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Brain scan. Coloured magnetic resonance imaging (MRI) scan of a sagittal (side view) section through a healthy brain superimposed on a head. Conscious brain activity occurs in the folded cerebrum (blue/purple/white), which is the largest part of the brain. The cerebellum (blue, triangular, centre right) controls balance, posture and muscle tone. To its left is the brainstem (purple, centre). This controls automatic processes such as breathing, and joins the brain to the spinal cord. MRI uses radio waves and a powerful magnetic field to create slice images through the body. (KEYSTONE/SCIENCE PHOTO LIBRARY/MARK THOMAS)