Getting the Gist
But what exactly does the shift in activity mean? Is the diminishing brain simply throwing out all lifelines, casting about desperately for assistance from any quarter? Or is the brain adroitly rewiring itself, adapting to one region's diminishment by arranging for other, stronger regions to take over certain functions?
Ten years ago, Roberto Cabeza, a neuroscientist at Duke University who advised Dennis when she was a postdoctoral fellow there, published an influential paper advancing the compensatory view. Imaging the brains of young adults, older adults who did well on memory tests, and older adults who did poorly on the tests, Cabeza showed that the first and last groups were using regions on only one side of the brain to complete the tasks, while the middle group -- what he called "high-performing" older adults -- were calling on both sides of the brain. The high-performers' brains, he concluded, had successfully adapted to age-related neural decline.
The shift in activity, Dennis says, reflects a successful change in cognitive strategy, as older adults tend to rely more on getting the gist of an experience than younger adults do, and less on grasping the details. But adaptation has its limits, she notes. The reliance on familiarity runs into trouble where two or more experiences are basically similar. "When memories share a common gist they can easily get confused," she says. This confusion can lead to what's known as false memory.
"When we think of memory problems, we tend to think only of forgetting," Dennis explains. "False memory is remembering things that never happened. You could swear you took the dog out last night, or swallowed this morning's pill, but you didn't."
In this case, the brain tends to fill in the specifics based on familiarity with past events. In one current study, Dennis notes, she shows photos of typical domestic scenes -- kitchens and bathrooms -- then asks subjects to recall their details. Was there a sink in that kitchen? "People tend to answer yes," she says, even if there is no sink present, "because their mind knows a kitchen is supposed to have a sink. It tries to fill in the blanks."
This tendency can have more serious implications in matters such as eyewitness identification. "There's a lot of research that shows that in the absence of the actual person in a line-up, people tend to endorse the person that most closely resembles the actual person," Dennis says. "They're relying on gist. The problem is that now you have a true memory of the wrong person, and your mind tends to build on that."
Here again, brain scans can offer insight, Dennis says. In younger adults monitored by fMRI, there is very little change in terms of brain activity between false memory and true memory. "Aside from a few select regions, such as the hippocampus, neurally, you almost can't tell the difference," she says. In older adults, however, the brain regions activated for false memories are different than those recruited for true memories, suggesting a different cognitive process.
Trying to Forget
One key to help in accurate remembering is the ability to forget what's inessential. Intentional forgetting, Dennis says, is an exercise of cognitive control. "It can be a very beneficial strategy. You forget where you parked your car yesterday so you can remember where you parked it today." This ability to tune out irrelevant information has also been shown to decline with age.
Even when older adults can intentionally forget, Dennis finds, the brain regions responsible are not the same ones that younger adults employ. "On the scans you can see that younger adults are using the regions associated with inhibition and suppression, while older adults are using those associated with attention and focus," she says. "It may be that older adults are switching their attention away while younger adults are suppressing the encoding of information."
A certain amount of age-related memory loss is to be expected, she says. In the case of normal decline, however, behavioral strategies may help. Dennis hopes soon to start work with a colleague at another university who is trying out different approaches to help older adults with association memory. Her role, she says, will be to assess the corresponding neural activity, helping to determine which strategies are most beneficial and why.
"Different strategies may work better for different tasks," she says. "But whether memory is stable or shows decline, there are strategies we can use to improve on performance."
That comes as welcome news. After all, you can't always rely on your kids to remember.
Nancy Dennis is an assistant professor of psychology. She can be reached at email@example.com. Her research on memory and aging is funded by the National Science Foundation and the American Federation for Aging Research, and conducted in the Social, Life, and Engineering Sciences Imaging Center on the University Park campus.