Memory centers in the brain may interact like kids on a seesaw, new research indicates. When test subjects performed learning tasks that called on two different types of memory, brain imaging revealed that heightened activity in structures responsible for one form of recall accompanied a corresponding reduction of activity in a region that controls the other form. One of these centers was dominant very early in both tasks, suggesting it may play an important role in learning of either kind. A report detailing these findings appears today in the journal Nature.

In one experiment, Russ Poldrack of Massachusetts General Hospital and colleagues had study participants learn to associate cards bearing a set of symbols to a specific weather pattern while they underwent functional magnetic resonance imaging (fMRI). A prompt either told subjects what the relationship was, which required their declarative memory for facts alone, or instructed them to guess the connection first, which invoked their nondeclarative memory for experiences. The first task raised activity in subjects' medial temporal lobes and lowered it in their basal ganglia, whereas the second task had the opposite effect. "The idea that these systems may compete with each other was suggested by animal research," Poldrack says, "and we wanted to see if this takes place in humans as well."

A second experiment looked at participants' brains for a longer period of time as they performed the guessing-first task. Here the researchers found that the medial temporal lobe alone was active at first, but the basal ganglia quickly took over. Scientists have traditionally thought of the hippocampus, which resides in the medial temporal lobe, as important for declarative memory only, says lead investigator Mark Gluck of Rutgers University. This result, he asserts, supports his theory that the hippocampus takes part in all learning and controls how the brain processes new information.