1,283 research outputs found
Numerical modelling of plasticity induced by transcranial magnetic stimulation
We use neural field theory and spike-timing dependent plasticity to make a simple but biophysically reasonable model of long-term plasticity changes in the cortex due to transcranial magnetic stimulation (TMS). We show how common TMS protocols can be captured and studied within existing neural field theory. Specifically, we look at repetitive TMS protocols such as theta burst stimulation and paired-pulse protocols. Continuous repetitive protocols result mostly in depression, but intermittent repetitive protocols in potentiation. A paired pulse protocol results in depression at short (∼ 100 ms) interstimulus intervals, but potentiation for mid-range intervals. The model is sensitive to the choice of neural populations that are driven by the TMS pulses, and to the parameters that describe plasticity, which may aid interpretation of the high variability in existing experimental results. Driving excitatory populations results in greater plasticity changes than driving inhibitory populations. Modelling also shows the merit in optimizing a TMS protocol based on an individual’s electroencephalogram. Moreover, the model can be used to make predictions about protocols that may lead to improvements in repetitive TMS outcomes
The evolution, distribution and diversity of endogenous circoviral elements in vertebrate genomes
Circoviruses (family Circoviridae) are small, non-enveloped viruses that have short, single-stranded DNA genomes. Circovirus sequences are frequently recovered in metagenomic investigations, indicating that these viruses are widespread, yet they remain relatively poorly understood. Endogenous circoviral elements (CVe) are DNA sequences derived from circoviruses that occur in vertebrate genomes. CVe are a useful source of information about the biology and evolution of circoviruses. In this study, we screened 362 vertebrate genome assemblies in silico to generate a catalog of CVe loci. We identified a total of 179 CVe sequences, most of which have not been reported previously. We show that these CVe loci reflect at least 19 distinct germline integration events. We determine the structure of CVe loci, identifying some that show evidence of potential functionalization. We also identify orthologous copies of CVe in snakes, fish, birds, and mammals, allowing us to add new calibrations to the timeline of circovirus evolution. Finally, we observed that some ancient CVe group robustly with contemporary circoviruses in phylogenies, with all sequences within these groups being derived from the same host class or order, implying a hitherto underappreciated stability in circovirus-host relationships. The openly available dataset constructed in this investigation provides new insights into circovirus evolution, and can be used to facilitate further studies of circoviruses and CVe
Flow equation analysis of the anisotropic Kondo model
We use the new method of infinitesimal unitary transformations to calculate
zero temperature correlation functions in the strong-coupling phase of the
anisotropic Kondo model. We find the dynamics on all energy scales including
the crossover behaviour from weak to strong coupling. The integrable structure
of the Hamiltonian is not used in our approach. Our method should also be
useful in other strong-coupling models since few other analytical methods allow
the evaluation of their correlation functions on all energy scales.Comment: 4 pages RevTeX, 2 eps figures include
MINIMUM EFFECTIVE LEVEL OF METHIOCARB FOR PROTECTING SPROUTING RICE IN LOUISIANA FROM BLACKBIRD DAMAGE
Blackbirds cause locally serious losses to rice. The Denver Wildlife Research Center, U. S. Fish and Wildlife Service, and the Rice Research Station, Louisiana State University Agricultural Center have been cooperating in tests to determine the efficacy of methiocarb seed treatments for protecting sprouting rice in Louisiana from blackbird damage. Results from four field tests (1980, 1982, 1983, and 1984) have shown that methiocarb provides good protection when applied to rice seed at the rate of 2.4 g and 1.25 g active ingredient (a.i.)/kg of rice seed (0.25 and 0.125%). Seed treated at 0.6 g a.i./kg appeared to be susceptible to damage; but, results were inconclusive due to low bird pressure on untreated fields. Fields planted with seed treated at 0.4 g a.i./kg were heavily damaged. Laboratory testing substantiated that \u3e1 g of methiocarb/kg was required to achieve acceptable repellency. We recommend that future field testing be restricted to treatments of \u3e1.0 g a.i./kg rice seed, and that \u3e1.0 g a.i./kg be accepted as the minimum seed treatment level for formulations under consideration for registration or use in conjunction with emergency (Section 18) exemptions to FIFRA by the Environmental Protection Agency
Exact perturbative solution of the Kondo problem
We explicitly evaluate the infinite series of integrals that appears in the
"Anderson-Yuval" reformulation of the anisotropic Kondo problem in terms of a
one-dimensional Coulomb gas. We do this by developing a general approach
relating the anisotropic Kondo problem of arbitrary spin with the boundary
sine-Gordon model, which describes impurity tunneling in a Luttinger liquid and
in the fractional quantum Hall effect. The Kondo solution then follows from the
exact perturbative solution of the latter model in terms of Jack polynomials.Comment: 4 pages in revtex two-colum
The Numerical Renormalization Group Method for correlated electrons
The Numerical Renormalization Group method (NRG) has been developed by Wilson
in the 1970's to investigate the Kondo problem. The NRG allows the
non-perturbative calculation of static and dynamic properties for a variety of
impurity models. In addition, this method has been recently generalized to
lattice models within the Dynamical Mean Field Theory. This paper gives a brief
historical overview of the development of the NRG and discusses its application
to the Hubbard model; in particular the results for the Mott metal-insulator
transition at low temperatures.Comment: 14 pages, 7 eps-figures include
Phase transitions in two-dimensional anisotropic quantum magnets
We consider quantum Heisenberg ferro- and antiferromagnets on the square
lattice with exchange anisotropy of easy-plane or easy-axis type. The
thermodynamics and the critical behaviour of the models are studied by the
pure-quantum self-consistent harmonic approximation, in order to evaluate the
spin and anisotropy dependence of the critical temperatures. Results for
thermodynamic quantities are reported and comparison with experimental and
numerical simulation data is made. The obtained results allow us to draw a
general picture of the subject and, in particular, to estimate the value of the
critical temperature for any model belonging to the considered class.Comment: To be published on Eur. Phys. J.
Entanglement between a qubit and the environment in the spin-boson model
The quantitative description of the quantum entanglement between a qubit and
its environment is considered. Specifically, for the ground state of the
spin-boson model, the entropy of entanglement of the spin is calculated as a
function of , the strength of the ohmic coupling to the environment,
and , the level asymmetry. This is done by a numerical
renormalization group treatment of the related anisotropic Kondo model. For
, the entanglement increases monotonically with , until it
becomes maximal for . For fixed , the entanglement
is a maximum as a function of for a value, .Comment: 4 pages, 3 figures. Shortened version restricted to groundstate
entanglemen
Oscillations of the magnetic polarization in a Kondo impurity at finite magnetic fields
The electronic properties of a Kondo impurity are investigated in a magnetic
field using linear response theory. The distribution of electrical charge and
magnetic polarization are calculated in real space. The (small) magnetic field
does not change the charge distribution. However, it unmasks the Kondo cloud.
The (equal) weight of the d-electron components with their magnetic moment up
and down is shifted and the compensating s-electron clouds don't cancel any
longer (a requirement for an experimental detection of the Kondo cloud). In
addition to the net magnetic polarization of the conduction electrons an
oscillating magnetic polarization with a period of half the Fermi wave length
is observed. However, this oscillating magnetic polarization does not show the
long range behavior of Rudermann-Kittel-Kasuya-Yosida oscillations because the
oscillations don't extend beyond the Kondo radius. They represent an internal
electronic structure of the Kondo impurity in a magnetic field. PACS: 75.20.Hr,
71.23.An, 71.27.+
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