Linear inverse source estimate of combined EEG and MEG data related to voluntary movements

A method for the modeling of human movement-related cortical activity from combined electroencephalography (EEG) and magnetoencephalography (MEG) data is proposed. This method includes a subject's multi-compartment head model (scalp, skull, dura mater, cortex) constructed from magnetic resonance ima ges, multi-dipole source model, and a regularized linear inverse source est imate based on boundary element mathematics. Linear inverse source estimate s of cortical activity were regularized by taking into account the covarian ce of background EEG and MEG sensor noise. EEG (121 sensors) and MEG (43 se nsors) data were recorded in separate sessions whereas normal subjects exec uted voluntary right one-digit movements. Linear inverse source solution of EEG, MEG, and EEG-MEG data were quantitatively evaluated by using three pe rformance indexes. The first two indexes (Dipole Localization Error [DLE] a nd Spatial Dispersion [SDis]) were used to compute the localization power f or the source solutions obtained. Such indexes were based on the informatio n provided by the column of the resolution matrix (i.e., impulse response). Ideal DLE values tend to zero (the source current was correctly retrieved by the procedure). In contrast, high DLE values suggest severe mislocalizat ion in the source reconstruction. A high value of SDis at a source space po int mean that such a source will be retrieved by a large area with the line ar inverse source estimation. The remaining performance index assessed the quality of the source solution based on the information provided by the row s of the resolution matrix R, i.e., resolution kernels. The i-th resolution kernels of the matrix R describe how the estimation of the i-th source is distorted by the concomitant activity of all other sources. A statistically significant lower dipole localization error was observed and lower spatial dispersion in source solutions produced by combined EEG-MEG data than from EEG and MEG data considered separately (P < 0.05). These effects were not due to an increased number of sensors in the combined EEG-MEG solutions. Th ey result from the independence of source information conveyed by the multi modal measurements. From a physiological point of view, the linear inverse source solution of EEG-MEG data suggested a contralaterally preponderant bi lateral activation of primary sensorimotor cortex from the preparation to t he execution of the movement. This activation was associated with that of t he supplementary motor area. The activation of bilateral primary sensorimot or cortical areas was greater during the processing of afferent information related to the ongoing movement than in the preparation for the motor act. In conclusion, the linear inverse source estimate of combined MEG and EEG data improves the estimate of movement-related cortical activity. (C) 2001 Wiley-Liss, Inc.