Some brain systems become less coordinated with age even in the absence of Alzheimer's disease, according to a new study from Harvard University. The results help to explain why advanced age is often accompanied by a loss of mental agility, even in an otherwise healthy individual.
The study was led by Jessica Andrews-Hanna, a doctoral candidate in the Department of Psychology in the Faculty of Arts and Sciences at Harvard.
"This research helps us to understand how and why our minds change as we get older, and why some individuals remain sharp into their 90s, while others' mental abilities decline as they age," says Andrews-Hanna. "One of the reasons for loss of mental ability may be that these systems in the brain are no longer in sync with one another."
Previous studies have focused on the specific structures and functions within the brain, and how their deterioration might lead to decreased cognitive abilities. However, this study examined the way that large-scale brain systems that support higher-level cognition correlate and communicate across the brain, and found that in older adults these systems are not in sync. In particular, widely separated systems from the front to the back of the brain were less correlated.
The human brain can be divided into major functional regions, each responsible for different kinds of "applications," such as memory, sensory input and processing, executive function or even one's own internal musing. The functional regions of the brain are linked by a network of white matter conduits. These communication channels help the brain coordinate and share information from the brain's different regions. White matter is the tissue through which messages pass from different regions of the brain.
Scientists have known that white matter degrades with age, but they did not understand how that decline contributes to the degradation of the large-scale systems that govern cognition.
"The crosstalk between the different parts of the brain is like a conference call," said Jessica Andrews-Hanna, a graduate student in Buckner's lab and the lead author of the study. "We were eavesdropping on this crosstalk and we looked at how activity in one region of the brain correlates with another."
The researchers studied 55 older adults, approximately age 60 and over, and 38 younger adults, approximately age 35 and younger. They used a neuroimaging technique called fMRI to obtain a picture of activity in the brain. The results showed that among the younger people, brain systems were largely in sync with one another, while this was not the case with the older individuals.
Among the older individuals, some of the subjects' brains systems were correlated, and older individuals that performed better on psychometric tests were more likely to have brain systems that were in sync. These psychometric tests, administered in addition to the fMRI scanning, measured memory ability, processing speed and executive function.
Among older individuals whose brain systems did not correlate, all of the systems were not affected in the same way. Different systems process different kinds of information, including the attention system, used to pay attention, and the default system, used when the mind is wandering. The default system was most severely disrupted with age. Some systems do remain intact; for example, the visual system was very well preserved. The study also showed that the white matter of the brain, which connects the different regions of the brain, begins to lose integrity with age.
One of the challenges to studying the aging brain is that the early signs of Alzheimer's disease are very subtle, and it is difficult to distinguish between the early stages of Alzheimer's disease and normal aging. In order to ensure that the researchers were only looking at healthy aging brains, the researchers used a PET scanning process to identify the presence of amyloid, a chemical present in individuals with Alzheimer's. When the presence of this chemical was detected, individuals were not included in the study. In this way, the researchers ensured that they were looking at a healthy aging brain.
"Understanding why we lose cognitive function as we age may help us to prolong our mental abilities later in life," says Buckner. "The results of this study help us to understand how the aging brain differs from the brain of a younger individual."
This research was published in the Dec. 6 issue of Neuron. Other researchers involved in this study include Justin Vincent, a graduate student in the Department of Psychology at Harvard and Randy Buckner, Harvard professor of psychology and an investigator with the Howard Hughes Medical Institute. Co-authors also include Andrew Snyder, Denise Head and Marcus Raichle of Washington University in St. Louis and Cindy Lustig of the University of Michigan.
The research was funded by the National Institutes of Health, the Alzheimer's Association, and the Howard Hughes Medical Institute.
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