513 research outputs found
A test-retest reliability analysis of diffusion measures of white matter tracts relevant for cognitive control
Recent efforts to replicate structural brain-behavior correlations have called into question the replicability of structural brain measures used in cognitive neuroscience. Here, we report an evaluation of test-retest reliability of diffusion tensor imaging (DTI) measures, including fractional anisotropy, mean diffusivity, axial diffusivity, and radial diffusivity, in several white matter tracts previously shown to be involved in cognitive control. In a data set consisting of 34 healthy participants scanned twice on a single day, we observe overall stability of DTI measures. This stability remained in a subset of participants who were also scanned a third time on the same day as well as in a 2-week follow-up session. We conclude that DTI measures in these tracts show relative stability, and that alternative explanations for the recent failures of replication must be considered
Young stars in Epsilon Cha and their disks: disk evolution in sparse associations
(abridge) The nearby young stellar association Epsilon Cha association has an
estimated age of 3-5 Myr, making it an ideal laboratory to study the disk
dissipation process and provide empirical constraints on the timescale of
planet formation. We combine the available literature data with our Spitzer IRS
spectroscopy and VLT/VISIR imaging data. The very low mass stars USNO-B120144.7
and 2MASS J12005517 show globally depleted spectral energy distributions
pointing at strong dust settling. 2MASS J12014343 may have a disk with a very
specific inclination where the central star is effectively screened by the cold
outer parts of a flared disk but the 10 micron radiation of the warm inner disk
can still reach us. We find the disks in sparse stellar associations are
dissipated more slowly than those in denser (cluster) environments. We detect
C_{2}H_{2} rovibrational band around 13.7 micron on the IRS spectrum of
USNO-B120144.7. We find strong signatures of grain growth and crystallization
in all Epsilon Cha members with 10 micron features detected in their IRS
spectra. We combine the dust properties derived in the Epsilon Cha sample with
those found using identical or similar methods in the MBM 12, Coronet cluster,
Eta Cha associations, and in the cores to disks (c2d) legacy program. We find
that disks around low-mass young stars show a negative radial gradient in the
mass-averaged grain size and mass fraction of crystalline silicates. A positive
correlation exists between the mass-averaged grain sizes of amorphous silicates
and the accretion rates if the latter is above ~10^{-9} Msun/yr, possibly
indicating that those disks are sufficiently turbulent to prevent grains of
several microns in size to sink into the disk interior.Comment: 17 pages, 18 figures, 6 tables, language revised; accepted to A&
Cortico-subthalamic connection predicts individual differences in value-driven choice bias
It has been suggested that a connection between the STN and value-sensitive areas of the prefrontal cortex might mediate value-based actions in perceptual decision making. In this study, we first seek to quantify a structural connection between the STN and a cortical region that was associated with mechanisms underlying bias in choice behavior (vmPFC). Next, we tested whether individual differences in the probabilistic tract-strength of this connection were predictive for individual differences in the magnitude of bias in a perceptual decision-making task. Probabilistic tractography was used to measure the tract-strength between the STN and the vmPFC. Bias was quantified using an accumulation-to-bound model where a shift in the starting point of the accumulation of sensory evidence causes faster and more choices for an alternative that is more likely or more valuable. Results show that vmPFC is structurally connected with the STN and that the strength of this connection is predictive for choice bias towards an alternative that is more valuable, but not for choice bias towards an alternative that is more likely. These findings confirm the involvement of the cortico-subthalamic circuit in mechanisms underlying value-based actions in perceptual decision making
Resolving HD 100546 disc in the mid-infrared: Small inner disc and asymmetry near the gap
A region of roughly half of the solar system scale around the star HD 100546
is largely cleared of gas and dust, in contrast to the bright outer disc.
However, some material is observed in the immediate vicinity of the star. We
investigate how the dust is distributed within and outside the gap, and
constrain the disc geometry with mid-infrared interferometric observations
using VLTI/MIDI. With baseline lengths of 40m, our long baseline observations
are sensitive to the inner few AU from the star, and we combined them with
observations at shorter, 15m baselines, to probe emission beyond the gap at up
to 20AU from the star. We modelled the mid-infrared emission using radial
temperature profiles. Our model is composed of infinitesimal concentric annuli
emitting as black bodies, and it has distinct inner and outer disc components.
We derived an upper limit of 0.7AU for the radial size of the inner disc, from
our longest baseline data. This small dusty disc is separated from the edge of
the outer disc by a large, roughly 10AU wide gap. Our short baseline data place
a bright ring of emission at 11+-1AU, consistent with prior observations of the
transition region between the gap and the outer disc, known as the disc wall.
The inclination and position angle are constrained by our data to i=53+-8deg
and PA=145+-5deg. Compared to the rim and outer disc geometry this suggests
co-planarity. Brightness asymmetry is evident in both short and long baseline
data, and it is unequivocally discernible from any atmospheric or instrumental
effects. The origin of the asymmetry is consistent with the bright disc wall,
which we find to be 1-2AU wide. The gap is cleared of micron-sized dust, but we
cannot rule out the presence of larger particles and/or perturbing bodies.Comment: 12 pages, 9 figures, accepted for publication in A&
Toward a model-based cognitive neuroscience of mind wandering
Published version also available at http://dx.doi.org/10.1016/j.neuroscience.2015.09.053People often ââmind wanderâ during everyday
tasks, temporarily losing track of time, place, or current task
goals. In laboratory-based tasks, mind wandering is often
associated with performance decrements in behavioral
variables and changes in neural recordings. Such empirical
associations provide descriptive accounts of mind
wandering â howit affects ongoing task performance â but fail
to provide true explanatory accounts â why it affects task
performance. In this perspectives paper, we consider mind
wandering as a neural state or process that affects the
parameters of quantitative cognitive process models, which
in turn affect observed behavioral performance. Our
approach thus uses cognitive process models to bridge
the explanatory divide between neural and behavioral data.
We provide an overview of two general frameworks for
developing a model-based cognitive neuroscience of mind
wandering. The first approach uses neural data to segment
observed performance into a discrete mixture of latent
task-related and task-unrelated states, and the second
regresses single-trial measures of neural activity onto
structured trial-by-trial variation in the parameters of
cognitive process models. We discuss the relative merits of
the two approaches, and the research questions they can
answer, and highlight that both approaches allow neural data
to provide additional constraint on the parameters of cognitive
models, which will lead to a more precise account of the
effect of mind wandering on brain and behavior. We conclude
by summarizing prospects for mind wandering as conceived
within a model-based cognitive neuroscience framework,
highlighting the opportunities for its continued study and
the benefits that arise from using well-developed quantitative techniques to study abstract theoretical constructs
The 10 micron amorphous silicate feature of fractal aggregates and compact particles with complex shapes
We model the 10 micron absorption spectra of nonspherical particles composed
of amorphous silicate. We consider two classes of particles, compact ones and
fractal aggregates composed of homogeneous spheres. For the compact particles
we consider Gaussian random spheres with various degrees of non-sphericity. For
the fractal aggregates we compute the absorption spectra for various fractal
dimensions. The 10 micron spectra are computed for ensembles of these particles
in random orientation using the well-known Discrete Dipole Approximation. We
compare our results to spectra obtained when using volume equivalent
homogeneous spheres and to those computed using a porous sphere approximation.
We conclude that, in general, nonspherical particles show a spectral signature
that is similar to that of homogeneous spheres with a smaller material volume.
This effect is overestimated when approximating the particles by porous spheres
with the same volume filling fraction. For aggregates with fractal dimensions
typically predicted for cosmic dust, we show that the spectral signature
characteristic of very small homogeneous spheres (with a volume equivalent
radius r_V<0.5 micron) can be detected even in very large particles. We
conclude that particle sizes are underestimated when using homogeneous spheres
to model the emission spectra of astronomical sources. In contrast, the
particle sizes are severely overestimated when using equivalent porous spheres
to fit observations of 10 micron silicate emission.Comment: Accepted for publication in A&
Variable accretion as a mechanism for brightness variations in T Tau S
(Note: this is a shortened version of the original A&A-style structured
abstract). The physical nature of the strong photometric variability of T Tau
Sa, the more massive member of the Southern "infrared companion" to T Tau, has
long been debated. Intrinsic luminosity variations due to variable accretion
were originally proposed but later challenged in favor of apparent fluctuations
due to time-variable foreground extinction. In this paper we use the timescale
of the variability as a diagnostic for the underlying physical mechanism.
Because the IR emission emerging from Sa is dominantly thermal emission from
circumstellar dust at <=1500K, we can derive a minimum size of the region
responsible for the time-variable emission. In the context of the variable
foreground extinction scenario, this region must be (un-) covered within the
variability timescale, which implies a minimum velocity for the obscuring
foreground material. If this velocity supercedes the local Kepler velocity we
can reject foreground extinction as a valid variability mechanism. The variable
accretion scenario allows for shorter variability timescales since the
variations in luminosity occur on much smaller scales, essentially at the
surface of the star, and the disk surface can react almost instantly on the
changing irradiation with a higher or lower dust temperature and according
brightness. We have detected substantial variations at long wavelengths in T
Tau S: +26% within four days at 12.8 micron. We show that this short-term
variability cannot be due to variable extinction and instead must be due to
variable accretion. Using a radiative transfer model of the Sa disk we show
that variable accretion can in principle also account for the much larger
(several magnitude) variations observed on timescales of several years. For the
long-term variability, however, also variable foreground extinction is a viable
mechanism.Comment: 15 pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
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