802 research outputs found
Motion blur and motion sharpening in the human visual system
AbstractThe effect of motion sharpening upon blur discrimination thresholds was examined for a range of speeds and blur widths. Blur discrimination thresholds were measured for drifting edges whose blur was either physically or perceptually constant. Under conditions where edges were kept at a constant physical blur width, discrimination thresholds rose as a function of speed as previously reported. However, when the perceived blur of edges was held constant, discrimination performance was more-or-less constant for speeds up to at least 6.3 deg sec−1. The results indicate that the deterioration of blur discrimination performance with speed may be due to motion sharpening and not motion blur as has previously been suggested. The results are discussed in terms of a scheme whereby a non-linearity in motion processing serves to sharpen moving edges, whilst the finite integration time of the system tends to smear them
A ratio model of perceived speed in the human visual system
The perceived speed of moving images changes over time. Prolonged viewing of a pattern (adaptation) leads to an exponential decrease in its perceived speed. Similarly, responses of neurones tuned to motion reduce exponentially over time. It is tempting to link these phenomena. However, under certain conditions, perceived speed increases after adaptation and the time course of these perceptual effects varies widely. We propose a model that comprises two temporally tuned mechanisms whose sensitivities reduce exponentially over time. Perceived speed is taken as the ratio of these filters' outputs. The model captures increases and decreases in perceived speed following adaptation and describes our data well with just four free parameters. Whilst the model captures perceptual time courses that vary widely, parameter estimates for the time constants of the underlying filters are in good agreement with estimates of the time course of adaptation of direction selective neurones in the mammalian visual system
The effect of contrast adaptation on briefly presented stimuli
AbstractWilson and Humanski (1993) have recently reported evidence that adapting to low temporal frequency sinewave gratings yields little threshold elevation for briefly presented test stimuli. We postulated that brief stimuli may be detected by a transient channel which would be minimally affected by a low temporal frequency adapting pattern. We therefore measured the effect of adaptation om briefly presented test stimuli for a wider range of adapting temporal frequencies. The results indicate that adaptation may yield threshold elevation for briefly presented stimuli and that threshold elevation is greater for high than low temporal frequency adapting patterns. These results are consistent with the hypothesis that briefly presented stimuli are detected by a transient channel
Direct multiscale coupling of a transport code to gyrokinetic turbulence codes
Direct coupling between a transport solver and local, nonlinear gyrokinetic
calculations using the multiscale gyrokinetic code TRINITY [M. Barnes, Ph.D.
thesis, arxiv:0901.2868] is described. The coupling of the microscopic and
macroscopic physics is done within the framework of multiscale gyrokinetic
theory, of which we present the assumptions and key results. An assumption of
scale separation in space and time allows for the simulation of turbulence in
small regions of the space-time grid, which are embedded in a coarse grid on
which the transport equations are implicitly evolved. This leads to a reduction
in computational expense of several orders of magnitude, making
first-principles simulations of the full fusion device volume over the
confinement time feasible on current computing resources. Numerical results
from TRINITY simulations are presented and compared with experimental data from
JET and ASDEX Upgrade plasmas.Comment: 12 pages, 13 figures, invited paper for 2009 APS-DPP meeting,
submitted to Phys. Plasma
Phase mixing vs. nonlinear advection in drift-kinetic plasma turbulence
A scaling theory of long-wavelength electrostatic turbulence in a magnetised,
weakly collisional plasma (e.g., ITG turbulence) is proposed, with account
taken both of the nonlinear advection of the perturbed particle distribution by
fluctuating ExB flows and of its phase mixing, which is caused by the streaming
of the particles along the mean magnetic field and, in a linear problem, would
lead to Landau damping. It is found that it is possible to construct a
consistent theory in which very little free energy leaks into high velocity
moments of the distribution function, rendering the turbulent cascade in the
energetically relevant part of the wave-number space essentially fluid-like.
The velocity-space spectra of free energy expressed in terms of Hermite-moment
orders are steep power laws and so the free-energy content of the phase space
does not diverge at infinitesimal collisionality (while it does for a linear
problem); collisional heating due to long-wavelength perturbations vanishes in
this limit (also in contrast with the linear problem, in which it occurs at the
finite rate equal to the Landau-damping rate). The ability of the free energy
to stay in the low velocity moments of the distribution function is facilitated
by the "anti-phase-mixing" effect, whose presence in the nonlinear system is
due to the stochastic version of the plasma echo (the advecting velocity
couples the phase-mixing and anti-phase-mixing perturbations). The partitioning
of the wave-number space between the (energetically dominant) region where this
is the case and the region where linear phase mixing wins its competition with
nonlinear advection is governed by the "critical balance" between linear and
nonlinear timescales (which for high Hermite moments splits into two
thresholds, one demarcating the wave-number region where phase mixing
predominates, the other where plasma echo does).Comment: 45 pages (single-column), 3 figures, replaced with version published
in JP
Kinetic Simulations of Magnetized Turbulence in Astrophysical Plasmas
This letter presents the first ab initio, fully electromagnetic, kinetic
simulations of magnetized turbulence in a homogeneous, weakly collisional
plasma at the scale of the ion Larmor radius (ion gyroscale). Magnetic and
electric-field energy spectra show a break at the ion gyroscale; the spectral
slopes are consistent with scaling predictions for critically balanced
turbulence of Alfven waves above the ion gyroscale (spectral index -5/3) and of
kinetic Alfven waves below the ion gyroscale (spectral indices of -7/3 for
magnetic and -1/3 for electric fluctuations). This behavior is also
qualitatively consistent with in situ measurements of turbulence in the solar
wind. Our findings support the hypothesis that the frequencies of turbulent
fluctuations in the solar wind remain well below the ion cyclotron frequency
both above and below the ion gyroscale.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Non-existence of normal tokamak equilibria with negative central current
Recent tokamak experiments employing off-axis, non-inductive current drive
have found that a large central current hole can be produced. The current
density is measured to be approximately zero in this region, though in
principle there was sufficient current drive power for the central current
density to have gone significantly negative. Recent papers have used a large
aspect-ratio expansion to show that normal MHD equilibria (with axisymmetric
nested flux surfaces, non-singular fields, and monotonic peaked pressure
profiles) can not exist with negative central current. We extend that proof
here to arbitrary aspect ratio, using a variant of the virial theorem to derive
a relatively simple integral constraint on the equilibrium. However, this
constraint does not, by itself, exclude equilibria with non-nested flux
surfaces, or equilibria with singular fields and/or hollow pressure profiles
that may be spontaneously generated.Comment: 5 pages, 3 figures. Submitted to Physics of Plasmas, Feb. 14, 2003.
Revised Feb. 24, 2003. Vers. 2: revised May 29 to clarify points raised by
referee, add references to recent work. July 18, accepted for publicatio
Implicit representations of luminance and the temporal structure of moving stimuli in multiple regions of human visual cortex revealed by multivariate pattern classification analysis
Motion sharpening and contrast: Gain control precedes compressive non-linearity?
AbstractBlurred edges appear sharper in motion than when they are stationary. We (Vision Research 38 (1998) 2108) have previously shown how such distortions in perceived edge blur may be accounted for by a model which assumes that luminance contrast is encoded by a local contrast transducer whose response becomes progressively more compressive as speed increases. If the form of the transducer is fixed (independent of contrast) for a given speed, then a strong prediction of the model is that motion sharpening should increase with increasing contrast. We measured the sharpening of periodic patterns over a large range of contrasts, blur widths and speeds. The results indicate that whilst sharpening increases with speed it is practically invariant with contrast. The contrast invariance of motion sharpening is not explained by an early, static compressive non-linearity alone. However, several alternative explanations are also inconsistent with these results. We show that if a dynamic contrast gain control precedes the static non-linear transducer then motion sharpening, its speed dependence, and its invariance with contrast, can be predicted with reasonable accuracy
Applications of large eddy simulation methods to gyrokinetic turbulence
The Large Eddy Simulation (LES) approach - solving numerically the large
scales of a turbulent system and accounting for the small-scale influence
through a model - is applied to nonlinear gyrokinetic systems that are driven
by a number of different microinstabilities. Comparisons between modeled, lower
resolution, and higher resolution simulations are performed for an experimental
measurable quantity, the electron density fluctuation spectrum. Moreover, the
validation and applicability of LES is demonstrated through a series of
diagnostics based on the free energetics of the system.Comment: 14 pages, 9 figure
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