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See Inside September / October 2009

How Memories are Maintained Over Time

As recollections age, different brain areas take charge of the upkeep

The brain’s ability to learn and form memories of day-to-day facts and events depends on the hippocampus, a structure deep within the brain. But is the hippocampus still maintaining the memory of, say, the commencement address at your college graduation 20 years ago? The latest evidence suggests that as memories age, the hippocampus’s participation wanes.

In a 2006 study, neuroscientist Larry R. Squire of the University of California, San Diego, and the Veterans Affairs San Diego Healthcare System studied patients who had hippocampal damage. These indi­viduals did not remember details of newsworthy events that occurred in the five to 10 years prior to their injuries, but they did recall older events.

Building on those results, Squire turned to healthy brains. His team questioned 15 people in their 50s and 60s about events in the news over the past 30 years while scanning the participants’ brains with functional MRI. To single out brain activity related to the date of the event, the researchers separately evaluated activity tied to learning and remem­bering the test questions. They also accounted for the richness of participants’ recollections of events, to make sure the degree to which someone was able to recall an event did not influence the data.

Squire’s team reported in January that activity in the hippocampus steadily declined as subjects remembered events that were up to 12 years old. With more remote memories, the structure’s activity leveled off. In contrast, areas in the frontal, temporal and parietal lobes displayed increasing activity for recalled events from those dozen years, then reached a plateau during older remembrances.

The biology behind how the brain makes and keeps memories is not fully understood, Squire notes, but it appears that, initially, a memory resides in the hippocampus and in areas the structure connects to in the neocortex, the outer part of the cerebral cortex. “A time comes when the cortical regions important to a memory are connected [to one another] heavily enough to form a stable representation,” Squire says. “Then the hippocampus isn’t needed to hold the whole thing together.”

Note: This article was originally printed with the title, "Memory Maintenance."

This article was originally published with the title "Memory Maintenance."

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