Interictal and ictal dipole modeling in patients with refractory epilepsy

Fifteen patients (7 men, 8 women) with mean age of 34 years and mean duration of refractory partial seizures of 17 years were included in a presurgical evaluation protocol. Neuroimaging (CAT, 1.5 T MR) demonstrated intracranial structural lesions (space-occupying: n = 9; atrophic: n = 6) and video-EEG monitoring showed complex partial seizures in all patients. Four patients underwent additional intracranial EEG monitoring that demonstrated hippocampal seizure onset in all. Voltage topography and spatiotemporal dipole mapping of interictal epileptic discharges revealed two distinct dipole types. Patients with lesions in the medial (and lateral) temporal lobe uniformly presented with a negative voltage held with a steep gradient over the inferior temporal area and a stable, combined dipole that consisted of a radial and a tangential component with a high degree of elevation relative to the axial plane. Patients with extratemporal lesions had a more diffuse, less dipolar voltage held and a corresponding dipole which was less stable and had a predominant radial component. Dipole modelling of epochs of early ictal discharges revealed a striking correspondence with the interictal findings in individual patients. Interictal spike voltage topography and corresponding dipole mapping provided additional and reliable information that was relevant in surgical candidates for refractory partial epilepsy, e.g. by suggesting in some patients that the medial temporal structures were not primarily involved. Ictal dipole modelling revealed concordant results with interictal data. It shows promising but needs further confirmation and validation in a larger patient population with intracranial EEG recordings. Despite intrinsic limitations, spike voltage topography and dipole mapping contributes to a better localisation of the underlying brain source of epileptic discharges

The realistic versus the spherical head model in EEG dipole source analysis in the presence of noise

The performance of the three-shell spherical head model versus the performance of the realistic head model is investigated when solving the inverse problem with a single dipole, in the presence of noise. This is evaluated by inspecting the average dipole location error when applying a spherical and a realistic head model, for 1000 noisy scalp potentials, originating from the same test dipole and having the same noise level. The location errors are obtained utilizing a local linearization, which is validated with a Monte Carlo simulation. For 27 electrodes, an EEG epoch of one time sample and spatially white Gaussian noise we found that the importance of the realistic head model over the spherical head model reduces by increasing the noise level