Normal Aging Leads To Brain Decline

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Main Category: Neurology / Neuroscience
Also Included In: Seniors / Aging
Article Date: 06 Dec 2007 - 3:00 PDT

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A new US study suggests that the normal aging process leads to brain decline as it becomes increasingly difficult for different regions of the brain to communicate with each other.

The study is published in the 6th December issue of the journal Neuron and is the work of researchers at the Howard Hughes Medical Institute at Harvard University, Cambridge, Massachusetts, and colleagues.

The researchers used advanced imaging techniques to examine the brains of 93 healthy volunteers aged 18 to 93 years and revealed that decline in brain function is normal even in the absence of serious conditions like Alzheimer's disease.

Howard Hughes Medical Institute investigator, Dr Randy L. Buckner, co-author of the study, said in a prepared statement:

"We may have caught the failure of communication in the act."

Scientists already knew that as people get older, the bundles of nerve axons in the brain degrade, as does the brain's white matter. These are critical for the transmission of signals from brain cell to brain cell, brain region to brain region.

"Our study now shows that cognitive decline in aging may be linked to disruption of communication between different regions of the brain," explained Buckner.

The brain has different regions for memory, perceiving and processing sensory inputs, executive functioning, and even internal reflection or "musing". These communicate with each other via a network of white matter channels that help the brain coordinate and share information among its various regions.

While scientists knew quite a lot about the biology of age related white matter decline, they did not know how that decline affected cognitive function.

"The crosstalk between the different parts of the brain is like a conference call," explained Jessica Andrews-Hanna, a graduate student in Buckner's lab and 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," she added.

The team looked at the brains of the young adult volunteers (18 to 34) and the older adults (60 - 93).

The volunteers performed tests of cognitive ability, including memory, executive functioning, and processing speed, while activity in different regions of their brains were examined using functional magnetic resonance imaging (fMRI).

fMRI maps enhanced blood flow in specific brain regions, reflecting greater activity in those areas occupied with mental tasks, such as deciding whether a word represented a living or non living object such as a dog or a house. As Bucker explained:

Such a task requires the participants to meaningfully process the words."

The brains of the young adult group showed a lot of "robust neural crosstalk", and the researchers wanted to see if aging reduced this in the older group.

They looked in particular at a part of the brain that processes information from the outside world, the "default network", which becomes active when we reflect on external information. This network is thought to rely on two regions of the brain linked by long-range white matter pathways.

They found a dramatic difference in activity in this part of the brain between the young and old volunteers.

"We found that in young adults, the front of the brain was pretty well in sync with the back of the brain," said Andrews-Hanna.

"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," she added.

Buckner reckoned that a young healthy brain transmits signals readily using white matter conduits. As aging kicks in, the conduits degrade.

"Measures of white matter integrity in the older adults point to decline," he said.

However, the researches pointed out there may be other reasons the aging brain diminishes cognitive performance. For instance, the ability of cells to express chemical messengers or neurotransmitters may also decline.

However, as Andrews-Hanna suggested, the study helps to better understand the physiology of cognitive decline among the elderly, and particularly individual differences:

"It may help explain why some people are just as sharp in their 90s as they were in their 40s," noted Andrews-Hanna.

"We all age differently and cognitive abilities vary considerably among individuals," she added.

Buckner explained that it was usual to see some change in individuals as they get into their 70s and 80s.

"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.

"Disruption of Large-Scale Brain Systems in Advanced Aging."
Jessica R. Andrews-Hanna, Abraham Z. Snyder, Justin L. Vincent, Cindy Lustig, Denise Head, Marcus E. Raichle, and Randy L. Buckner.
Neuron, Vol 56, 924-935, 06 December 2007.

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Written by: Catharine Paddock
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