Electrocorticography and stereo EEG provide distinct measures of brain connectivity: Implications for network models

Abstract

Brain network models derived from graph theory have the potential to guide functional neurosurgery, and to improve rates of post-operative seizure freedom for patients with epilepsy. A barrier to applying these models clinically is that intracranial EEG electrode implantation strategies vary by center, region and country, from cortical grid & strip electrodes, to purely stereotactic depth electrodes, to a mixture of both. To determine whether models derived from one type of study are broadly applicable to others, we investigate the differences in brain networks mapped by electrocortiography (ECoG) and stereoelectroencephalography (SEEG) in a matched cohort of patients who underwent epilepsy surgery. We show that ECoG and SEEG map broad network structure differently, and demonstrate substantial disparity in the ability of node strength to localize the epileptogenic zone in SEEG compared to ECoG. We demonstrate that eliminating white matter contacts and reducing network nodes to anatomical regions of interest rather than individual contacts improves the ability of these models to distinguish between epileptogenic and non-epileptogenic regions in SEEG. Our findings suggest that effectively applying computational models to localize epileptic networks requires accounting for the effects of spatial sampling, particularly when analyzing both ECoG and SEEG recordings in the same cohort. Finally, we share all code and data in this study, aiming for our findings to accelerate research in brain network connectivity in epilepsy and beyond.

White Matter Signals Reflect Information Transmission Between Brain Regions During Seizures

“…Clinicians in many epilepsy centers typically exclude white matter contacts when creating iEEG montages to interpret, diagnose, and localize the seizure onset zone (SOZ). Research studies 21–25 utilizing SEEG will typically exclude white matter contacts. The exclusion of white matter contacts is perhaps not unexpected given the fact that no validated studies exist that demonstrate a rigorous approach to incorporating white matter contacts.…”

“…The inefficacy of the surgery could be due to two factors: (1) the SOZ is still a focal region of the pathology, but was missed in the ablation, or (2) the SOZ appears focal to clinicians, but the underlying pathophysiology in some patients is more distributed across brain regions and interconnecting white matter 40 . This latter factor can occur when a patient does not have a focal pathophysiology, but one that stems from an “epileptogenic” 41 or “epileptic” 21 network.…”

“…This implantation modality is now favored in clinical practice because it decreases morbidity and is better tolerated by patients 20 . Yet, SEEG differs from more traditional implantation modalities, such as electrocorticography (ECoG), creating potential problems for clinical translation of the literature based on ECoG 21 . For example, SEEG provides coverage to deeper brain structures and captures neural activity in many tissue types, including white matter (Fig.…”

“…Clinicians in many epilepsy centers typically exclude white matter contacts when creating intracranial EEG (iEEG) montages to interpret, diagnose, and localize the SOZ. Research studies 21–25 utilizing SEEG will also typically exclude white matter contacts; their signals may prove problematic when creating computational models sensitive to EEG features if white matter signals are functionally unique. Indeed, analyses on interictal recordings of select EEG features have shown differences in white matter (Fig.…”

Synchronizability predicts effective responsive neurostimulation for epilepsy prior to treatment

“…In this study, we take the first step towards translating our work into clinical practice by preparing a framework to support large-scale validation of our biomarker across centers. Finally, we are limited by the variability in electrode coverage between patients 46 , variability in RNS targets, and heterogeneity in the device settings and medication regimens that may have contributed to responder status.…”