Sunday, November 10, 2002
DURHAM, N.C. -- Elderly adults who perform as well as younger adults on certain cognitive tests appear to enlist the otherwise underused left half of the prefrontal cortex of their brain in order to maintain performance, Duke University neuroscientists have found. In contrast, elderly people who are not "high performers" on the tests resemble younger adults in showing a preferred usage of the right side of the prefrontal cortex.
The researchers said that, although their finding is basic, it raises the potential of using either training or drugs to enhance cognitive function in the elderly by increasing "recruitment" of the left prefrontal cortex.
The evidence for compensatory brain activity in the high-performing elderly was reported in the November 1, 2002, issue of NeuroImage, by a research team led by Roberto Cabeza of Duke's Center for Cognitive Neuroscience. Other co-authors are Nicole Anderson, Jill Locantore and Anthony McIntosh of the University of Toronto.
In their study, Cabeza and his colleagues sought to better understand a phenomenon seen in previous studies, in which older people tend to show a reduced "hemispheric asymmetry" in activity of the two sides of the prefrontal cortex with aging. The prefrontal cortex, the brain region just behind the forehead, is where higher-level cognitive processing takes place.
Past research has shown that in young people certain cognitive tasks are carried out by the right frontal cortex, but older people tend to lose that asymmetry of processing. They tend to show more bilateral activity of the two brain hemispheres during such tasks.
Neuroscientists have proposed two divergent theories to explain this change with age. According to the "dedifferentiation" theory, age-related reduction in asymmetry occurs because older adults become less able to recruit specialized neural mechanisms. In contrast, the "compensation" theory holds that the asymmetry reduction occurs largely because the elderly compensate for reduced capacity in one hemisphere by activating the other hemisphere.
"Basically, the dedifferentiation theory holds that this bilateral activation reflects a detrimental process in aging, like other forms of age-related decline, such as atrophy or cell loss," said Cabeza, who is an assistant professor of psychological and brain sciences. "In contrast, according to the compensation view, these contralateral activations could help performance, like recruiting additional brain tissue in order to support cognitive processes," he said.
In their study, the scientists identified two groups of healthy older adults, in their 60s and 70s, who performed at either high or low levels on a range of cognitive tests. The scientists then administered two of the tests to the two older groups, as well as to a group of young adults aged 20-35, as their brains were being scanned using positron-emission tomography (PET).
PET scans display brain activity by measuring the radioactivity in brain tissues from small amounts of injected glucose tagged with a tracer. Since glucose is the energy source for brain cells, the concentration of glucose reflects brain activity in a region.
The cognitive tests that the researchers gave the subjects were -- A recall test, in which they were asked to remember the second of pairs of words memorized before the test; and -- A source recognition test, in which they were asked to recall whether words presented earlier were heard or read.
Cabeza and his colleagues chose the two kinds of tests because of the tests' ability to distinguish the subjects' usage of one side of the prefrontal cortex from the other.
"In previous studies, the source memory task has been associated in young adults with greater activation of the right prefrontal cortex, compared to recall memory," said Cabeza. "So, because this task was lateralized in the young, it was a good paradigm to investigate the reduction in asymmetry in older adults."
Analyzing the results of their tests, the scientists found that the high-performing older adults showed significantly more bilateral activity in the prefrontal cortex than either the low-performing older adults or the young adults. Both the low-performing older adults and young adults showed similar preferential activity in the right prefrontal cortex.
Thus, said Cabeza, "Our results are more consistent with the compensation view then with the dedifferentiation view."
The findings suggest the possibility of methods to maintain cognitive abilities in the elderly, he said. "Although we are still far from practical applications, if we can understand the differences between those elderly who show little or no cognitive decline, versus those who are clearly impaired, then we could try to find methods to help control age-related memory decline," said Cabeza. Such methods will depend on the origin of the compensation, he said.
"One possibility is that this bilateral recruitment reflects the use of alternative cognitive strategies," said Cabeza. "In that case, the goal would be to understand those strategies and try to use them to develop training regimes for seniors to enhance cognitive abilities.
"However, these changes might also reflect more of a neurobiological mechanism, rather than a cognitive change," he said. "If so, we need to understand how to facilitate this contralateral recruitment, perhaps by using drugs."
Further studies will explore whether older adults show such bilateral brain activity during other kinds of cognitive tasks, such as attention, said Cabeza. Also, he said, further research should explore how compensatory changes affect the course of such dementias as Alzheimer's disease. Training or drugs to enhance compensation might alleviate some of the cognitive impairment of such diseases, he said.