Coloured diffusion tensor imaging (DTI) magnetic resonance imaging (MRI) scan showing white matter fibres in the brain. The back of the brain is at bottom left. The structures shown here are the fornix, the inferior longitudinal fibres and the cingulum hippocampal fibres. They form part of the limbic system, a primitive part of the brain involved in emotions, learning and the formation of memories. 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.