Tactile stimulation. Coloured positron emission tomography (PET) scan of blood flow in the brain during tactile (touch) stimulation of the right hand. 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. The stimulation leads to increased activity (red & white) in the sensory cortex of the left hemisphere (at left) of the brain and the area leading to it (blue, at centre). The left hemisphere controls the right side of the body. PET scans use radioactively- labelled substances introduced into the blood to view metabolic activity.