PhD Thesis Defense: Spatio-Temporal Analysis of Functional Imaging
Speaker: Wanmei Ou , CSAIL, MITContact:
Date: December 2 2009
Time: 2:00PM to 3:00PM
Location: 32-G449 Kiva
Host: Professor Polina Golland, CSAIL, MIT
Wanmei Ou, 3-4143, email@example.comRelevant URL:
Localizing sources of activity from electroencephalography (EEG) and magnetoencephalography (MEG) measurements involves solving an ill-posed inverse problem, where infinitely many source distribution patterns can give rise to identical measurements. This thesis aims to improve the accuracy of source localization by incorporating spatio-temporal models of the signal into the reconstruction procedure.
First, we introduce a novel method for current source estimation, which we call the L1L2-norm source estimator. The underlying model captures the sparseness of the active areas in space while encouraging smooth temporal dynamics. We compute the current source estimates efficiently by solving a second-order cone programming problem. By considering all time points simultaneously in the estimation procedure, we achieve accurate and stable results as confirmed by the experiments using simulated and human MEG data.
Although it enables accurate source estimation, the L1L2-norm estimator still faces challenges when the current sources are close to each other in space. To alleviate problems caused by the limited spatial resolution of EEG/MEG measurements, we propose to incorporate information from functional magnetic resonance imaging (fMRI) into the estimation algorithm. Whereas EEG/MEG measures neural activity, fMRI records hemodynamic activity in the brain with high spatial resolution. As part of this thesis, we examine empirically the neurovascular coupling in simultaneously recorded MEG and diffuse optical imaging (DOI) data. Our results suggest that the neural activity and hemodynamic responses are aligned in space. However, the relationship between the temporal dynamics of the two types of signals is non-linear and varies from region to region.
Based on these findings, we develop the fMRI-informed regional EEG/MEG source estimator (FIRE). This method is based on a generative model that encourages similar spatial patterns but allows for differences in time courses across imaging modalities. Our experiments with both Monte Carlo simulation and human fMRI-EEG/MEG data indicate that FIRE significantly reduces ambiguities in source localization and accurately captures the timing of activation in adjacent functional regions.
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