Reply to "Comment on 'Universal Behavior of Load Distribution in Scale-Free Networks'"

Reply to "Comment on 'Universal Behavior of Load Distribution in Scale-Free Networks.'"Comment: 1 page, 1 figur

Spatial Contrast Sensitivity of Birds

Contrast sensitivity (CS) is the ability of the observer to discriminate between adjacent stimuli on the basis of their differences in relative luminosity (contrast) rather than their absolute luminances. Prior to this study, birds had been thought to have low contrast detection thresholds relative to mammals and fishes. This was a surprising phenomenon because birds had been traditionally attributed with superior vision. In addition, the low CS of birds could not be explained by retinal or optical factors, or secondary stimulus characteristics. Unfortunately, avian contrast sensitivity functions (CSFs) were sparse in the literature, so it was unknown whether low contrast sensitivity was a general phenomenon in birds. This study measured CS in six species of birds sampled across different taxa and different ecological backgrounds in order to answer this very question. The species chosen for this experiment were American kestrels (Falco sparverius), Barn owls (Tyto alba), Japanese quail (Coturnix coturnix japonica), White Carneaux Pigeons (Columba livia), Starlings (Sturnus vulgaris), and Red-bellied woodpeckers (Melanerpes carolinus). CSFs were obtained from these birds using the pattern electroretinogram (PERG), and compared with CSFs from the literature. The quail and pigeon data obtained in this experiment fit well with existing CS data for these species. The kestrel data were not similar to kestrel data in the literature; however the data in the literature were collected from a single subject. All of the birds studied had contrast sensitivities that were consistent with their retinal or optical morphologies relative to other birds (in species for which such data exists) and seem well suited for their natural environments. In addition, all of these birds exhibited low CS relative to humans and most mammals, which suggests that low CS is a general phenomenon of birds. Explanations for this avian low CS phenomenon include a possible trade-off between contrast mechanisms and UV mechanisms in cone systems, and lateral inhibitory mechanisms that are perhaps categorically different from mammals. Lateral inhibition affects contrast gain, and has been shown to differ according to ganglion cell types, which in turn will differ in vertebrate species

A novel method for segmenting growth of cells in sheared endothelial culture reveals the secretion of an anti-inflammatory mediator

Background Effects of shear stress on endothelium are important for the normal physiology of blood vessels and are implicated in the pathogenesis of atherosclerosis. They have been extensively studied in vitro. In one paradigm, endothelial cells are cultured in devices that produce spatially varying shear stress profiles, and the local profile is compared with the properties of cells at the same position. A flaw in this class of experiments is that cells exposed to a certain shear profile in one location may release mediators into the medium that alter the behaviour of cells at another location, experiencing different shear, thus obscuring or corrupting the true relation between shear and cell properties. Methods Surface coating methods were developed for attaching cells only to some areas of culture-ware and preventing them from spreading into other regions even during prolonged culture. Results Segmenting the growth of cells had no effect on cell shape, alignment and number per unit area compared to culturing cells in the whole well, but there were differences in tumour-necrosis-factor-α (TNF-α)-induced expression of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1), and monocyte adherence to the monolayer. Conclusions The results are consistent with the release of a mediator from cells exposed to high-magnitude uniaxial shear stress that has anti-inflammatory effects on activated endothelium; the mediator may be of importance in atherogenesis. Hence the new methods revealed an important property that would not have been observed without growth segmentation, suggesting that they could find more widespread application

Symmetry breaking in MAST plasma turbulence due to toroidal flow shear

The flow shear associated with the differential toroidal rotation of tokamak plasmas breaks an underlying symmetry of the turbulent fluctuations imposed by the up-down symmetry of the magnetic equilibrium. Using experimental Beam-Emission-Spectroscopy (BES) measurements and gyrokinetic simulations, this symmetry breaking in ion-scale turbulence in MAST is shown to manifest itself as a tilt of the spatial correlation function and a finite skew in the distribution of the fluctuating density field. The tilt is a statistical expression of the "shearing" of the turbulent structures by the mean flow. The skewness of the distribution is related to the emergence of long-lived density structures in sheared, near-marginal plasma turbulence. The extent to which these effects are pronounced is argued (with the aid of the simulations) to depend on the distance from the nonlinear stability threshold. Away from the threshold, the symmetry is effectively restored

Comparison of BES measurements of ion-scale turbulence with direct, gyrokinetic simulations of MAST L-mode plasmas

Observations of ion-scale (k_y*rho_i <= 1) density turbulence of relative amplitude dn_e/n_e <= 0.2% are available on the Mega Amp Spherical Tokamak (MAST) using a 2D (8 radial x 4 poloidal channel) imaging Beam Emission Spectroscopy (BES) diagnostic. Spatial and temporal characteristics of this turbulence, i.e., amplitudes, correlation times, radial and perpendicular correlation lengths and apparent phase velocities of the density contours, are determined by means of correlation analysis. For a low-density, L-mode discharge with strong equilibrium flow shear exhibiting an internal transport barrier (ITB) in the ion channel, the observed turbulence characteristics are compared with synthetic density turbulence data generated from global, non-linear, gyro-kinetic simulations using the particle-in-cell (PIC) code NEMORB. This validation exercise highlights the need to include increasingly sophisticated physics, e.g., kinetic treatment of trapped electrons, equilibrium flow shear and collisions, to reproduce most of the characteristics of the observed turbulence. Even so, significant discrepancies remain: an underprediction by the simulations of the turbulence amplituide and heat flux at plasma periphery and the finding that the correlation times of the numerically simulated turbulence are typically two orders of magnitude longer than those measured in MAST. Comparison of these correlation times with various linear timescales suggests that, while the measured turbulence is strong and may be `critically balanced', the simulated turbulence is weak.Comment: 27 pages, 11 figure