MEG/EEG vs. other functional brain imaging modalities…

MEG and EEG can be measured simultaneously (M/EEG) and reveal complementary properties of the electrical fields. The two techniques have temporal resolutions of about the millisecond, which is the typical granularity of the measurable electrical phenomena that arise in the brain. This high temporal resolution is what makes MEG and EEG attractive for the functional study of the brain. The spatial resolution, on the contrary, is rather poor as only a few hundred simultaneous data positions can be acquired simultaneously (about 300-400 sensors for MEG and up to 256 electrodes for EEG). With appropriate models and methods, localization of activity from MEG and EEG is nevertheless possible.

MEG and EEG are complementary with fMRI, PET and SPECT, which provide a very good spatial resolution but a rather poor temporal one (of the order of a second for fMRI and of a minute for SPECT). Contrarily to fMRI, which “only” measures an haemodynamic response linked to the metabolic demand, MEG and EEG measure a direct consequence of the electrical activity of the brain: it is admitted that the MEG and EEG measure signals corresponding to the variations of the post-synaptic potentials of the pyramidal cells in the cortex. Pyramidal neurons constitute approximately 80% of the neurons of the cortex (the proportion varies with cortical regions), and at least 50,000 such neurons are required to be active simultanously in order to generate some measurable signal.