In a nutshell: This finding rules out a key theory that the parts of the brain most susceptible to Alzheimer’s disease are the ones that are larger in humans.

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The big picture:

Alzheimer’s disease is the most common form of dementia. As the disease progresses, more and more areas of the brain become damaged and begin to shrink. But what makes some parts of the brain succumb to Alzheimer’s disease sooner than others remains a mystery.

“We don’t know exactly what the fingerprint is that says the brain is going to start to degenerate in [one] particular area,” says Marcello Rosa, one of the study authors.

Because Alzheimer’s disease is almost exclusively seen in humans, one theory is that the condition targets areas of the brain that are uniquely human as judged by their size relative to other species. The assumption is that these brain regions have become bigger over the course of evolution because they play a critical role in human brain function.

To investigate that theory, Rosa and his colleagues compared MRI brain scans of people with and without Alzheimer’s disease with those of other primates – capuchin monkeys, macaques and marmosets.

The theory was wrong: while some brain regions that are larger in humans were more vulnerable to Alzheimer’s disease, other areas did not fit this trend.

One region that was particularly susceptible, was a primitive area of the brain called the medial temporal lobe, which is no larger in humans compared with other primates.

The medial temporal lobe is involved in forming new memories.

The team also discovered that this and other regions affected by Alzheimer’s were the very same regions that are vulnerable to breakdown in normal ageing.

Rosa now suspects that the vulnerability to Alzheimer’s may have something to do with how active an area of the brain is. “Those areas that have to work the hardest are the ones that are more likely to fail first,” he suggests.

In the medial temporal lobe, for example, memory formation means that neurons must continuously make and break connections.

Next steps:
Researchers can now focus on other factors that might play a role in susceptibility to Alzheimer’s, such as how active a brain region is.


Fjell, A. M., Amlien, I. K., Sneve, M. H., Grydeland, H., Tamnes, C. K., Chaplin, T. A., Rosa, M. G. P., and Walhovd, K. B. (2014). The roots of Alzheimer’s disease: are high-expanding cortical areas preferentially targeted?. Cerebral Cortex, bhu055.

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