1,273 research outputs found
Commentary on âUltrasound-guided Intralesional Diode Laser Treatment of Congenital Extratruncular Venous Malformations: Mid-term Resultsâ
Processing of natural temporal stimuli by macaque retinal ganglion cells
This study quantifies the performance of primate retinal ganglion cells in response to natural stimuli. Stimuli were confined to the temporal and chromatic domains and were derived from two contrasting environments, one typically northern European and the other a flower show. The performance of the cells was evaluated by investigating variability of cell responses to repeated stimulus presentations and by comparing measured to model responses. Both analyses yielded a quantity called the coherence rate (in bits per second), which is related to the information rate. Magnocellular (MC) cells yielded coherence rates of up to 100 bits/sec, rates of parvocellular (PC) cells were much lower, and short wavelength (S)-cone-driven ganglion cells yielded intermediate rates. The modeling approach showed that for MC cells, coherence rates were generated almost exclusively by the luminance content of the stimulus. Coherence rates of PC cells were also dominated by achromatic content. This is a consequence of the stimulus structure; luminance varied much more in the natural environment than chromaticity. Only approximately one-sixth of the coherence rate of the PC cells derived from chromatic content, and it was dominated by frequencies below 10 Hz. S-cone-driven ganglion cells also yielded coherence rates dominated by low frequencies. Below 2â3 Hz, PC cell signals contained more power than those of MC cells. Response variation between individual ganglion cells of a particular class was analyzed by constructing generic cells, the properties of which may be relevant for performance higher in the visual system. The approach used here helps define retinal modules useful for studies of higher visual processing of natural stimuli
Geometry, Scaling and Universality in the Mass Distributions in Heavy Ion Collisions
Various features of the mass yields in heavy ion collisions are studied. The
mass yields are discussed in terms of iterative one dimensional discrete maps.
These maps are shown to produce orbits for a monomer or for a nucleus which
generate the mass yields and the distribution of cluster sizes. Simple
Malthusian dynamics and non-linear Verhulst dynamics are used to illustrate the
approach. Nuclear cobwebbing, attractors of the dynamics, and Lyapanov
exponents are discussed for the mass distribution. The self-similar property of
the Malthusian orbit offers a new variable for the study of scale invariance
using power moments of the mass distribution. Correlation lengths, exponents
and dimensions associated with scaling relations are developed. Fourier
transforms of the mass distribution are used to obtain power spectra which are
investigated for a behavior.Comment: 29 pages in REVTEX, 9 figures (available from the authors), RU-92-0
Multifractal Analysis on the Return Series of Stock Markets Using MF-DFA Method
Part 3: Finance and Service ScienceInternational audienceAnalyzing the daily returns of NASDAQ Composite Index by using MF-DFA method has led to findings that the return series does not fit the normal distribution and its leptokurtic indicates that a single-scale index is insufficient to describe the stock price fluctuation. Furthermore, it is found that the long-term memory characteristics are a main source of multifractality in time series. Based on the main reason causing multifractality, a contrast of the original return series and the reordered return series is made to demonstrate the stock price index fluctuation, suggesting that the both return series have multifractality. In addition, the empirical results verify the validity of the measures which illustrates that the stock market fails to reach the weak form efficiency
Anisotropic Flow from RHIC to the LHC
Anisotropic flow is recognized as one of the main observables providing
information on the early stage of a heavy-ion collision. At RHIC the large
observed anisotropic flow and its successful description by ideal hydrodynamics
is considered evidence for an early onset of thermalization and almost ideal
fluid properties of the produced strongly coupled Quark Gluon Plasma. This
write-up discusses some key RHIC anisotropic flow measurements and for
anisotropic flow at the LHC some predictions.Comment: 4 pages, 6 figures, hotquarks 200
Allen Telescope Array Multi-Frequency Observations of the Sun
We present the first observations of the Sun with the Allen Telescope Array
(ATA). We used up to six frequencies, from 1.43 to 6 GHz, and baselines from 6
to 300 m. To our knowledge, these are the first simultaneous multifrequency
full-Sun maps obtained at microwave frequencies without mosaicing. The
observations took place when the Sun was relatively quiet, although at least
one active region was present each time. We present multi-frequency flux
budgets for each sources on the Sun. Outside of active regions, assuming
optically thin bremsstrahlung (free--free) coronal emission on top of an
optically thick ~10 000 K chromosphere, the multi-frequency information can be
condensed into a single, frequency-independent, "coronal bremsstrahlung
contribution function" [EM/sqrt(T)] map. This technique allows the separation
of the physics of emission as well as a measurement of the density structure of
the corona. Deviations from this simple relationship usually indicate the
presence of an additional gyroresonance-emission component, as is typical in
active regions.Comment: 16 pages, 11 figures. Accepted for publication in Solar Physic
Coherence lengths and anisotropy in MgB2 superconductor
Field and temperature microwave measurements have been carried out on MgB2
thin film grown on Al2O3 substrate. The analysis reveals the mean field
coherence length xi_{MF} in the mixed state and a temperature independent
anisotropy ratio gamma_{MF} = xi_{MF}^{ab} / xi_{MF}^c approximately 2. At the
superconducting transition, the scaling of the fluctuation conductivity yields
the Ginzburg-Landau coherence length with a different anisotropy ratio
gamma_{GL} = 2.8, also temperature independent.Comment: submitted to PR
The sign problem in Monte Carlo simulations of frustrated quantum spin systems
We discuss the sign problem arising in Monte Carlo simulations of frustrated
quantum spin systems. We show that for a class of ``semi-frustrated'' systems
(Heisenberg models with ferromagnetic couplings along the -axis
and antiferromagnetic couplings in the -plane, for
arbitrary distances ) the sign problem present for algorithms operating in
the -basis can be solved within a recent ``operator-loop'' formulation of
the stochastic series expansion method (a cluster algorithm for sampling the
diagonal matrix elements of the power series expansion of
to all orders). The solution relies on identification of operator-loops which
change the configuration sign when updated (``merons'') and is similar to the
meron-cluster algorithm recently proposed by Chandrasekharan and Wiese for
solving the sign problem for a class of fermion models (Phys. Rev. Lett. {\bf
83}, 3116 (1999)). Some important expectation values, e.g., the internal
energy, can be evaluated in the subspace with no merons, where the weight
function is positive definite. Calculations of other expectation values require
sampling of configurations with only a small number of merons (typically zero
or two), with an accompanying sign problem which is not serious. We also
discuss problems which arise in applying the meron concept to more general
quantum spin models with frustrated interactions.Comment: 13 pages, 16 figure
Quantum railroads and directed localization at the juncture of quantum Hall systems
The integer quantum Hall effect (QHE) and one-dimensional Anderson
localization (AL) are limiting special cases of a more general phenomenon,
directed localization (DL), predicted to occur in disordered one-dimensional
wave guides called "quantum railroads" (QRR). Here we explain the surprising
results of recent measurements by Kang et al. [Nature 403, 59 (2000)] of
electron transfer between edges of two-dimensional electron systems and
identify experimental evidence of QRR's in the general, but until now entirely
theoretical, DL regime that unifies the QHE and AL. We propose direct
experimental tests of our theory.Comment: 11 pages revtex + 3 jpeg figures, to appear in Phys. Rev.
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