Table 1.
Summary of Studies on Interictal Functional Connectivity Using Intracranial EEG
| Authors | Population | Recording modality | Methods | Main results |
|---|---|---|---|---|
| ECoG ± depth electrodes | ||||
| Towle et al (1998) | Twenty-five patients Tumors in the sensory-motor region or temporal lobectomy |
ECoG During surgery (five with also presurgical recording) |
Coherence | Local zone of increased FC nearby/within EZ |
| Arnhold et al (1999) | One patient with mesial TLE One patient with neocortical epilepsy |
ECoG+depth electrodes (during presurgical monitoring) | “Interdependence” | Greater FC within the EZ than within the NIZ |
| Schevon et al (2007) | Nine patients with neocortical epilepsy | ECoG (during presurgical monitoring) | Mean phase coherence | Area of local hypersynchrony (stable in time) overlapping with the EZ |
| Ortega et al (2008) | Twenty-nine patients with TLE | ECoG (during surgery) | Linear correlation, mutual information, and phase synchronization | Cluster of local hypersynchrony in the epileptic temporal lobe (no good correlation with the EZ) |
| Dauwels et al (2009) | Six patients with neocortical epilepsy | ECoG+depth electrodes (during presurgical monitoring) | Cross-correlation, Granger causality (directed transfer function), phase synchrony, magnitude coherence | Area of hypersynchrony correlates with the EZ |
| Warren et al (2010) | Comparison of patients with chronic pain (n = 2) and with epilepsy (n = 4) | ECoG (during presurgical monitoring) | Linear correlation, mean phase coherence | Disconnection of EZ from the NIZ and decrease of connectivity within the NIZ |
| Wilke et al (2011) | Twenty-five patients with neocortical epilepsy | ECoG (during presurgical monitoring) | Directed transfer function (gamma band) | Correlation between “active node” and EZ (lower than interictal spikes) |
| Park and Madsen (2018) | Twenty-five patients (10 TLE) | ECoG+depth electrodes | Granger causality | FC predicts better than chance the EZ/RZ |
| Shah et al (2019b) | Twenty-seven patients (18 TLE) | ECoG+depth electrodes | Linear correlation | Gradual decrease of FC: within-RZ > RZ-OUT > within-OUT |
| SEEG | ||||
| Mormann et al (2000) | Seventeen patients with TLE | SEEG (bilateral temporal mesial sampling) | Mean phase coherence | Increased FC in the epileptogenic side |
| Bettus et al (2008) | Twenty-one patients with mesial TLE +14 with nonmesial TLE | SEEG | Nonlinear correlation (h2) | FC within mesiotemporal structures is higher when these structures belong to EZ |
| Bettus et al (2011) | Five patients with TLE | SEEG | Nonlinear correlation (h2) | Higher FC within the EZ and the irritative zone than within the NIZ (in beta only) Leading role of the EZ |
| Varotto et al (2012) | Ten patients with FCD II (neocortical epilepsy) | SEEG | Partial directed coherence | Increase in outgoing connections (30–80 Hz) in FCD compared with other structures of the EZ Increase in the betweenness centrality within the FCD |
| Van Diessen et al (2013) | Twelve patients with TLE | SEEG (bilateral temporal mesial sampling) | Phase Lag Index | Disconnection of the epileptogenic mesiotemporal structures from their contralateral nonepileptogenic homologous |
| Bartolomei et al (2013) | Eleven patients with mesial TLE + eight with nonmesial TLE | SEEG | Synchronization likelihood | FC within temporal structures is higher when these structures belong to EZ |
| Lagarde et al (2018) | Fifty-nine patients with FCD or NDT (20 TLE) | SEEG | Nonlinear correlation (h2) | Gradual decrease of FC: EZ > PZ > NIZ FC between EZ-PZ > PZ-NIZ Leading role of the EZ |
| Goodale et al (2020) | Fifteen patients (12 TLE) | SEEG | Imaginary coherence | FC in EZ and EZ-NIZ higher than NIZ Higher clustering coefficient, betweenness centrality within EZ Predictive accuracy = 80.4% |
| Narasimhan et al (2020) | Twenty-five patients (18 TLE) | SEEG | Mutual information, imaginary coherence, partial directed coherence, directed transfer entropy | Gradual decrease of FC: EZ > PZ > Irritative zone > NIZ EZ with higher inward FC Predictive accuracy = 88%, better if combination of connectivity methods |
| Paulo et al (2022) | Thirty-two patients (18 TLE) | SEEG | Imaginary coherence, partial directed coherence (in alpha band) | Stability across time (days) Higher FC in EZ than in NIZ This difference decreased with the antiseizure medication withdrawal when using imaginary coherence |
| Jiang et al (2022) | Twenty-seven patients (23 TLE) | SEEG | Directed transfer function, cross-frequency directionality | Higher inward strength in EZ than in NIZ Information flow from NIZ high-frequency activity to EZ low-frequency activity |
Studies are presented in chronological order. If the definition of the epileptic areas (epileptogenic, seizure-onset zone, etc. …) is variable from one study to another, for simplicity, we summarized the data with the terms EZ and NIZ. For studies using ECoG recordings, we precise if the recordings were performed during a surgery procedure (with anesthesia) or long-term monitoring.
ECoG, electrocorticography; EZ, epileptogenic zone; FC, functional connectivity; FCD, focal cortical dysplasia; h2, nonlinear correlation coefficient; irritative zone, area with interictal spikes but no ictal discharge; NDT, neurodevelopmental tumor; NIZ, noninvolved zone; PZ, propagation zone; RZ, resection zone; SEEG, stereo-EEG; TLE, temporal lobe epilepsy.