Abstracts

Rationale: Source estimates performed using a single equivalent current dipole (ECD) model for interictal epileptiform discharges (IEDs) which appear unifocal have proven highly accurate in neocortical epilepsies, falling within millimeters of that demonstrated by electrocorticography. Despite this 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 data has recently been proposed as a means to estimate the current motion field of cortical activity, or "cortical flow". This technique can be used to estimate local kinetic energy of cortical surface currents, and has been used to characterize correspondence between the speed and direction of the surface current flow within the visual cortex and the dynamical properties of the visual stimulus itself. The motion field so defined can be used to identify dynamic features of interest such as patterns of directional flow, current sources and sinks. The Helmholtz-Hodge Decomposition (HHD) is a technique frequently applied in fluid dynamics to separate a flow pattern into three components: 1) a non-rotational scalar potential U describing sinks and sources, 2) a non-diverging scalar potential A accounting for vortices, and 3) an harmonic vector field H. As IEDs seem likely to represent periods of highly correlated directional flow of cortical currents, the U component of the HHD suggests itself to characterize spikes in terms of current sources and sinks. Methods: Source Localization Cortical surface segmentation and tessellation from brain MRI was obtained using BrainSuite software (http://www.loni.ucla.edu/Software/BrainSuite/). The BrainStorm Toolbox (http://neuroimage.usc.edu/brainstorm/) was used for all subsequent source analyses. The computed head and cortex models were used in combination with the MEG fields to compute an estimate of current-source density distribution over the cortex based on a minimum norm estimate. The ECD location, orientation and moment was calculated for each spike. For each spike identified, a time period covering spike onset, peak and offset was subjected to optical flow analysis, and subsequent HHD. The time of the maximum value of the global U component prior to peak occurrence was used to calculate the HHD source map. Results: MEG data corresponding to 24 spike or sharp wave discharges from six patients with refractory epilepsy was subjected to the HHD analysis. Current sources and sinks with a location proximal to that of the calculated dipole were identified for approximately one third of the events. Figure 1 illustrates the ECD corresponding to the current source identified and and projected in figure 2 (along with the original spike). Conclusions: For MEG detected spikes, the HHD offers an additional means of characterization of the spatial distribution of the discharge over time. For a subset of spike discharges, there appears to be a good anatomic correlation with the calculated spike dipole.