Multiple source analysis of interictal spikes: Goals, requirements, and clinical value

When evaluating interictal spikes using dipole source analysis it is import ant to account for multiple sources and the overlapping background EEG. Ana lyses of spike peaks may be modeling only propagated sources. Careful filte ring of averaged spike data and multiple source analysis can provide useful information about the onset of epileptiform activity. A forward high-pass filter can help to enhance the initial spike activity during onset over the propagated activity. These points are illustrated with examples of a tempo ral, a parietal, and a frontal averaged spike. Multiple source analysis was applied using a genetic algorithm and a sequential strategy, in one case i ncluding a model of background alpha activity. Multiple source analysis cou ld model sources describing the onset activity that were distinct in locati on and orientation from the propagated activity. In all cases, the prominen t peak on the scalp was dominated by the contribution of propagated sources . Clinical interpretation benefits from an approach that combines the tempo ral evolution of EEG scalp topography and multiple source activities with t he information from localization and orientation of equivalent dipole sourc es to identify the cortical generators underlying the earliest phase of int erictal spikes.