Activation of the brain is measured by electrophysiological techniques using an electrical or magnetic field that the nervous system produces. These types of measures include may include magneto-encephalography (MEG) or electroencephalography (EEG). Brain scans, such as MRI (magnetic resonance imaging), fMRI (functional magnetic resonance imaging), positron emission tomography (PET) and computed axial tomography (CAT or CT), are used in investigations of the function and structure of the brain.