| Summary: | Source estimates performed using a single equivalent current dipole (ECD) model forinterictal epileptiform discharges (IEDs) which appear unifocal have proven highly accurate inneocortical epilepsies, falling within millimeters of that demonstrated by electrocorticography. Despitethis success, the single ECD solution is limited, best describing sources which are temporally stable.Adapted from the field of optics, optical flow analysis of distributed source models of MEG or EEG datahas been proposed as a means to estimate the current motion field of cortical activity, or "cortical flow".The motion field so defined can be used to identify dynamic features of interest such as patterns ofdirectional flow, current sources and sinks. The Helmholtz-Hodge Decomposition (HHD) is a techniquefrequently applied in fluid dynamics to separate a flow pattern into three components: 1) a nonrotationalscalar potential U describing sinks and sources, 2) a non-diverging scalar potential Aaccounting for vortices, and 3) an harmonic vector field H. As IEDs seem likely to represent periods ofhighly correlated directional flow of cortical currents, the U component of the HHD suggests itself as away to characterize spikes in terms of current sources and sinks. In a series of patients with refractoryepilepsy who were studied with magnetoencephalography as part of their evaluation for possibleresective surgery, spike localization with ECD was compared to HHD applied to an optical flow analysisof the same spike. Reasonable anatomic correlation between the two techniques was seen in themajority of patients, suggesting that this method may offer an additional means of characterization ofepileptic discharges.
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