The findings are reported in the on-line edition of the Proceedings of the National Academy of Sciences.
"When you need to remember people you've just met at a meeting, the brain probably doesn't memorize each person's facial features to help you identify them later," says Sam Deadwyler, Ph.D., a Wake Forest neuroscientist and study investigator. "Instead, it records vital information, such as their hairstyle, height, or age, all classifications that we are familiar with from meeting people in general. Our research suggests how the brain might do this, which could lead to ways to improve memory in humans."
The researchers found that when monkeys were taught to remember computer clip art pictures, their brains reduced the level of detail by sorting the pictures into categories for recall, such as images that contained "people," "buildings," "flowers," and "animals." The categorizing cells were found in the hippocampus, an area of the brain that processes sensory information into memory. It is essential for remembering all things including facts, places, or people, and is severely affected in Alzheimer's disease.
"One of the intriguing questions is how information is processed by the hippocampus to retain and retrieve memories," said Robert Hampson, Ph.D., co-investigator. "The identification of these cells in monkeys provides evidence that information can be remembered more effectively by separating it into categories. It is likely that humans use a similar process."
The researchers measured individual cell activity in the hippocampus while the monkeys performed a video-game-like memory task. Each monkey was shown one clip art picture, and after a delay of one to 30 seconds, picked the original out of two to six different images to get a juice reward.
By recording cell activity during hundreds of these trials in which the pictures were all different, the researchers noticed that certain cells were more active when the pictures contained similar features, such as images of people - but not other objects. They found that different cells coded images that fit different categories.
"Unlike other cells in the brain that are devoted to recording simply an object's shape, color or brightness, the category cells grouped images based on common features - a strategy to improve memory," said Terry Stanford, Ph.D., study investigator. "For example, the same cell responded to both tulips and daisies because they are both flowers."
The researchers found, however, that different monkeys classified the same pictures differently. For example, with a picture of a man in a blue coat, some monkeys placed the image in the "people" category, while others appeared to encode the image based on features that were not related to people such as "blue objects" or "types of coats."
While such categorization is a highly efficient memory process, it may also have a downside, said the researchers.
"The over generalization of a category could result in errors," said Deadwyler. "For example, when the trials included more than one picture with people in it, instead of different images, the monkeys often confused the image with a picture of other people." The researchers said that learning more about how the brain remembers could have far reaching benefits.
"If we can understand in advance how the brain works when decisions are made, we can predict when the brain will make a mistake, and correct it," said Tim Pons, Ph.D., an expert in monkey research and team member. "This finding about how large amounts of information are processed by the brain will help us to ultimately achieve that goal."
"This discovery by the Wake Forest team could be the solution to a big puzzle," said Mortimer Mishkin, Chief of the Section on Cognitive Neuroscience, National Institutes of Mental Health. "Recollection -- bringing back to mind a past event -- depends critically on the hippocampus, but we haven't known how this works. The team's new findings suggest that hippocampal 'category ' neurons are some of the ones that help remind us of things we experienced before."
The research was funded by the National Institute on Drug Abuse, the National Institute of Mental Health and the Defense Advanced Research Projects Agency.