent. For weak or vanishing disorder correlations the system is
in the random singlet phase, in which the dynamical exponent is formally
infinity. We discuss consequences of our results for the experimentally
measured low-temperature susceptibility of
CuCl_{2x}Br_{2(1-x)}(\gamma-{pic})_2
Three-body interactions with cold polar molecules
We show that polar molecules driven by microwave fields give naturally rise
to strong three-body interactions, while the two-particle interaction can be
independently controlled and even switched off. The derivation of these
effective interaction potentials is based on a microscopic understanding of the
underlying molecular physics, and follows from a well controlled and systematic
expansion into many-body interaction terms. For molecules trapped in an optical
lattice, we show that these interaction potentials give rise to Hubbard models
with strong nearest-neighbor two-body and three-body interaction. As an
illustration, we study the one-dimensional Bose-Hubbard model with dominant
three-body interaction and derive its phase diagram.Comment: 8 pages, 4 figure
Numerical analysis of the magnetic-field-tuned superconductor-insulator transition in two dimensions
Ground state of the two-dimensional hard-core-boson model subjected to
external magnetic field and quenched random chemical potential is studied
numerically. In experiments, magnetic-field-tuned superconductor-insulator
transition has already come under through investigation, whereas in computer
simulation, only randomness-driven localization (with zero magnetic field) has
been studied so far: The external magnetic field brings about a difficulty that
the hopping amplitude becomes complex number (through the gauge twist), for
which the quantum Monte-Carlo simulation fails. Here, we employ the exact
diagonalization method, with which we demonstrate that the model does exhibit
field-tuned localization transition at a certain critical magnetic field. At
the critical point, we found that the DC conductivity is not universal, but is
substantially larger than that of the randomness-driven localization transition
at zero magnetic field. Our result supports recent experiment by Markovi'c et
al. reporting an increase of the critical conductivity with magnetic field
strengthened
Retrorectal endometrioid cyst: a case report
<p>Abstract</p> <p>Introduction</p> <p>Developmental cysts are the most common retrorectal cystic lesions in adults, whereas reports of endometrioid cysts in this anatomic location are extremely rare.</p> <p>Case presentation</p> <p>A 21-year-old nulliparous Greek woman presented with chronic noncyclic pelvic pain, and a retrorectal cyst was diagnosed. The lesion was resected through a laparotomy and, on histologic examination, was found to be an endometrioid cyst. The treatment was completed with a six-month course of a gonadotropin-releasing hormone analogue. One year after surgery, the woman remained free of symptoms, and pelvic imaging showed no recurrence of the lesion. Reviewing the literature, we found only three previous reports of an endometrioid cyst in this anatomic location.</p> <p>Conclusion</p> <p>In women of reproductive age, endometriosis must be included in the differential diagnosis of retrorectal cysts.</p
Percolation in random environment
We consider bond percolation on the square lattice with perfectly correlated
random probabilities. According to scaling considerations, mapping to a random
walk problem and the results of Monte Carlo simulations the critical behavior
of the system with varying degree of disorder is governed by new, random fixed
points with anisotropic scaling properties. For weaker disorder both the
magnetization and the anisotropy exponents are non-universal, whereas for
strong enough disorder the system scales into an {\it infinite randomness fixed
point} in which the critical exponents are exactly known.Comment: 8 pages, 7 figure
From elastomers to thermoplasts – Precise control of isotactic propylene structure and properties and the role of different structural elements in its mechanical behaviour
Studies of polypropylene with different degrees of isotacticity have shown a way of the rational design of material with predetermined mechanical properties starting from the synthesis stage already – controlled introducement of stereodefects will allow the smooth adjustment of the Young's modulus and elasticity in the range from plastic to elastomer materials. It was also revealed that modern theoretical models of the elasticity can be successfully applied not only for the description of the mechanical behaviour of polymers, but also for better understanding of the mechanism of elasticity in them. While in the low crystalline materials deformation has Gaussian nature, in the materials of the intermediate crystallinity (30–40%) percolation takes place, and the cross-linking network becomes harder, manifesting the switch to the thermotropic behaviour of the material. Simultaneously the divide between cross- and slip-links becomes substantial, as an extensibility grows sharply
Exact renormalization of the random transverse-field Ising spin chain in the strongly ordered and strongly disordered Griffiths phases
The real-space renormalization group (RG) treatment of random
transverse-field Ising spin chains by Fisher ({\it Phys. Rev. B{\bf 51}, 6411
(1995)}) has been extended into the strongly ordered and strongly disordered
Griffiths phases and asymptotically exact results are obtained. In the
non-critical region the asymmetry of the renormalization of the couplings and
the transverse fields is related to a non-linear quantum control parameter,
, which is a natural measure of the distance from the quantum critical
point. , which is found to stay invariant along the RG trajectories and
has been expressed by the initial disorder distributions, stands in the
singularity exponents of different physical quantities (magnetization,
susceptibility, specific heat, etc), which are exactly calculated. In this way
we have observed a weak-universality scenario: the Griffiths-McCoy
singularities does not depend on the form of the disorder, provided the
non-linear quantum control parameter has the same value. The exact scaling
function of the magnetization with a small applied magnetic field is calculated
and the critical point magnetization singularity is determined in a simple,
direct way.Comment: 11 page
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