Visual cortex brain areas. 3-D magnetic resonance imaging (MRI) scans of the brain mapped to reveal areas of the cerebral visual cortex, which controls vision. This mapping is used in virtual reality assisted surgery. Electric currents are first applied to parts of the visual cortex to supress vision and create a map. The map is shown by colour-coded cylinders: red (vision suppressed to right visual field), blue (vision suppressed to left visual field) and green (no effect). By knowing these regions, surgery such as tumour removal can be done with minimum damage to the visual cortex.