Watching the remembering brain at work.

Neuroscientist taking pictures of the human brain in action have confirmed for the first time that people use different brain areas to perform different types of memory tasks. They have also uncovered the first evidence in living brains that the hippocampus -- a banana-shaped region deep within the brain -- plays a key role in memory.

Last week, Larry R. Squire of the University of California, San Diego, and Marcus Raichle of Washington University Medical Center in St. Louis reported their observations of 18 volunteers performing word-completion tasks. Using an imaging technique called positron emission tomography (PET), Squire and Raichle found that the study participants used different areas of their brains to provide the endings of word fragments briefly flashed before them by the researchers. The brain areas used varied according to whether the volunteers were asked to provide the first word that came to mind, or to remember a word from a list they had scanned previously.

During the experiment, the researchers used PET scans to monitor changes in blood flow in the volunteer's brains. Areas of increased flow reveated the brain regions used during the various tasks.

When subjects drew upon their memories of previous lists to complete the fragment "mot-" as "motor," for example, the right sides of their hippocampi flooded with blood. This indicated that each subject used nerve cells there to remember the word, even though researchers usually attribute such verbal processing to the left brain. If the subjects were not searching their brains for a word they had already seen and instead gave the first word that came to them, blood flow did not increase significantly to either side of their hippocampi.

Interestingly, participants sometimes spontaneously recalled words from the lists even if they did not remember having seen the words before. During this phenomenon, which psychologists term "priming," PET scans revealed that the volunteers primarily used a brain area called the visual cortex.

"We are finding [in the living ] that there is more than one kind of memory, and that separate neural regions are involved in each," Squire concludes.