1,518 research outputs found
Mapping the 3-D dark matter with weak lensing in COMBO-17
We present a 3-dimensional lensing analysis of the z=0.16 supercluster
A901/2, resulting in a 3-D map of the dark matter distribution within a 3 X
10^{5} [Mpc]^3 volume from the COMBO-17 survey. We perform a chi^2-fit of
isothermal spheres to the tangential shear pattern around each cluster as a
function of redshift to estimate the 3-D positions and masses of the main
clusters in the supercluster from lensing alone. We then present the first 3-D
map of the dark matter gravitational potential field, Phi, using the
Kaiser-Squires (1993) and Taylor (2001) inversion methods. These maps clearly
show the potential wells of the main supercluster components, including a new
cluster behind A902, and demonstrates the applicability of 3-D dark matter
mapping and projection free-mass-selected cluster finding to current data.
Finally, we develop the halo model of dark matter and galaxy clustering and
compare this with the auto-and cross-correlation functions of the 3-D
gravitational potential, galaxy number densities and galaxy luminosity
densities measured in the A901/2 field. We find significant anti-correlations
between the gravitational potential field and the galaxy number density and
luminosities, as expected due to baryonic infall into dark matter
concentrations. We find good agreement with the halo model for the number
densities and luminosity correlation functions.Comment: Submitted to MNRAS; 21 pages, 18 figure
Structure Learning in Coupled Dynamical Systems and Dynamic Causal Modelling
Identifying a coupled dynamical system out of many plausible candidates, each
of which could serve as the underlying generator of some observed measurements,
is a profoundly ill posed problem that commonly arises when modelling real
world phenomena. In this review, we detail a set of statistical procedures for
inferring the structure of nonlinear coupled dynamical systems (structure
learning), which has proved useful in neuroscience research. A key focus here
is the comparison of competing models of (ie, hypotheses about) network
architectures and implicit coupling functions in terms of their Bayesian model
evidence. These methods are collectively referred to as dynamical casual
modelling (DCM). We focus on a relatively new approach that is proving
remarkably useful; namely, Bayesian model reduction (BMR), which enables rapid
evaluation and comparison of models that differ in their network architecture.
We illustrate the usefulness of these techniques through modelling
neurovascular coupling (cellular pathways linking neuronal and vascular
systems), whose function is an active focus of research in neurobiology and the
imaging of coupled neuronal systems
Speech rhythms and multiplexed oscillatory sensory coding in the human brain
Cortical oscillations are likely candidates for segmentation and coding of continuous speech. Here, we monitored continuous speech processing with magnetoencephalography (MEG) to unravel the principles of speech segmentation and coding. We demonstrate that speech entrains the phase of low-frequency (delta, theta) and the amplitude of high-frequency (gamma) oscillations in the auditory cortex. Phase entrainment is stronger in the right and amplitude entrainment is stronger in the left auditory cortex. Furthermore, edges in the speech envelope phase reset auditory cortex oscillations thereby enhancing their entrainment to speech. This mechanism adapts to the changing physical features of the speech envelope and enables efficient, stimulus-specific speech sampling. Finally, we show that within the auditory cortex, coupling between delta, theta, and gamma oscillations increases following speech edges. Importantly, all couplings (i.e., brain-speech and also within the cortex) attenuate for backward-presented speech, suggesting top-down control. We conclude that segmentation and coding of speech relies on a nested hierarchy of entrained cortical oscillations
An F-ratio-Based Method for Estimating the Number of Active Sources in MEG
Magnetoencephalography (MEG) is a powerful technique for studying the human
brain function. However, accurately estimating the number of sources that
contribute to the MEG recordings remains a challenging problem due to the low
signal-to-noise ratio (SNR), the presence of correlated sources, inaccuracies
in head modeling, and variations in individual anatomy. To address these
issues, our study introduces a robust method for accurately estimating the
number of active sources in the brain based on the F-ratio statistical
approach, which allows for a comparison between a full model with a higher
number of sources and a reduced model with fewer sources. Using this approach,
we developed a formal statistical procedure that sequentially increases the
number of sources in the multiple dipole localization problem until all sources
are found. Our results revealed that the selection of thresholds plays a
critical role in determining the method`s overall performance, and appropriate
thresholds needed to be adjusted for the number of sources and SNR levels,
while they remained largely invariant to different inter-source correlations,
modeling inaccuracies, and different cortical anatomies. By identifying optimal
thresholds and validating our F-ratio-based method in simulated, real phantom,
and human MEG data, we demonstrated the superiority of our F-ratio-based method
over existing state-of-the-art statistical approaches, such as the Akaike
Information Criterion (AIC) and Minimum Description Length (MDL). Overall, when
tuned for optimal selection of thresholds, our method offers researchers a
precise tool to estimate the true number of active brain sources and accurately
model brain function
Cognitive Impairments in Schizophrenia as Assessed Through Activation and Connectivity Measures of Magnetoencephalography (MEG) Data
The cognitive dysfunction present in patients with schizophrenia is thought to be driven in part by disorganized connections between higher-order cortical fields. Although studies utilizing electroencephalography (EEG), PET and fMRI have contributed significantly to our understanding of these mechanisms, magnetoencephalography (MEG) possesses great potential to answer long-standing questions linking brain interactions to cognitive operations in the disorder. Many experimental paradigms employed in EEG and fMRI are readily extendible to MEG and have expanded our understanding of the neurophysiological architecture present in schizophrenia. Source reconstruction techniques, such as adaptive spatial filtering, take advantage of the spatial localization abilities of MEG, allowing us to evaluate which specific structures contribute to atypical cognition in schizophrenia. Finally, both bivariate and multivariate functional connectivity metrics of MEG data are useful for understanding how these interactions in the brain are impaired in schizophrenia, and how cognitive and clinical outcomes are affected as a result. We also present here data from our own laboratory that illustrates how some of these novel functional connectivity measures, specifically imaginary coherence (IC), are quite powerful in relating disconnectivity in the brain to characteristic behavioral findings in the disorder
MERLiN: Mixture Effect Recovery in Linear Networks
Causal inference concerns the identification of cause-effect relationships
between variables, e.g. establishing whether a stimulus affects activity in a
certain brain region. The observed variables themselves often do not constitute
meaningful causal variables, however, and linear combinations need to be
considered. In electroencephalographic studies, for example, one is not
interested in establishing cause-effect relationships between electrode signals
(the observed variables), but rather between cortical signals (the causal
variables) which can be recovered as linear combinations of electrode signals.
We introduce MERLiN (Mixture Effect Recovery in Linear Networks), a family of
causal inference algorithms that implement a novel means of constructing causal
variables from non-causal variables. We demonstrate through application to EEG
data how the basic MERLiN algorithm can be extended for application to
different (neuroimaging) data modalities. Given an observed linear mixture, the
algorithms can recover a causal variable that is a linear effect of another
given variable. That is, MERLiN allows us to recover a cortical signal that is
affected by activity in a certain brain region, while not being a direct effect
of the stimulus. The Python/Matlab implementation for all presented algorithms
is available on https://github.com/sweichwald/MERLi
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