EEG

Technique for recording electrical activity in the brain, whose cells emit distinct patterns of rhythmic electrical impulses. Pairs of electrodes on the scalp transmit signals to an electroencephalograph, which records them as peaks and troughs on a tracing called an electroencephalogram (EEG). Different wave patterns on the EEG are associated with normal and abnormal waking and sleeping states. They help diagnose conditions such as tumours, infections, and epilepsy. The electroencephalograph was invented in the 1920s by Hans Berger (1873–1941).

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Principle

Scalp EEG reflects the brain’s electrical activity, and in particular post-synaptic potentials (see Inhibitory postsynaptic current and Excitatory postsynaptic potential) in the cerebral cortex, whereas fMRI is capable of detecting haemodynamic changes throughout the brain through the BOLD effect. EEG-fMRI therefore enables the direct correlation of these two important measures of brain activity.

Methodology

The simultaneous acquisition of EEG and fMRI data of sufficient quality requires solutions to problems linked to potential health risks (due to currents induced by the MR image forming process in the circuits created by the subject and EEG recording system) and EEG and fMRI data quality. There are two degrees of integration of the data acquisition, reflecting technical limitations associated with the interference between the EEG and MR instruments. These are: interleaved acquisitions, in which each acquisition modality is interrupted in turn (periodically) to allow data of adequate quality to be recorded by the other modality; continuous acquisitions, in which both modalities are able to record data of adequate quality continuously. The latter can be achieved using real-time or post-processing EEG artifact reduction software. EEG was first recorded in an MR environment around 1993. The first continuous EEG-fMRI experiment was performed in 2000.

Applications

In principle, the technique combines the EEG’s well documented ability to characterise certain brain states with high temporal resolution and to reveal pathological patterns, with fMRI’s (more recently discovered and less well understood) ability to image blood dynamics through the entire brain with high spatial resolution. Up to now, EEG-fMRI has been mainly seen as an fMRI technique in which the synchronously acquired EEG is used to characterise brain activity (‘brain state’) across time allowing to map (through statistical parametric mapping, for example) the associated haemodynamic changes.

The initial motivation for EEG-fMRI was in the field of research into epilepsy, and in particular the study of interictal epileptiform discharges (IED, or interictal spikes), and their generators, and of seizures. IED are unpredictable and sub-clinical events in patients with epilepsy that can only be observed using EEG (or MEG). Therefore recording EEG during fMRI acquisition allows to study their haemodynamic correlates. The method can reveal haemodynamic changes linked to IED and seizures, and has proven a powerful scientific tool; the clinical value of these findings is the subject of ongoing investigations. Outside the field of epilepsy, EEG-fMRI has been used to study event-related (triggered by external stimuli) brain responses and provided important new insights into baseline brain activity in during resting wakefulness and sleep.