Coloured diffusion tensor imaging (DTI) magnetic resonance imaging (MRI) scan showing the white matter fibres of the brain's visual pathways. The eyes (grey spheres) and optic nerves (grey tubes) are at right and centre right respectively. The optic nerves cross at the chiasma and impulses from the left side of each eye are sent to the left of the brain and impulses from the right sides of the eyes to the right of the brain. The impulses are processed in the visual cortex at the rear of the brain. Diffusion tensor imaging measures the direction of water diffusion, which in the brain reveals the orientation of nerve fibres. Red fibres have a left to right orientation, green a front to back orientation and blue an up and down orientation.