Fundamentals of electroencefalography, magnetoencefalography, and functional magnetic resonance imaging

This review introduces readers to fundamentals of electroencephalography (EEG), magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI). EEG and MEG signals are mainly produced by postsynaptic ionic currents of synchronically active pyramidal cortical neurons. These signals reflect the integrative information processing of neurons representing the output of cortical neural modules. EEG and MEG signals have a high temporal resolution (<1 ms) ideal to investigate an emerging propriety of brain physiology, namely the brain rhythms. A background spontaneous oscillatory activity of brain neurons at about 10 Hz generates dominant alpha rhythms of resting-state EEG and MEG activity. This background activity is blocked during sensory and cognitive-motor events. Standard EEG shows a low spatial resolution (5-9 cm), which partially improves by high-resolution EEG including 64-128 channels and source estimation techniques (1-3 cm); source estimation of MEG data shows a better spatial resolution (0.5-2 cm). fMRI is an indirect measurement of regional brain activity based on the ratio between deoxyhemoglobin and oxyhemoglobin blood (BOLD) during events referenced to baseline conditions. Event-related BOLD response has low temporal resolution (>1 s) and quite high spatial resolution (<1 cm), and is especially suitable to investigate spatial details of both cortical and subcortical activation.