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Familiar face. As people try to remember which face pairs with a garden scene, the length of their stare (size of green circles) reveals what their brain already knows.

Deborah Hannula

Remembering Without Knowing It

Try this trick at home: Have a friend remove an object from a room you know well--say, a napkin holder from your kitchen--and then see if you can guess what he's taken away. Even if you don't know the answer, your eyes will unconsciously fixate on the stretch of countertop next to the toaster where the holder usually sits. Remembering what goes where in your kitchen is called relational memory, and now scientists have shown that your unknowing stare may be a sign that your brain remembers even when you don't.

What we think of as traditional memories are known as declarative memories. If someone asks us what color shirt we wore yesterday, for example, we say "green." Scientists know that a region of the brain called the hippocampus is responsible for these memories. But they've debated whether this region can also recall unconscious relational memories. Recent studies suggest it does: When people with hippocampal damage were given a test analogous to the napkin-holder theft, their eyes didn't fixate on the region where the object was missing.

To further probe the role of the hippocampus in unconscious relational memories, cognitive neuroscientists Deborah Hannula and Charan Ranganath of the University of California, Davis, utilized functional magnetic resonance imaging, which measures changes in brain activity. They asked volunteers to view 216 photo pairs that showed a person's face and a scene such as the Grand Canyon. Later, the subjects had to pick which of three faces went with a scene they had viewed. While the subjects made their decision, the researchers monitored where the subjects looked.

Hannula and Ranganath found that when subjects' eyes focused on the correct match, the hippocampus and related memory areas in the medial temporal lobe, which the hippocampus is a part of, lit up. Even if subjects picked the wrong face, the hippocampus was still more active when they stared at the correct face. The scientists conclude today in Neuron that the hippocampus may be retrieving the link between face and scene even when the person doesn't seem to remember it.

The researchers also found that regions of the prefrontal cortex (PFC), which is involved in decision-making, lit up more when subjects made correct decisions than when they made incorrect ones. Because the level of PFC activity mirrored hippocampus activity during correct matches, the scientists believe that interactions between the two regions may be necessary to make us aware of connections the hippocampus has recalled. So your hippocampus may have made the connection that the napkin holder is missing, but your PFC must get involved for you to realize it. "The idea is that recollection may be a two-stage process," Hannula says. "First you have retrieval of the memory, and then you have a conscious appreciation of what's been retrieved."

The study provides strong support for the idea that the hippocampus can process relational memories without a person being aware of it, says Boston University neuroscientist Howard Eichenbaum. Neuroscientist Anthony Wagner of Stanford University in Palo Alto, California, agrees; however, he believes that to strengthen the hypothesis, future experiments need to address the possibility that subjects may have a brief inkling of a memory but then ignore it and pick the wrong face. Wagner and Eichenbaum also think that the study supports using eye movements as a way to explore people's memories when they can't describe them explicitly, such as with infants or mentally ill patients.