Epileptic networks in action: Synchrony between distant hemodynamic responses
Simultaneous electroencephalography (EEG)–functional magnetic resonance imaging (fMRI) is a technique that measures noninvasively and with high spatial resolution the changes in hemodynamic activity (the blood oxygen level–dependent [BOLD] responses) across the whole brain during epileptic discharges. This technique aids identification of the EZ9 and has the potential to become a clinical test for presurgical workup.11 EEG‐fMRI analyses in patients with epilepsy often show distributed patterns of BOLD response related to IEDs detected on scalp. They show responses not only in the EZ but also in remote regions, and it is often assumed that this response map represents an epileptic network.13 However, most studies of EEG‐fMRI in epilepsy have concentrated on the maximum BOLD responses, probably because of the complexity of analyzing all responses and because the maximum appears the most important, and have not studied the meaning of distributed responses. Furthermore, the BOLD response is a hemodynamic activity that does not directly represent neuronal activity and its neuronal underpinnings are uncertain. Consequently, little is known about the relationship between neuronal activity in these distributed brain regions and the BOLD response.
We want to understand the neuronal basis of the distributed BOLD responses often seen in EEG‐fMRI studies, and in particular whether the neuronal activities of these regions are synchronized and correspond to an epileptic network in which the nodes interact with each other. We sought to examine the electrophysiological underpinnings of distributed IED‐related BOLD responses during interictal periods using synchrony of IED activity recorded by intracerebral EEG (iEEG) in the regions of the brain showing BOLD responses. Our findings provide insights into the neuronal correlates of hemodynamic‐based imaging of epileptic events, especially the basis of the presumed interictal epileptic networks identified by EEG‐fMRI.