Even seniors fortunate enough to avoid the horrors of Alzheimer's disease typically experience some declines in memory and other cognitive abilities. Little is known about why this happens, but a new study suggests that cognitive declines in healthy older adults may result when brain regions that normally work together become out of sync, perhaps because the connections between them break down.
A team led by Harvard neuroscientists Jessica Andrews-Hanna and Randy Buckner used functional magnetic resonance imaging (fMRI) to monitor brain activity in 38 young adults, mostly 20-somethings, and 55 older adults, age 60 or above. The researchers focused on a "default" network of brain regions that are active when the brain is just idling, not working on any particular task (ScienceNOW, 18 January). The fMRI scans revealed coordinated activity in the default network in younger subjects: For example, two particular brain regions in the network tended to be active at the same time even though one is at the front of the brain and the other is near the back. In the older subjects, however, activity in these areas was poorly coordinated. In nine older adults, the researchers also performed a positron emission tomography (PET) scan that can detect amyloid protein in the brain--a hallmark of Alzheimer's disease. The PET scans were negative, suggesting that an out-of-sync default network is a part of normal aging, not a sign of disease, Buckner says.
Additional experiments using a method called diffusion tensor imaging revealed evidence of deteriorated white matter--the cables of axons connecting one brain region to another--in older adults whose default network activity was poorly coordinated. Although the role of the default network in cognition is poorly understood, coordinated activity made a difference in how people performed on tests of memory and other mental skills. Those with the least coordinated default network activity tended to get the lowest scores, the researchers report in the 6 December issue of Neuron.
"I think it's a great contribution to the field of cognitive aging," says neuroscientist Adam Gazzaley of the University of California, San Francisco. The findings, he notes, add to previous hints that the cognitive declines that happen with age result from changes in the way brain regions interact. Deteriorating white matter may turn out to be the root problem, breaking down communication links between brain regions and impairing their ability to work in a coordinated manner, says Gazzaley.