Combining EEG and fMRI: A multimodal tool for epilepsy research

Abstract

Patients with epilepsy often present in their electroencephalogram (EEG) short electrical potentials (spikes or spikewave bursts) that are not accompanied by clinical manifestations but are of important diagnostic significance. They result from a population of abnormally hyperactive and hypersynchronous neurons. It is not easy to determine the location of the cerebral generators and the other brain regions that may be involved as a result of this abnormal activity. The possibility to combine EEG recording with functional MRI (fMRI) scanning opens the opportunity to uncover the regions of the brain showing changes in the fMRI signal in response to epileptic spikes seen in the EEG. These regions are presumably involved in the abnormal neuronal activity at the origin of epileptic discharges. This paper reviews the methodology involved in performing such studies, particularly the challenge of recording a good quality EEG inside the MR scanner while scanning is taking place, and the methods required for the statistical analysis of the combined EEG and fMRI time series. We review the results obtained in patients with different types of epileptic disorders and discuss the difficult theoretical problems raised by the interpretation of an increase (activation) and decrease (deactivation) in blood oxygen level dependent (BOLD) signal, both frequently seen in response to spikes.

Feasibility of simultaneous intracranial EEG-fMRI in humans: A safety study

“…These limitations could potentially be mitigated if icEEG recording were used in combination with functional MRI (fMRI). Indeed, despite its own limitations scalp EEG has previously been successfully combined with fMRI in study patients with epilepsy, providing unique new information on the haemodynamic correlates of paroxysmal discharges in a good proportion of cases studied to date [4][5][6][7][8]. Importantly, these include brain areas thought to be primarily responsible for the generation of interictal and ictal epileptic discharges visible on scalp EEG recordings [5,7,9,10].…”

“…Indeed, despite its own limitations scalp EEG has previously been successfully combined with fMRI in study patients with epilepsy, providing unique new information on the haemodynamic correlates of paroxysmal discharges in a good proportion of cases studied to date [4][5][6][7][8]. Importantly, these include brain areas thought to be primarily responsible for the generation of interictal and ictal epileptic discharges visible on scalp EEG recordings [5,7,9,10]. However, EEG-fMRI studies in epilepsy have raised a number of important questions, for example; 1) is normal neurovascular coupling maintained, as in cognitive fMRI experiments [12][13][14][15][16]?…”

Effects of fluctuating physiological rhythms during prolonged EEG-fMRI studies

“…With simultaneous EEG recording, BOLD correlates of EEG events can also be studied, such as epileptiform events (Gotman et al, 2006) or evoked potential (Horovitz et al, 2002;Bénar et al, 2007). In the same way, EEG-fMRI can be used to analyze the BOLD effect of each type of EEG oscillation.…”

Variability of the hemodynamic response as a function of age and frequency of epileptic discharge in children with epilepsy

“…Using the interictal spikes detected on scalp EEG as events, we can identify hemodynamic changes in the whole brain. Several studies demonstrated that this method provides valuable information about the generators of interictal spikes (Gotman et al, 2006), i.e. the irritative zone (Rosenow and Lueders, 2001).…”

“…The effect of these two variables on the shape of the HRF is unknown. However, there is evidence that studies with larger numbers of spikes are more likely to result in BOLD responses than those with small numbers (Gotman et al, 2006). On the other hand, the general linear model (GLM) used for statistical analyses in EEG-fMRI may fail or be inaccurate with high event numbers, as this model postulates that the response of two closely spaced events will sum linearly (Friston et al, 1998).…”