Investigation of Physiological and Pathological Brain Activity using EEG-Correlated fMRI

Since it has become possible to acquire EEG und fMRI data simultaneously (EEG-correlated fMRI, EEG/fMRI), this technique has been applied mainly to four fields: 1) mapping of fMRI activation associated with interictal epileptiform discharges (IED) in patients with epilepsy, 2) mapping of fMRI activation associated with physiological EEG activity, like alpha rhythm; 3) monitoring vigilance and sleep stages during fMRI experiments; and 4) simultaneous acquisition of fMRI data and evoked potentials. Here, EEG/fMRI studies of IED and alpha- and beta-rhythm are presented. Several studies have shown that EEG-correlated fMRI is a practicable method to be applied to patients with epilepsy showing frequent IED on scalp EEG. However, the clinical interpretation of the fMRI maps remains difficult; mainly because EEG/fMRI results have not been systematically validated with a gold standard, which are intracranial recordings and outcome after epilepsy surgery. Currently, EEG-correlated fMRI has to be considered as a research tool providing insights to the pathophysiological processes underlying epileptic disorders and to the effects of epileptiform activity on cognition. It remains unclear if the method will also be used for the routine clinical work-up of epilepsy patients in the future. EEG/fMRI studies of alpha- and beta-rhythm in normal controls have shown that spontaneous fluctuations of the band power during rest are associated with fMRI signal changes, possibly representing alterations of attention-modulating networks (retrosplenial and fronto-parietal cortices). Similar fMRI activation patterns are found during generalized and focal IED in epileptic patients and might represent an fMRI signature of the cognitive effects of IED.