Memory processing. Coloured positron emission tomography (PET) scan of the brain during memory processing. The PET scan has been superimposed onto a grey magnetic resonance imaging (MRI) scan. In this axial (horizontal) section the front of the brain is at top, with ventricles (cavities, black) at centre. The scan shows brain activity associated with memory processing occurring in three areas (red & white) of the prefrontal cortex. PET scans use radioactively-labelled substances introduced into the blood to view metabolic activity.