You might not realize it, but each time you're introduced to a friend of a friend, you probably make a subconscious generalization about that person based on memory. Similarly, when you visit a new restaurant owned by a familiar chef, you likely rely on unconscious memory to make a prediction about the experience.

Images of midbrain dopomine region, from top view (left) and side view. Rows B and C show enhanced activity during the memorization stage of the study. Participants with higher activation were more likely to make subsequent generalizations. [Image credit: Daphna Shohamy]
Images of midbrain dopomine region, from top view (left) and side view. Rows B and C show enhanced activity during the memorization stage of the study. Participants with higher activation were more likely to make subsequent generalizations.

Generalizations and predictions don't always involve rational thought processes; in fact, they may entail automatic judgments made possible by linking existing memories—or what one Columbia researcher refers to as memory integration.

Using magnetic imaging technology, a Columbia neuroscientist and her colleague from Stanford University found that memory integration is controlled by the hippocampus, a brain region already known to encode and retrieve memories, and the midbrain dopamine region, which is commonly associated with reward and unexpected outcomes. The two have published a study based on their findings that appeared in the Oct. 23 issue of the journal Neuron.

"Memories don't exist in isolation; they are constantly getting linked, and they affect the way we make judgments," said Daphna Shohamy, an assistant professor of psychology at Columbia, who co-authored the study with Stanford neuroscientist Anthony Wagner.

With the newfound knowledge that the hippocampus interacts with the midbrain dopamine region, doctors might one day treat neurological diseases associated with one of the brain areas by manipulating the other. Alzheimer's disease involves neural damage in the hippocampus, and Parkinson's disease involves neural damage to the dopamine system.

In addition, psychologists and educators can explore the interaction between the two brain regions to develop strategies for integrating memories. This, in turn, could help people make generalizations in day-to-day situations, which can assist with decision-making.

"People's ability to generalize is strongly correlated with their level of hippocampal and midbrain-dopamine-region activity during the memory-encoding process," said Shohamy. "This has never been shown before." The study also suggested that the hippocampus and the midbrain dopamine region cooperate with each other as memories are encoded. "It seems that for generalization to take place, the two regions must talk to each other," said Shohamy.

The researchers studied 24 participants in a Stanford MRI laboratory. The participants viewed a series of images while their brains were monitored by a scanner. Each image portrayed a human face and two outdoor scenes, such as a beach and a mountain. As participants viewed each image, they were asked to guess which scene each face "prefers." For example, "Joe" might prefer the beach over the mountain, and "Jane" might prefer the sunshine over the snow. After each guess, participants were told by a computer whether they were right or wrong.

After being repeatedly exposed to the faces, participants began to memorize the preferences of each face. Toward the end of the experiment, most participants made predictions about one face's unknown preference based on another face's known preference. Example: Joe and Sally each prefer the beach over the mountain, and Joe prefers the lake over the forest. Therefore, Sally prefers the lake over the forest, even though the computer never said so.

"We see from the imaging data that the brain has the capability to link separate memories, which gives people the ability to make generalizations," said Shohamy. "The next step is determining why some people have the ability to use memory integration to generalize and others don't. Is it genetic? Can it be carried over the long-term? These are the questions we're beginning to work on now."

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