Abstracts

Non-invasive localization of epileptogenic foci by means of EEG dipole source modeling depends on there being a definable relationship between the cerebral source of an EEG spike or seizure potential and the scalp EEG voltage field that is used for the inverse solution. Validating this relationship can best be performed by simultaneously recording both intracranial and scalp EEG and correlating source location to scalp field. Determining scalp voltage fields correlated with selective sublobar sources would benefit from a technique to increase the [ldquo]signal[rdquo] of the epileptiform activity from the [ldquo]noise[rdquo], which in this case is the ongoing EEG. We selected data from eight patients who had simultaneous EEG recordings from both an array of subdural electrodes encircling the temporal lobe and at least 24 scalp electrodes, including subtemporal locations bilaterally. Intracranial EEG spikes or seizure potentials of similar morphology from various circumscribed temporal lobe sources (base, tip, anterior infero-lateral, anterior and posterior lateral) were identified and used as a trigger to average the scalp EEG and thus enhance the signal to noise of any correlated scalp potential. Voltage topographic maps and single moving dipole models of the averaged scalp potentials were then calculated. Discrete, sublobar intracranial EEG spikes and seizure potentials often did not have a scalp EEG correlate that was recognizable in ongoing EEG activity. Signal averaging usually resulted in a definable scalp voltage field for even small cortical sources. Temporal sources from different sublobar areas produced distinctive scalp voltage fields that were modeled by dipoles of different location and orientation. Dipole orientation, in particular, distinguished the various sublobar surfaces. There is a direct and definable relation between the location and orientation of a cortical spike or seizure source and the scalp voltage field it produces. Dipole and other source models of scalp EEG are therefore reasonable approaches to non-invasive localization of epileptogenic foci. Sublobar resolution should be possible given our findings.