Brain's speech centre. Coloured positron emission tomography (PET) scan of the brain during a speech exercise. The exercise involved reciting synonyms. The PET scan is superimposed onto a black and white three-dimensional magnetic resonance imaging (MRI) scan. The front of the brain is at left. The scan shows the areas of brain activity (colour) associated with speech. These areas are in the speech cortex of the brain's frontal lobe. PET scans use radioactively-labelled substances introduced into the blood to view metabolic activity.