Neocortical seizure foci localization by means of a directed transfer function method

Abstract

Purpose: Determination of the origin of extratemporal neocortical onset seizures is often challenging due to the rapid speed at which they propagate throughout the cortex. Typically, these patients are poor surgical candidates and many times experience recurrences of seizure activity following resection of the assumed seizure focus.

Methods: We applied a causal measurement technique--the directed transfer function (DTF)--in an effort to determine the cortical location responsible for the propagation of the seizure activity. Intracranial seizure recordings were obtained from a group of 11 pediatric patients with medically intractable neocortical-onset epilepsy. Time windows were selected from the recordings following onset of the ictal activity. The DTF was applied to the selected time windows, and the frequency-specific statistically significant source activity arising from each cortical recording site was quantified. The DTF-estimated source activity was then compared with the seizure-onset zone(s) identified by the epileptologists.

Results: In an analysis of the 11 pediatric patients, the DTF was shown to identify estimated ictal sources that were highly correlated with the clinically identified foci. In addition, it was observed that in the patients with multiple ictal foci, the topography of the casual source activity from the analyzed seizures was associated with the separate clinically identified seizure-onset zones.

Discussion: Although localization of neocortical-onset seizures is typically challenging, the causal measures employed in this study-namely the directed transfer function-identified generators of the ictal activity that were highly correlated with the cortical regions identified as the seizure-onset zones by the epileptologists. This technique could prove useful in the identification of seizure-specific propagation pathways in the presurgical evaluation of patients with epilepsy.

Figure 1

A diagram outlining the DTF source localization method utilized in the present study. First, a time segment following the ictal onset was selected from the ECoG recordings. The DTF method was applied to the time series and the connectivity pattern between the ECoG electrodes was obtained. Significance testing by means of a surrogate data method was performed to obtain the causal interactions which were statistically significant. From here, the amount of information leaving each electrode (strength of the outgoing arrows) was summed and the electrode with the maximum amount of source activity for each seizure was noted. This process was repeated for each seizure and the statistically significant source activity was summed to obtain the total DTF-calculated source activity for each patient.

Figure 2

(A)The significant source activity obtained from each of the six seizures analyzed in Patient 1. The red color indicates a high degree of source activity in each seizure. (B) The source activity from each electrode was normalized such that the maximum source activity for each seizure had unit strength. This source activity was summed over all of the analyzed seizures in each patient and thresholding was performed at 50% and 80% of the maximum summed activity. Here, the source activity obtained from the six seizures analyzed in Patient 1 are shown.

Figure 3

(a) The SOZs in Patient 1 identified clinically by the epileptologists; (b) the DTF-calculated source activity obtained by selecting the cortical regions having the maximum source activity in each of the analyzed seizures; (c) the summed source activity exceeding the 50% threshold; (d) the summed source activity exceeding the 80% threshold.

Figure 4

The DTF-calculated source activity for Patient 2 (a) through Patient 11 (j). The layout of the results for each patient is the same as shown in Figure 3. In Patients 4 (c) and 11 (j), whole-brain MRI scans were not available. In these two patients, the causal source activity and SOZ were projected onto an averaged smoothed cortical model.

Figure 4

The DTF-calculated source activity for Patient 2 (a) through Patient 11 (j). The layout of the results for each patient is the same as shown in Figure 3. In Patients 4 (c) and 11 (j), whole-brain MRI scans were not available. In these two patients, the causal source activity and SOZ were projected onto an averaged smoothed cortical model.

Figure 5

The correlation of the post-surgical seizure outcome in the analyzed patients to the percentage of the causal source activity within the clinical SOZ calculated from the 50% thresholded spatial maps.