title: Unifying Blind Separation and Clustering for Resting-State EEG/MEG Functional Connectivity Analysis.
abstract: Unsupervised analysis of the dynamics (nonstationarity) of functional brain connectivity during rest has recently received a lot of attention in the neuroimaging and neuroengineering communities. Most studies have used functional magnetic resonance imaging, but electroencephalography (EEG) and magnetoencephalography (MEG) also hold great promise for analyzing nonstationary functional connectivity with high temporal resolution. Previous EEG/MEG analyses divided the problem into two consecutive stages: the separation of neural sources and then the connectivity analysis of the separated sources. Such nonoptimal division into two stages may bias the result because of the different prior assumptions made about the data in the two stages. We propose a unified method for separating EEG/MEG sources and learning their functional connectivity (coactivation) patterns. We combine blind source separation (BSS) with unsupervised clustering of the activity levels of the sources in a single probabilistic model. A BSS is performed on the Hilbert transforms of band-limited EEG/MEG signals, and coactivation patterns are learned by a mixture model of source envelopes. Simulation studies show that the unified approach often outperforms conventional two-stage methods, indicating further the benefit of using Hilbert transforms to deal with oscillatory sources. Experiments on resting-state EEG data, acquired in conjunction with a cued motor imagery or nonimagery task, also show that the states (clusters) obtained by the proposed method often correlate better with physiologically meaningful quantities than those obtained by a two-stage method.
authors.affiliation: Advanced Telecommunications Research Institute International (ATR), Soraku-gun, Kyoto, 619-0288, Japan hirayama@atr.jp.
authors.forename: Jun-Ichiro
authors.initials: J
authors.lastname: Hirayama
authors.affiliation: Advanced Telecommunications Research Institute International (ATR), Soraku-gun, Kyoto, 619-0288, Japan t.ogawa@atr.jp.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Ogawa
authors.affiliation: Department of Computer Science and HIIT, University of Helsinki, 00560 Helsinki, Finland; and Advanced Telecommunications Research Institute International (ATR), Soraku-gun, Kyoto 619-0288, Japan aapo.hyvarinen@helsinki.fi.
authors.forename: Aapo
authors.initials: A
authors.lastname: Hyvärinen