For centuries, scientists dreamed of being able to peer into a human brain as it performs various activitiesfor example, while a person is seeing, hearing, smelling, tasting, or touching something.
Now several imaging techniques such as PET (positron emission tomography) and the newer fMRI (functional magnetic resonance imaging) make it possible to observe human brains at work.
The PET scan on the left shows two areas of the brain (red and yellow) that become particularly active when volunteers read words on a video screen: the primary visual cortex and an additional part of the visual system, both in the back of the left hemisphere.
Other brain regions become especially active when subjects hear words through ear-phones, as seen in the PET scan on the right.
To create these images, researchers gave volunteers injections of radioactive water and then placed them, head first, into a doughnut-shaped PET scanner. Since brain activity involves an increase in blood flow, more bloodand radioactive waterstreamed into the areas of the volunteers' brains that were most active while they saw or heard words.
The radiation counts on the PET scanner went up accordingly. This enabled the scientists to build electronic images of brain activity along any desired "slice" of the subjects' brains. The images above were produced by averaging the results of tests on nine different volunteers.
Much excitement surrounds a newer technique, fMRI, that needs no radioactive materials and produces images at a higher resolution than PET.
In this system, a giant magnet surrounds the subject's head. Changes in the direction of the magnetic field induce hydrogen atoms in the brain to emit radio signals. These signals increase when the level of blood oxygen goes up, indicating which parts of the brain are most active.
Since the method is non-invasive, researchers can do hundreds of scans on the same person and obtain very detailed information about a particular brain's activity, as well as its structure. They no longer need to average the results from tests on different subjects, whose brains are as individual as fingerprints.
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