The Role of EEG-fMRI in Studying Cognitive Network Alterations in Epilepsy

Abstract

Brain functions do not arise from isolated brain regions, but from interactions in widespread networks necessary for both normal and pathological conditions. These Intrinsic Connectivity Networks (ICNs) support cognitive processes such as language, memory, or executive functions, but can be disrupted by epileptic activity. Simultaneous EEG-fMRI can help explore the hemodynamic changes associated with focal or generalized epileptic discharges, thus providing information about both transient and non-transient impairment of cognitive networks related to spatio-temporal overlap with epileptic activity. In the following review, we discuss the importance of interictal discharges and their impact on cognition in different epilepsy syndromes. We explore the cognitive impact of interictal activity in both animal models and human connectivity networks in order to confirm that this effect could have a possible clinical impact for prescribing medication and characterizing post-surgical outcome. Future work is needed to further investigate electrophysiological changes, such as amplitude/latency of single evoked responses or spontaneous epileptic activity in either scalp or intracranial EEG and determine its relative change in hemodynamic response with subsequent network modifications.

In vivo calcium imaging reveals disordered interictal network dynamics in epileptic stxbp1b zebrafish

“…Random monitoring of electrographic activity in stxbp1b larvae (5–7 days post fertilization [dpf]) using single electrode site LFP recording indicates a relatively low frequency of large amplitude ictal-like epileptiform discharges (1–2 events per 15 min recording in ∼20% of mutants; n = 28). Periods between ictal seizure events are classified as “interictal” and potentially relate to cognitive processing and epileptogenesis ( Fisher et al., 2014 ; Shamshiri et al., 2019 ). In patients with epilepsy, interictal recordings are commonly used for presurgical evaluation and localization of epileptogenic brain regions ( Lascano et al., 2012 ; Stefan et al., 2003 ; Wang et al., 2011 ).…”

Simultaneous EEG-fMRI: What Have We Learned and What Does the Future Hold?

“…Using the time course of these discharges from the EEG can inform the fMRI analysis to reveal which brain regions generate the IEDs and sometimes identify more widespread responses associated with the IED, for example, how they propagate [ 103 , 104 , 105 , 108 ]. For a review, please see Shamshiri [ 109 ]. Figure 8 provides an example of IEDs in EEG data and BOLD responses to these IEDs.…”

Accelerated long‐term forgetting in focal epilepsy: Do interictal spikes during sleep matter?

“…On the other hand, some types of epilepsy are associated with cognitive impairment due to the massive occurrence of interictal spikes during sleep (Landau–Kleffner syndrome, continuous spikes and waves during sleep) 28,44,45 . Interictal discharges have been shown to disturb long‐range brain connectivity 46–48 . The rate of interictal spikes significantly increases during non–rapid eye movement (NREM) sleep, 49,50 mostly in medial temporal regions whether these regions belong to the epileptogenic zone (EZ) or not, 51 thus possibly disrupting sleep‐dependent processes of long‐term memory consolidation.…”

“…28,44,45 Interictal discharges have been shown to disturb long-range brain connectivity. [46][47][48] The rate of interictal spikes significantly increases during non-rapid eye movement (NREM) sleep, 49,50 mostly in medial temporal regions whether these regions belong to the epileptogenic zone (EZ) or not, 51 thus possibly disrupting sleep-dependent processes of long-term memory consolidation.…”