Scientists have revealed that aging disrupts communication between different regions of the brain even if a person is not suffering from Alzheimer's disease.
The study was carried out by a team of Howard Hughes Medical Institute researchers and was led by Jessica Andrews-Hanna.
The research used advanced medical imaging techniques to look at the brain function of 93 healthy individuals from 18 to 93 years old, and it was disclosed that this decline occurs even if serious pathologies like Alzheimer's disease are absent.
It was known for quite some time that normal aging slowly degrades bundles of axons in the central nervous system that transmit critical signals.
"Our study now shows that cognitive decline in aging may be linked to disruption of communication between different regions of the brain," said Buckner, who is a Howard Hughes Medical Institute investigator at Harvard University.
The study began to reveal how simply growing old can affect the higher-level brain systems that govern cognition.
"We may have caught the failure of communication in the act," said Buckner.
Human brain can be divided into major functional regions, each responsible for various "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 its different regions. White matter is the tissue through which messages pass from different regions of the brain.
Scientists were aware that white matter degrades with age, but they did not understand how that decline was responsible in contributing 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.
We were eavesdropping on this crosstalk and we looked at how activity in one region of the brain correlates with another," said Jessica Andrews-Hanna
The team analyzed the crosstalk in the brains of 93 people aged 18 to 93, divided roughly into a young adult group (18-34 years old) and an old adult group (60-93 years old).
The older participants were given a series of tests to measure their cognitive abilities -- including memory, executive function and processing speed.
Each person was studied using functional magnetic resonance imaging (fMRI) exams to measure activity in different parts of the brain. fMRI can precisely map enhanced blood flow in specific regions of the brain.
Increased blood flow reflects greater activity in regions of the brain that are utilized during mental tasks.
The group looked at whether aging in the older group caused a loss of correlation between the regions of the brain that on the other hand engage in robust neural crosstalk, in young adults.
They paid attention on the links within two significant networks, one responsible for processing information from the outside world and the other known as the default network, which is more internal and is triggered when we muse to ourselves.
For example, the default network is presumed to depend on two regions of the brain linked by long-range white matter pathways. The new study revealed a dramatic difference in these regions between young and old subjects.
"We found that in young adults, the front of the brain was pretty well in sync with the back of the brain.In older adults this was not the case. The regions became out of sync and they were less correlated with each other. Interestingly, the older adults with normal, high correlations performed better on cognitive tests," said Andrews-Hanna.
According to Buckner, it is inferred that in a young, healthy brain, signals are readily transmitted by white-matter conduits. As we age, those conduits are compromised.
"Measures of white matter integrity in the older adults point to decline," he said.
Depending on the networks at play, the result may be impaired memory, reasoning or other important cognitive functions.
Buckner and Andrews-Hanna emphasized that other changes in the aging brain may affect cognitive decline. For example, cells' ability to express chemical neurotransmitters may also be compromised.
Generally, the new work assures a better physiological understanding of cognitive decline in the elderly and may help in explaining differences among individuals.
"It may help explain why some people are just as sharp in their 90s as they were in their 40s. We all age differently and cognitive abilities vary considerably among individuals," indicated Andrews-Hanna.
Buckner said that usually as individuals get into their 70s and 80s, you see some degree of change.
"We can use this new approach (correlating the activities of different regions of the brain) as a tool to understand variation between individuals. We can also explore risk factors for breakdowns (in these pathways) like cardiovascular health," he said.
The new research is published in the journal Neuron.