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People lose their ability to memorize and recognize new things as they grow older, and researchers say this appears to be due to an age-related decrease in activity in certain areas of the brain.

Tests measuring blood flow to areas of the brain while people perform memory tasks indicate that younger people exhibit more activity in different areas of the brain when encoding new information than do elderly people.These results suggest that age-related memory impairment may result from a failure of older brains to register stimuli adequately, indicating that older people may require longer or repeated exposure to new things to memorize them and recall them, researchers said.

Scientists with the National Institute on Aging and the National Institute of Mental Health said tests involving memorizing new faces showed that younger subjects, who were 23 to 27, had much more activity in the hippocampus and adjacent structures of the brain when viewing new things than did subjects who were 64 to 76.

The hippocampal region of the brain is important for memory, experts say. A prominent theory holds that the hippocampus helps in the storage of new memories by collecting and organizing activities from various other regions of the brain, like the cerebral cortex, so that memories can be recalled more easily.

In a report in the current issue of the journal Science, the researchers said they monitored increased blood flow to parts of the brain during memory exercises as an indication of increased activity in those areas. These brain scans, using a technique called positron emission tomography, showed blood flow in the brain as subjects memorized and matched pictures of unfamiliar faces.

"Our results suggest that it is the learning phase that is a major memory problem for the elderly," Dr. Cheryl L. Grady of the aging institute, the lead researcher, said in an interview. She said older people did not take in and process adequate amounts of information on new things that they saw and therefore could not recall enough detail for good recognition later.

She likened this type of memory problem to an underexposed image on film, which lacks details. The image would be improved if the film had a longer exposure to light, and an older person might require more exposure to the subject to remember it than would a younger person.

"Older people have a harder time storing new memories than do younger people," Grady said. "What we have with our research is the first indication of what the neurophysiological problem with memory might be in elderly individuals."

The researchers showed two groups of research subjects, divided into young and old age groups, pictures of 32 faces taken from a yearbook. For the memory test, the subjects saw each face for four seconds. The researchers found that older people had less ability to memorize the faces and to recognize them later when they were mixed with "distractor" faces of people whose pictures were not previously seen.

During this test, blood flow increased in the hippocampus and several other areas of the brain in the younger subjects, the report said, but brain scans of the elderly participants did not show similar increases.

In contrast, on perception tests that involved matching up two pictures that were the same from a group of three, the two age groups performed the same, the researchers said. The researchers said this suggested that the brains of older people were somehow compensating to maintain perception skills, but not using a similar mechanism for memory.