ESTIMATION OF OBSCURE ICTAL EPILEPTIC ACTIVITY IN SCALP EEG

We have produced a method to estimate ictal localized epileptic activi ty hidden among the background in scalp EEGs. When the visually comple tely different waveforms of the epileptic and background activities ar e nearly orthogonal, epileptic activity may be approximately extracted from the EEG data matrix by singular value decomposition with subsequ ent orthogonal rotation to match the distribution of one component wit h that of the epileptic source. A simulation study was carried out usi ng a matrix mimicking the scalp EEC with an inconspicuous ictal epilep tic activity from a dipole source. This hidden epileptic activity was approximately recovered by matching the dipole of interest with the ep ileptic dipole, even when the simulated waveforms of the epileptic and background activities were not exactly orthogonal. High linear correl ation between these two types of waveforms hampered the recovery of th e epileptic activity. In another simulation study employing two epilep tic dipoles producing activities with the same waveform and a brief ti me lag, it was indicated that the temporal relationship between the ep ileptic activities could be also estimated using the cross-correlation function. In the preliminary clinical application of this method to t he ictal EEGs of complex partial seizures, rhythmic activities with se emingly epileptic waveforms were estimated at the dipoles which were l ocated in the vicinity of cortical lesions revealed by neuroimaging st udies. These activities were indicated to appear before any change in the scalp EEG. We hope for the clinical application of this method for noninvasive estimation of inconspicuous ictal epileptic activity.