Numerical study of the correspondence between the dissipative and fixed-energy Abelian sandpile models.

We consider the Abelian sandpile model (ASM) on the square lattice with a single dissipative site (sink). Particles are added one by one per unit time at random sites and the resulting density of particles is calculated as a function of time. We observe different scenarios of evolution depending on the value of initial uniform density (height) h(0). During the first stage of the evolution, the density of particles increases linearly. Reaching a critical density ρ(c)(h(0)), the system changes its behavior and relaxes exponentially to the stationary state of the ASM with density ρ(s). Considering initial heights -1 ≤ h(0) ≤ 4, we observe a dramatic decrease of the difference ρ(c)(h(0)) - ρ(s) when h(0) is zero or negative. In parallel with the ASM, we consider the conservative fixed energy sandpile (FES). The extensive Monte Carlo simulations show that the threshold density ρ(th)(h(0)) of the FES converges rapidly to ρ(s) for h(0) < 1.

Jamming probabilities for a vacancy in the dimer model.

Following the recent proposal made by [J. Bouttier, Phys. Rev. E 76, 041140 (2007)], we study analytically the mobility properties of a single vacancy in the close-packed dimer model on the square lattice. Using the spanning web representation, we find determinantal expressions for various observable quantities. In the limiting case of large lattices, they can be reduced to the calculation of Toeplitz determinants and minors thereof. The probability for the vacancy to be strictly jammed and other diffusion characteristics are computed exactly.

Estimate of B(B(over) -->Xgamma) at O(alpha(s)2).

Combining our results for various O(alpha[s]) corrections to the weak radiative B-meson decay, we are able to present the first estimate of the branching ratio at the next-to-next-to-leading order in QCD. We find B(B[over ]-->X[s]gamma)=(3.15+/-0.23) x 10(-4) for Egamma>1.6 GeV in the B[over ]-meson rest frame. The four types of uncertainties:nonperturbative (5%), parametric (3%), higher-order (3%), and m(c)-interpolation ambiguity (3%) have been added in quadrature to obtain the total error.

The human primary somatosensory cortex response contains components related to stimulus frequency and perception in a frequency discrimination task.

Somatosensory stimulation of primary somatosensory cortex (SI) using frequency discrimination offers a direct, well-defined and accessible way of studying cortical decisions at the locus of early input processing. Animal studies have identified and classified the neuronal responses in SI but they have not yet resolved whether during prolonged stimulation the collective SI response just passively reflects the input or actively participates in the comparison and decision processes. This question was investigated using tomographic analysis of single trial magnetoencephalographic data. Four right-handed males participated in a frequency discrimination task to detect changes in the frequency of an electrical stimulus applied to the right-hand digits 2+3+4. The subjects received approximately 600 pairs of stimuli with Stim1 always at 21 Hz, while Stim2 was either 21 Hz (50%) or varied from 22 to 29 Hz in steps of 1 Hz. Both stimuli were 1 s duration, separated by a 1 s interval of no stimulation. The left-SI was the most consistently activated area and showed the first activation peak at 35-48 ms after Stim1 onset and sustained activity during both stimulus periods. During the Stim2 period, we found that the left-SI activation started to differ significantly between two groups of trials (21 versus 26-29 Hz) within the first 100 ms and this difference was sustained and enhanced thereafter (approximately 600 ms). When only correct responses from the above two groups were used, the difference was even higher at later latencies (approximately 650 ms). For one subject who had enough trials of same perception to different input frequencies, e.g. responded 21 Hz to Stim2 at 21 Hz (correct) and 26-29 Hz (error), we found the sustained difference only before 650 ms. Our results suggest that SI is involved with the analysis of an input frequency and related to perception and decision at different latencies.

Early (N70m) neuromagnetic signal topography and striate and extrastriate generators following pattern onset quadrant stimulation.

The MEG signal generated by sinusoidal grating pattern onset at 1 and 3 cpd, presented randomly to the four quadrants, was analyzed in terms of gross signal properties and current dipole modeling and for a subset of subjects with magnetic field tomography (MFT). In all subjects a prominent wave was identified with a peak latency around 70 ms (N70m), modulated by spatial frequency and varying systematically with the stimulation quadrant. Sensors over occipital areas recorded stronger responses with lower field quadrants, while the signal for sensors a few centimeters more superior was stronger with upper quadrant stimuli. A strong signal in inferior occipitotemporal areas was less sensitive to upper and lower field stimulation and was stronger in the left hemisphere with contralateral (right) visual field stimulation. For lower visual field stimulation a good fit to the average data was obtained with a single dipole for 3 cpd, but was less consistent across run repetitions for 1 cpd. Neither the single-dipole model nor the two-dipole model produced a good fit across runs with the upper field stimuli. MFT solutions identified overlapping activity in striate and extrastriate areas in all conditions. The MFT solutions in the V1/V2 at the N70m were highly reproducible across run repetitions for 1 and 3 cpd, and consistent with the cruciform model, even though they were often weaker than simultaneously activated extrastriate generators. Extrastriate generators in V5 and the human homologue of V6, which were variable across run repetitions at N70m, settled to highly reproducible activations between 100 and 200 ms.