title: Assessing direct paths of intracortical causal information flow of oscillatory activity with the isolated effective coherence (iCoh).
abstract: Functional connectivity is of central importance in understanding brain function. For this purpose, multiple time series of electric cortical activity can be used for assessing the properties of a network: the strength, directionality, and spectral characteristics (i.e., which oscillations are preferentially transmitted) of the connections. The partial directed coherence (PDC) of Baccala and Sameshima (2001) is a widely used method for this problem. The three aims of this study are: (1) To show that the PDC can misrepresent the frequency response under plausible realistic conditions, thus defeating the main purpose for which the measure was developed; (2) To provide a solution to this problem, namely the "isolated effective coherence" (iCoh), which consists of estimating the partial coherence under a multivariate autoregressive model, followed by setting all irrelevant associations to zero, other than the particular directional association of interest; and (3) To show that adequate iCoh estimators can be obtained from non-invasively computed cortical signals based on exact low resolution electromagnetic tomography (eLORETA) applied to scalp EEG recordings. To illustrate the severity of the problem with the PDC, and the solution achieved by the iCoh, three examples are given, based on: (1) Simulated time series with known dynamics; (2) Simulated cortical sources with known dynamics, used for generating EEG recordings, which are then used for estimating (with eLORETA) the source signals for the final connectivity assessment; and (3) EEG recordings in rats. Lastly, real human recordings are analyzed, where the iCoh between six cortical regions of interest are calculated and compared under eyes open and closed conditions, using 61-channel EEG recordings from 109 subjects. During eyes closed, the posterior cingulate sends alpha activity to all other regions. During eyes open, the anterior cingulate sends theta-alpha activity to other frontal regions.
authors.affiliation: The KEY Institute for Brain-Mind Research, University of Zurich Zurich, Switzerland ; Department of Neuropsychiatry, Kansai Medical University Osaka, Japan.
authors.forename: Roberto D
authors.initials: RD
authors.lastname: Pascual-Marqui
authors.affiliation: CIMFAV, Universidad de Valparaiso Valparaiso, Chile.
authors.forename: Rolando J
authors.initials: RJ
authors.lastname: Biscay
authors.affiliation: Neuroinformatic Department, Cuban Neuroscience Center Havana, Cuba.
authors.forename: Jorge
authors.initials: J
authors.lastname: Bosch-Bayard
authors.affiliation: The KEY Institute for Brain-Mind Research, University of Zurich Zurich, Switzerland.
authors.forename: Dietrich
authors.initials: D
authors.lastname: Lehmann
authors.affiliation: The KEY Institute for Brain-Mind Research, University of Zurich Zurich, Switzerland.
authors.forename: Kieko
authors.initials: K
authors.lastname: Kochi
authors.affiliation: Department of Neuropsychiatry, Kansai Medical University Osaka, Japan.
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Kinoshita
authors.affiliation: Department of Psychiatry, Shiga University of Medical Science Shiga, Japan.
authors.forename: Naoto
authors.initials: N
authors.lastname: Yamada
authors.affiliation: Division of Cerebral Integration, National Institute for Physiological Sciences Okazaki, Japan.
authors.forename: Norihiro
authors.initials: N
authors.lastname: Sadato
keywords: LORETA; alpha oscillation connectivity; causal intracortical connectivity; isolated effective coherence; resting state electriphysiological connectivity