In the 1970's the CAT scan (or CT scan), the first of completely new kinds of tools for studying the brain (or other body structures), was introduced. These methods are called non-invasive because they can visualize (~show) the inside of the body without opening it up. Therefore, they can be used on normal people as well as on people with brain damage. The CAT scan sweeps a beam of X-rays around the head (or other body structure). Electronic X-ray detectors send their signals to a computer programmed to convert the signals into an X-ray picture of a slice through the brain. These pictures show the grey matter, the white matter, and the fluid-filled ventricles in the middle of the brain, which lets you see the size and shape of many brain structures. The figure at the right shows a CAT scan as a horizontal slice through the brain. (The top of the slice is the front of the brain.)

More recently, PET and fMRI scanning methods have been developed to show brain function non- invasively. They measure blood flow in the brain, which changes very quickly as activity in an area of the brain increases or decreases. For example, many studies using these methods have showed that different parts of the brain are activated during different psychological actions. The figure at the left shows PET scans as horizontal slices through the cerebral hemispheres (top of each slice is the front of the brain) taken under different levels of visual stimulation. Note the increasing dark red areas (which stands for increasing activity) at the back of the brain where the primary visual areas are located.

To go to an explanation of how NMR works, click HERE. To go to an explanation of how PET works, click HERE. (Some background in physics is very helpful.)

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