Mapping for brain surgery. Coloured 3-D magnetic resonance imaging (MRI) scan to map motor cortex areas of the brain before surgery to remove a tumour (green). Blood vessels are red. The motor cortex (under blue diamonds) controls voluntary muscle action. The mapping is used in virtual reality assisted brain surgery. The motor cortex is mapped by coils (red and blue rods, top right) which fire electric pulses into the brain at given points. The map is created by measuring the body's response to the pulses. This enables the surgeon to correctly attack the tumour without causing damage to the motor cortex of the brain.