White matter fibres. Coloured 3D diffusion tensor imaging (DTI) scan of the bundles of white matter nerve fibres in the brainstem. The fibres transmit nerve signals between brain regions and between the brain and the spinal cord. The brainstem controls automatic functions, sleep and arousal, and relays messages from the brain to the spinal cord. DTI scans show the diffusion of water along white matter fibres, allowing their orientations and the connections between brain regions to be mapped.