Sie befinden Sich nicht im Netzwerk der Universität Paderborn. Der Zugriff auf elektronische Ressourcen ist gegebenenfalls nur via VPN oder Shibboleth (DFN-AAI) möglich. mehr Informationen...
P17. Isolated effective coherence (iCoh) based on estimated electric neuronal activity using exact low resolution brain electromagnetic tomography (eLORETA)
A problem of great interest in real world systems, where multiple time series measurements are available, is the estimation of the intra-system casual relations. For instance, electric cortical signals are used for studying functional connectivity between different brain areas, the directionality, the direct or indirect nature of the connections, and the spectral characteristics such as which frequencies are transmitted. The earliest spectral measure of causality was Akaike’s (1968) seminal work on the noise contribution ratio (NCR), reflecting direct and indirect connections. Later, the partial directed coherence (PDC) of Baccala and Sameshima (2001) was proposed for direct connections. In this study, the partial coherence is estimated under a stable, stationary multivariate auto-regressive model with stable isolated self-auto-regressions. The new measure of direct causal coherence, namely the isolated causal coherence (iCC), is defined under the “a posteriori” assumption that all irrelevant connections are zero. It is shown that when these assumptions are applied to the NCR, it produces the iCC, thus enriching its interpretability. In comparison with the iCC, it is shown that the PDC is affected by irrelevant connections to such an extent that it can misrepresent the frequency response. Toy examples are included to demonstrate these properties. Analysis of real EEG data is also included.