4,138 research outputs found
Time Series Analysis of fMRI Data: Spatial Modelling and Bayesian Computation
Time series analysis of fMRI data is an important area of medical statistics
for neuroimaging data. The neuroimaging community has embraced mean-field
variational Bayes (VB) approximations, which are implemented in Statistical
Parametric Mapping (SPM) software. While computationally efficient, the quality
of VB approximations remains unclear even though they are commonly used in the
analysis of neuroimaging data. For reliable statistical inference, it is
important that these approximations be accurate and that users understand the
scenarios under which they may not be accurate.
We consider this issue for a particular model that includes spatially-varying
coefficients. To examine the accuracy of the VB approximation we derive
Hamiltonian Monte Carlo (HMC) for this model and conduct simulation studies to
compare its performance with VB. As expected we find that the computation time
required for VB is considerably less than that for HMC. In settings involving a
high or moderate signal-to-noise ratio (SNR) we find that the two approaches
produce very similar results suggesting that the VB approximation is useful in
this setting. On the other hand, when one considers a low SNR, substantial
differences are found, suggesting that the approximation may not be accurate in
such cases and we demonstrate that VB produces Bayes estimators with larger
mean squared error (MSE). A real application related to face perception is also
carried out. Overall, our work clarifies the usefulness of VB for the
spatiotemporal analysis of fMRI data, while also pointing out the limitation of
VB when the SNR is low and the utility of HMC in this case
Necessity for quantum coherence of nondegeneracy in energy flow
In this work, we show that the quantum coherence among non-degenerate energy
subspaces (CANES) is essential for the energy flow in any quantum system. CANES
satisfies almost all of the requirements as a coherence measure, except that
the coherence within degenerate subspaces is explicitly eliminated.We show that
the energy of a system becomes frozen if and only if the corresponding CANES
vanishes, which is true regardless of the form of interaction with the
environment. However, CANES can remain zero even if the entanglement changes
over time. Furthermore, we show how the power of energy flow is bounded by the
value of CANES. An explicit relation connecting the variation of energy and
CANES is also presented. These results allow us to bound the generation of
system-environment correlation through the local measurement of the system's
energy flow
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