Frequency domain EEG source localization of ictal epileptiform activity in patients with partial complex epilepsy of temporal lobe origin

Abstract

The aim of this study was to investigate whether EEG source localization in the frequency domain, using the FFT dipole approximation (Lehmann, D. and Michel, C.M. Electroenceph. clin. Neurophysiol., 1990, 76: 271-276), would be useful for quantifying the frequency content of epileptic seizure activity. Between one and 7 extracranially recorded seizures were analyzed in each of 7 patients with mesolimbic epilepsy, who were seizure-free after temporal lobe resection. The full scalp frequency spectrum for the first 4 s after seizure onset, as well as for subsequent periods, was determined. Power peaks in the spectra were identified, and an instant dipole fit was performed for the frequencies corresponding to these peaks. Ictal frequencies, ranging between 3.5 and 8.5 Hz, showed a variable degree of stability over time in the different patients. For a particular frequency, dipole results were similar during the different phases of seizure development. In patients with more than one prominent frequency, dipole results for the different frequencies were similar. Dipole results were also similar between patients. We conclude that dipole localization of dominant frequencies, as obtained from full scalp FFT analysis, gives quite reproducible results for seizures originating in the mesial temporal area. The method may become a useful tool for the pre-surgical identification of patients with mesolimbic epilepsy.

Ictal source analysis: Localization and imaging of causal interactions in humans

“…Seizure localization has been advanced by source localization and imaging techniques using both EEG (Assaf & Ebersole, 1997;Lantz et al, 1999;Michel et al, 1999;Worrell et al, 2000) and MEG (Ebersole, 1997;Assaf et al, 2003) to characterize seizures in the source space instead of the sensor space. Such techniques handle the volume conductor effect and, thus, are able to identify the internal sources behind the seizures by solving the so-called inverse problem (von Helmholtz, 1853).…”

Temporal and spatial determination of EEG-seizure onset in the frequency domain

“…from 5 to 7 Hz) can also be appreciated in the ®gures of the exemplary seizures of mesiotemporal origin in the paper by Ebersole and Pacia (1996). Finally, Lantz et al (1999) have also observed in some of their patients that during different time periods following the ESO different frequencies become dominant. Even if our analysis con®rms the importance of ictal frequencies .5 Hz in mesiotemporal lobe epilepsy since they could be detected in all patients, we note that in 4 of 6 patients in whom two dominant frequencies were determined the lower frequencies from 3.0 to 5.5 Hz appeared earlier than or at the same time as the higher`typical mesiotemporal ' frequency patterns.…”

“…We did not include the onset of ictal behavior to select the ictal time period, since it has been shown in previous studies that ictal EEG analysis alone can reliably de®ne the seizure onset (Risinger et al, 1989;Ebersole and Pacia, 1996;Lantz et al, 1999). The selected 46 s time period was divided into 23 non-overlapping 2 s epochs.…”

“…2, Table 2; Risinger et al, 1989;Ebersole and Pacia, 1996;Lantz et al, 1999). Over all patients the mean of this frequency was determined at 7.0^0.8 Hz).…”

“…Recent investigations have shown that the spectral properties during the phase which follows rhythmic seizure onset can be retrieved (Gotman et al, 1993;Alcaron et al, 1995). By approximating phase corrected voltage maps in the frequency domain, Lantz et al (1999) have calculated the full scalp spectrum for ictally dominant frequencies, by using the so-called FFT Approximation method (Lehmann and Michel, 1990) and subsequently used these voltage maps to localize the mesiotemporal epileptic focus. Moreover, a recent preliminary report from our group suggests that the calculation of the full scalp spectrum in the frequency domain can detect pathological ictal rhythms earlier in time than revealed by pure visual inspection of the EEG .…”