Study Probes Memory's Mechanics

You remember his face, but can’t remember where you saw him. However, he not only remembers your face, but the details of your first encounter.

With a new insight into the way memories form in the brain, a team of researchers from Harvard and MIT may be able to explain how such embarrassing situations arise.

In a study to be published in the Feb. 18 issue of Proceedings of the National Academies of Sciences, the researchers identify the specific areas of the brain thought to be responsible for different modes of memory formation and recollection.

“We can now see in what way certain parts of brain tissue are important for different types of memory,” said Jason Mitchell, a graduate student in psychology at Harvard Medical School who worked on the study. “It is the interface of psychology and the brain.”

Mitchell, who is also a resident tutor in Cabot House, worked with Lila Davachi, a post-doctoral fellow in brain and cognitive science at MIT.

“This is research into the kind of processes that occur every day,” Davachi said.

Using functional magnetic resonance imaging (fMRI), which employs large magnets to make images of the brain, researchers could track the flow of blood to different parts of the brain during memory formation and recall.

Subjects were asked to identify 400 adjectives by either reading them backwards or creating imaginary scenes they could associate with the words.

The following day, the subjects were asked to recall the context in which they learned or saw the word.

The researchers found that two different regions of the brain are responsible for what they call “item memory” (the recognition of a face, for instance) and “recollective memory” (the association of the image with events).

“It appears that the hippocampus, the area of the brain responsible for long-term memory, binds together pieces of experience into a unified memory trace,” said Anthony Wagner, a professor of cognitive neuroscience at MIT who was the principle investigator of the study. “Later, when you get one bit of information, your brain retrieves all the details.”

The tissue beneath the hippocampus, an area called the perirhinal cortex, appears to create and recall memories based on images rather than experiences.

Davachi explained that this discovery provided insight into the field of amnesia study.

“This helps explained why amnesiacs, who usually have damage to the hippocampus region, have deficits in recollection as opposed to item familiarity,” she said.

—Staff writer Rebecca D. O’Brien can be reached at robrien@fas.harvard.edu.