fMRI: Functional Magnetic Resonance Imaging
This is a research tool for studying the brain "in action". Here's a very simple explanation of the technique. More detail can be found in the links below, although they get a bit thick with technical lingo.
You are probably already familiar with the idea of MRI, the imaging process which uses a large (really large) magnetic field. If not, try this site for more basics.
Functional MRI differs by adding another step to basic MRI imaging: it looks for regions of the brain that are more active than others, or activated by a particular task that the research participant does while lying in the scanner. Fortunately, a good MRI scanner can tell how much oxygen is being used in a particular region, by the difference in the appearance of the blood in that area (when the oxygen has been used up, the blood "looks" different). So in addition to getting a picture of the brain, fMRI also gets a picture of which parts of the brain are using a lot of oxygen.
This oxygen-use information can be projected as a picture, laid on top of the MRI image. The result is a picture of the active areas of the brain. Now, just one more step: using the computer which gathers all of this information, we can ask for a "subtraction". We ask the computer to show the difference between an fMRI image that was taken when a research participant was doing some active task, such as watching a complex image move around on a TV screen, versus the fMRI image taken when that same person was in the scanner watching a black screen.
The difference between these two images reflects what the brain is doing while following a moving target, you see? The only parts that "light up" are those which are active during the moving-target task, compared to the black-screen task. Here is an example of such an fMRI image.
back of the head front of the head
This image comes from the Wikipedia page on fMRI, which offers more detail on the process and the physics. And here's a longer essay with several interesting examples of fMRI use.