Brain viewing images. Coloured positron emission tomography (PET) scan of the area of the human brain active while seeing words or pictures. The left side of the brain is seen, with the front at left. Active parts of the cerebral hemisphere are red/orange. The visual stimuli have activated the visual area in the occipital cortex, part of the brain's cerebrum. The cerebrum also carries out conscious thought processes. The scan involved injecting the radioactive isotope oxygen-15 into the bloodstream to allow areas of the brain with high metabolic activity to be seen.