Age-related declines in mental function

October 25, 1999

ANN ARBOR---When young adults are asked to remember a list of words, then switch tasks and do a math problem, they use areas in the front of the brain and towards the back of both hemispheres---the visual cortices. But when older adults perform the same pair of concurrent tasks, they are more likely than young adults to use an area on the left front side of the brain---the dorsolateral prefrontal cortex (DLPFC).

That is one of the findings from a University of Michigan study presented Tuesday (Oct. 26) in Miami Beach at the annual meeting of the Society for Neuroscience. The study, funded by the National Institute on Aging, was presented by Anat Geva, a Ph.D. candidate in cognitive and clinical psychology.

"Several studies have shown that older people have more difficulty than younger people at switching tasks," says Geva. "But the underlying reasons for this difficulty have remained unclear. Insofar as our study illuminates the brain mechanisms that are involved in this age-related performance decline, it may allow us, in the long run, to target specific areas of the brain for intervention."

For the study, Geva and colleagues used positron emission tomography (PET) to measure the brain activation of 12 young subjects, with an average age of 23, and 12 older subjects, whose average age was 67. They were tested four times each under four different conditions: a memory task, consisting of remembering the order of a string of unrelated words; a math task, consisting of verifying the truth of a simple equation such as (8 x 5) - 8 = 33; a combined task, consisting of doing math while remembering a list of words; and a control task substituting circles and squares for letters and digits, to see which areas of the brain activate during visual processing without using working memory.

The researchers analyzed the resulting PET images, and also recorded and analyzed subjects' speed and accuracy on the memory task, the math task, and the combined tasks.

In general, they found that older subjects were less accurate and slower than younger subjects on both math and verbal tasks, with a disproportionate impairment on the combined task. For example, while the young subjects were 15 percent less accurate on the dual task than on the memory only task, the older subjects were 21 percent less accurate on the combined tasks, the researchers found.

But Geva and colleagues also analyzed differences in the PET scans of the young subjects who performed best compared with the young who performed worst. "When we divided the young subjects into poor performers and good performers, we found that the neuroimages of poor performers looked very similar to those of seniors," says Geva. "Both showed much more activation in the left DLPFC than the good young performers, who showed much more activation in the visual cortices."

The reason for this difference between good and poor young performers isn't clear, although it could be that good young performers are using more of a visually-based strategy, while poor young performers and older subjects are relying on less efficient strategies which make it more difficult for them to complete the complex combined tasks, according to Geva.

"Even though older and younger subjects in general seem to use different regions of the brain to carry out the same types of tasks, knowing someone's age doesn't tell you everything about their performance and how their brain works," says Geva, whose research interests include the cognitive declines associated with Alzheimer's Disease. "There's something about higher cognitive abilities---such as the functions of working memory that are employed in activities that require task switching---that seems to have a protective function. But it's not clear yet whether the protective effect lies in the fact that you're continuously using the brain or that you have a better brain to begin with."

University of Michigan

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