18 December 2009. Researchers often divide their subjects by age in studies of memory, but it looks like they must also divide them by performance, according to a study published online in PNAS this week. Researchers led by Irene Nagel at the Max Planck Institute for Human Development in Berlin, Germany, studied 60 adults, half of them young and half of them old, to determine that performance level can confound studies of memory. This fact was well established for long-term, episodic memory—such as remembering where you grew up—but the new study shows that the same applies to short-term, working memory—such as recalling the words in the sentence you just read.
The work “demonstrates how important it is to control for performance,” said Reisa Sperling of Harvard Medical School, who was not involved in the work. If researchers do not take performance into account in, say, functional MRI studies of brain activity, they “are mixing the idea of what the brain is doing when it attempts a task with what it is doing when it fails a task,” she said. It is not just that failure and success may be different outcomes—but that the execution of the task could also be different.
Nagel and colleagues recruited 30 young (20-30 years old) and 30 old (60-70 years old) subjects to complete a memory task. The participants first saw a computer screen briefly display a pattern of dots. After a short delay, the screen presented a single dot, and subjects had to decide if it was in the same position as in the initial pattern. The program had three levels: easy tasks started out with only one dot to remember, medium tasks had three, and hard tasks presented seven dots to keep track of.
Following the trials, the researchers used their data to stratify participants in each age group into the top, middle, and bottom 10 performers. When they graphed accuracy against the difficulty of the task, they found that the slope of the graph varied not only between young and old, but also between high and low performers in each age group. The work suggests that the brains of high and low performers may work a bit differently.
To explore brain function, the scientists used fMRI to image the blood oxygen level dependant (BOLD) signal as the participants completed their task. They saw differences in the brain regions activated in low and high performers. Among the older group, the low performers had the least BOLD response, and their levels dragged down the group’s average. The pattern of activation in old high performers actually resembled that of the younger participants.
“These findings underscore the need of taking performance level into account when studying changes in functional brain activation patterns from early to late adulthood,” the authors wrote. While the results are not particularly surprising, Sperling said, they may have implications for researchers designing memory experiments in people with mild cognitive impairment or Alzheimer disease. Sperling suggested that there are three methods to control for differences in performance. One is that taken by the current study—to include performance as a separate parameter in the analysis. Another method is to tailor the task to the subjects, so older subjects, or those with cognitive impairment, have an easier puzzle, giving them success rates comparable to other subjects. Third, researchers can selectively choose the data they analyze, for example, by only using data from successful trials. “I think all three are valid,” Sperling said.—Amber Dance.
Nagel IE, Preuschhof C, Li S-C, Nyberg L, Bäckman L, Lindenberger U, Heekeren HR. Performance level modulates adult age differences in brain activation during spatial working memory. PNAS. 2009 Dec.