Simplified projection of EEG dipole sources onto human brain anatomy

This study was aimed at determining an easy way to project dipole modelling results onto brain anatomy. This simplified projection is based on the estimation of the mean location of the centre of the dipole sphere according to internal brain landmarks. The mean values for the centre location were calculated from ten epileptic patients. To define the axes of the dipole model frame on the patient's magnetic resonance image (MRI), markers were pasted at some electrode positions during the acquisition. An estimation was then made of the mean position of the model centre from the bicommissural line (anterior commissure-posterior commissure [AC-PC]), and a simple transformation to pass from the model cartesian coordinates to the anatomical correlates either in the subject MRI or in the Talairach atlas. These data were then tested in four additional subjects in whom no markers had been placed during the MRI acquisition. On average, the horizontal plane of the sphere model was pitched up 1.9 degrees +/- 1.8 only with respect to the AC-PC horizontal plane, which allowed the projection of dipoles directly onto the Talairach atlas, without pitch. The mean sphere centre was located 7.4 +/- 4.2 mm above the bicommissural line, and 8.2 +/- 1 mm in front of the posterior commissure. In the four additional subjects, projections on MRI and atlas indicated the same anatomical regions and showed high congruence with the physiology or the pathology. This simplified way we report herein has proved to give reliable results. We believe that this method will be useful as a first approximation to project dipole coordinated onto MRI data; moreover, when MRI is unavailable, our results show that dipole modelling results can be superimposed onto atlas slices provided that they are represented according to the AC-PC plane.