Seeing or hearing words. Colour Positron Emission Tomography (PET) scan showing areas of the human brain activated by words which are seen or heard. The external left-side of the brain is shown. Brain regions which are active are colour-coded red/white. At top, the patient is seeing words that make sense; the visual cortex is activated in the temporal region of the brain. At bottom, the patient is hearing words; the auditory area is activated in the temporal region of the brain. PET scanning involves the injection of a radioactive tracer substance to detect blood flow and areas of high metabolic activity in the brain.