The human brain creates memories by linking complex details into manageable chunks. It remembers a set of words as an acronym, and recognizes someone by the face as a whole instead of a separate set of eyes, nose and mouth.
UC Davis researchers showed in a recent study that an area of the brain called the perirhinal cortex (PRc) is involved in forming these simple associations so they will be familiar to us in the future. The finding may resolve a major debate among researchers about how brain areas important for memory are organized.
“One [side] says the PRc and a brain area called the hippocampus are a large system that … links [units of a memory together],” said lead study author Charan Ranganath, a professor at the UC Davis Center for Neuroscience and the department of psychology. “To remember where you left your keys, for example, you need to link them to a particular place and time.”
“The other [side] says the hippocampus is really important for gluing these units of information together…but the PRc just encodes these units and [determines] how familiar they are,” he said.
Resolution of this inconsistency has implications for developing strategies to improve the memory of patients with hippocampus damage by utilizing other brain areas, said first author Logan Haskins, a psychology graduate student, in an e-mail interview.
In the study, volunteers viewed pairs of unrelated nouns while their brain activity was scanned using functional magnetic resonance imaging. For each pair, they were prompted to rank either how well the two words fit into a sentence or how well a definition fit a combination of the two words – for example, “slopebread,” was defined as a pastry for mountain climbers. After a half-hour break outside the scanner, researchers gauged how confidently the volunteers remembered seeing the word pairs.
“The idea is when you process the pair as a sentence, you treat it as two different words…. The only way to remember them together is if you recollect [the sentence],” Ranganath said. “If you think of these [words] as a single unit, your memory performance goes up…. You can tell that you’ve seen this pairing because it’s more familiar to you.”
Imaging showed that brain activity in the PRc increased when volunteers encountered word pairs as a single compound word. The stronger the activity, the more successfully they recognized the word pairs later on.
The results suggest that the PRc can unitize, or process bits of associated information as a single unit, to help in future recognition, Ranganath said.
“We have observed these behavioral effects even after a 48-hour delay [between study and test phases]. So they do seem quite long-term,” said co-author Andrew Yonelinas, a professor of psychology and associate director of the Center for Mind and Brain.
Normal healthy adults were tested in the study, but the question remains whether changes in the brain that occur with aging could affect the ability to unitize information.
Age-related decline in a brain area called the prefrontal cortex impairs the mind’s ability to focus attention for learning and remembering new associations, Haskins said. So the elderly might be less likely to employ the PRc in this task and to benefit from unitizing.
The study suggests that unitized information processed by the PRc can be rapidly recognized and used as a cue to recall specific details about the association that are stored in the hippocampus, Haskins said.
For example, unitizing could be a useful study strategy for recognizing a link between a question and an answer choice during a multiple-choice test, Haskins said. But the strategy alone may not be as helpful for recalling details to answer an essay exam question.
The study was published in the Aug. 28 issue of the journal Neuron.
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