224,056 research outputs found
Two independent mechanisms for motion-in-depth perception : evidence from individual differences
Our forward-facing eyes allow us the advantage of binocular visual information: using the tiny differences between right and left eye views to learn about depth and location in three dimensions. Our visual systems also contain specialized mechanisms to detect motion-in-depth from binocular vision, but the nature of these mechanisms remains controversial. Binocular motion-in-depth perception could theoretically be based on first detecting binocular disparity and then monitoring how it changes over time. The alternative is to monitor the motion in the right and left eye separately and then compare these motion signals. Here we used an individual differences approach to test whether the two sources of information are processed via dissociated mechanisms, and to measure the relative importance of those mechanisms. Our results suggest the existence of two distinct mechanisms, each contributing to the perception of motion-in-depth in most observers. Additionally, for the first time, we demonstrate the relative prevalence of the two mechanisms within a normal population. In general, visual systems appear to rely mostly on the mechanism sensitive to changing binocular disparity, but perception of motion-in-depth is augmented by the presence of a less sensitive mechanism that uses interocular velocity differences. Occasionally, we find observers with the opposite pattern of sensitivity. More generally this work showcases the power of the individual differences approach in studying the functional organization of cognitive systems.Publisher PDFPeer reviewe
Subsurface Flows in and Around Active Regions with Rotating and Non-rotating Sunspots
The temporal variation of the horizontal velocity in subsurface layers
beneath three different types of active regions is studied using the technique
of ring diagrams. In this study, we select active regions (ARs) 10923, 10930,
10935 from three consecutive Carrington rotations: AR 10930 contains a
fast-rotating sunspot in a strong emerging active region while other two have
non-rotating sunspots with emerging flux in AR 10923 and decaying flux in AR
10935. The depth range covered is from the surface to about 12 Mm. In order to
minimize the influence of systematic effects, the selection of active and quiet
regions is made so that these were observed at the same heliographic locations
on the solar disk. We find a significant variation in both components of the
horizontal velocity in active regions as compared to quiet regions. The
magnitude is higher in emerging-flux regions than in the decaying-flux region,
in agreement with earlier findings. Further, we clearly see a significant
temporal variation in depth profiles of both zonal and meridional flow
components in AR 10930, with the variation in the zonal component being more
pronounced. We also notice a significant influence of the plasma motion in
areas closest to the rotating sunspot in AR 10930 while areas surrounding the
non-rotating sunspots in all three cases are least affected by the presence of
the active region in their neighborhood.Comment: Solar Physics (in press), includes 11 figure
Andreev bound states versus Majorana bound states in quantum dot-nanowire-superconductor hybrid structures: Trivial versus topological zero-bias conductance peaks
Motivated by an important recent experiment [Deng et al., Science 354, 1557
(2016)], we theoretically consider the interplay between Andreev bound
states(ABSs) and Majorana bound states(MBSs) in quantum dot-nanowire
semiconductor systems with proximity-induced superconductivity(SC), spin-orbit
coupling and Zeeman splitting. The dot induces ABSs in the SC nanowire which
show complex behavior as a function of Zeeman splitting and chemical potential,
and the specific question is whether two such ABSs can come together forming a
topological MBS. We consider physical situations involving the dot being
non-SC, SC, or partially SC. We find that the ABSs indeed tend to coalesce
together producing near-zero-energy midgap states as Zeeman splitting and/or
chemical potential are increased, but this mostly happens in the
non-topological regime although there are situations where the ABSs could come
together forming a topological MBS. The two scenarios(two ABSs forming a
near-zero-energy non-topological ABS or a zero-energy topological MBS) are
difficult to distinguish by tunneling conductance spectroscopy due to
essentially the same signatures. Theoretically we distinguish them by knowing
the critical Zeeman splitting for the topological quantum phase transition or
by calculating the topological visibility. We find that the "sticking together"
propensity of ABSs to produce a zero-energy midgap state is generic in class D
systems, and by itself says nothing about the topological nature of the
underlying SC nanowire. One must use caution in interpreting tunneling
conductance measurements where the midgap sticking-together behavior of ABSs
cannot be construed as definitive evidence for topological SC with non-Abelian
MBSs. We also suggest some experimental techniques for distinguishing between
trivial and topological ZBCPs.Comment: 32 pages, 29 figure
Magnetic properties of G-band bright points in a sunspot moat
We present simultaneous spectropolarimetric observations of four visible and
three infrared spectral lines from the VTT (Tenerife), together with
speckle-reconstructed filtergrams in the G band and the CaII H line core from
the DOT (La Palma). After alignment of the data sets, we used the G-band
intensity to locate bright points (BPs) in the moat of a regular sunspot. With
the cospatial and cotemporal information provided by the polarimetric data, we
characterize the magnetic, kinematic, and thermal properties of the BPs. We
find that (a) 94 % of the BPs are associated with magnetic fields; (b) their
field strengths range between 500 and 1400 G, with a rather flat distribution;
(c) the contrast of BPs in the G band depends on the angle between the vector
magnetic field and the line of sight; (d) the BPs harbor downflows of
magnetized plasma and exhibit Stokes V profiles with large area and amplitude
asymmetries; (e) the magnetic interior of BPs is hotter than the immediate
field-free surroundings by about 1000 K at equal optical depth; and (f) the
mean effective diameter of BPs in our data set is 150 km, with very few BPs
larger than 300 km. Most of these properties can be explained by the classical
magnetic flux tube model. However, the wide range of BP parameters found in
this study indicates that not all G-band BPs are identical to stable long-lived
flux tubes or sheets of kG strength.Comment: Accepted in A&A, 20 pages, 21 figures in main text, 6 fig. in the
Appendices, 3 figures as jpg (fig. 5, C1, C2
Why one-dimensional models fail in the diagnosis of average spectra from inhomogeneous stellar atmospheres
We investigate the feasibility of representing a structured multi-dimensional
stellar atmosphere with a single one-dimensional average stratification for the
purpose of spectral diagnosis of the atmosphere's average spectrum. In
particular we construct four different one-dimensional stratifications from a
single snapshot of a magneto-hydrodynamic simulation of solar convection: one
by averaging its properties over surfaces of constant height, and three
different ones by averaging over surfaces of constant optical depth at 500 nm.
Using these models we calculate continuum, and atomic and molecular line
intensities and their center-to-limb variations. From analysis of the emerging
spectra we identify three main reasons why these average representations are
inadequate for accurate determination of stellar atmospheric properties through
spectroscopic analysis. These reasons are: non-linearity in the Planck function
with temperature, which raises the average emergent intensity of an
inhomogeneous atmosphere above that of an average-property atmosphere, even if
their temperature-optical depth stratification is identical; non-linearities in
molecular formation with temperature and density, which raise the abundance of
molecules of an inhomogeneous atmosphere over that in a one-dimensional model
with the same average properties; the anisotropy of convective motions, which
strongly affects the center-to-limb variation of line-core intensities. We
argue therefore that a one-dimensional atmospheric model that reproduces the
mean spectrum of an inhomogeneous atmosphere necessarily does not reflect the
average physical properties of that atmosphere, and are therefore inherently
unreliable.Comment: 27 pages, 9 figure
The 2D Continuum Radiative Transfer Problem: Benchmark Results for Disk Configurations
We present benchmark problems and solutions for the continuum radiative
transfer (RT) in a 2D disk configuration. The reliability of three Monte-Carlo
and two grid-based codes is tested by comparing their results for a set of
well-defined cases which differ for optical depth and viewing angle. For all
the configurations, the overall shape of the resulting temperature and spectral
energy distribution is well reproduced. The solutions we provide can be used
for the verification of other RT codes.We also point out the advantages and
disadvantages of the various numerical techniques applied to solve the RT
problem.Comment: 13 pages, 10 figures, To appear in Astronomy and Astrophysic
Bright single photon emission from a quantum dot in a circular Bragg grating microcavity
Bright single photon emission from single quantum dots in suspended circular
Bragg grating microcavities is demonstrated. This geometry has been designed to
achieve efficient (> 50 %) single photon extraction into a near-Gaussian shaped
far-field pattern, modest (~10x) Purcell enhancement of the radiative rate, and
a spectral bandwidth of a few nanometers. Measurements of fabricated devices
show progress towards these goals, with collection efficiencies as high as ~10%
demonstrated with moderate spectral bandwidth and rate enhancement. Photon
correlation measurements are performed under above-bandgap excitation (pump
wavelength = 780 nm to 820 nm) and confirm the single photon character of the
collected emission. While the measured sources are all antibunched and
dominantly composed of single photons, the multi-photon probability varies
significantly. Devices exhibiting tradeoffs between collection efficiency,
Purcell enhancement, and multi-photon probability are explored and the results
are interpreted with the help of finite-difference time-domain simulations.
Below-bandgap excitation resonant with higher states of the quantum dot and/or
cavity (pump wavelength = 860 nm to 900 nm) shows a near-complete suppression
of multi-photon events and may circumvent some of the aforementioned tradeoffs.Comment: 11 pages, 12 figure
What visual information is used for stereoscopic depth displacement discrimination?
There are two ways to detect a displacement in stereoscopic depth, namely by monitoring the change in disparity over time (CDOT) or by monitoring the inter-ocular velocity difference (IOVD). Though previous studies have attempted to understand which cue is most significant for the visual system, none have designed stimuli that provide a comparison in terms of relative efficiency between them. Here we used two-frame motion and random dot noise to deliver equivalent strengths of CDOT and IOVD information to the visual system. Using three kinds of random dot stimuli, we were able to isolate CDOT or IOVD or deliver both simultaneously. The proportion of dots delivering CDOT or IOVD signals could be varied, and we defined discrimination threshold as the proportion needed to detect the direction of displacement (towards or away)1. Thresholds were similar for stimuli containing CDOT only, and containing both CDOT and IOVD, but only one participant was able to consistently perceive the displacement for stimuli containing only IOVD. We also investigated the effect of disparity pedestals on discrimination. Performance was best when the displacement crossed the reference plane, but was not significantly different for stimuli containing CDOT only, or containing both CDOT and IOVD. When stimuli are specifically designed to provide equivalent two-frame motion or disparity-change, few participants can reliably detect displacement when IOVD is the only cue. This challenges the notion that IOVD is involved in the discrimination of direction of displacement in two-frame motion displays.PreprintPeer reviewe
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