The five ages of the brain

Connections between brain regions undergo profound reorganizations at specific stages of life. This could be the key to better understanding mental disorders and dementia.

Brain functions are not constant throughout life. We already know that learning ability and the risk of cognitive decline vary from birth to ninety years of age. Now a study published in Nature Communications may have discovered why: it seems that brain connections undergo four major changes around the ages of 9, 32, 66 and 83.

Previous research suggests that the body ages rapidly around the ages of 40, 60 and 80. The complexity of the brain, however, makes it more difficult to understand. This organ is made up of distinct regions that exchange information through bundles of white matter, composed of extensions called axons that branch out from neurons. These connections influence our cognitive abilities, such as memory. But we did not know whether they undergo major changes during life, says Alexa Mousley of the University of Cambridge, in the United Kingdom.

To fill this gap, she and her colleagues analyzed MRI scans of about 3,800 people from the United Kingdom and the United States, mostly white and aged between zero and ninety. The scans had been carried out for previous studies, many of which did not include people affected by neurodegenerative diseases or mental disorders.

The researchers found that brain connections go through five phases separated by four important turning points. In the first phase, from birth to nine years of age, the white matter bundles between brain regions appear to become longer and more intricate, becoming less efficient. “Information takes longer to pass from one region to another”, Mousley explains. One possible reason is that a newborn’s brain is full of connections, but as a person grows, unused ones are gradually eliminated. The priority seems to be creating a wide range of connections, useful for tasks such as learning to play the piano, even at the cost of lower efficiency.

During the second phase, between ages 9 and 32, the trend seems to reverse, perhaps due to puberty and hormonal changes. “The brain increases the efficiency of connections, which become shorter so that information passes more quickly from one region to another”. This likely supports the development of skills such as programming and decision making, and improves cognitive performance, such as working memory.

The next phase, the longest, lasts from 32 to 66 years. “In this phase the brain continues to change, but much less”, says Mousley. Connections between brain regions gradually begin to lose efficiency again. “We do not know why, but the thirties correspond to many important lifestyle changes”, she adds. Or the cause may simply be the wear and tear of the body, says Katya Rubia of King’s College London.

From 66 to 83 years, connections between neurons within the same brain region appear more stable than those between different regions. “This is interesting because, in this phase, the risk of dementia and other disorders increases”, says Mousley.

In the final phase, from 83 to 90 years, connections between regions weaken and increasingly pass through “hubs” that link many of them. “This suggests that in this phase there are fewer resources to preserve connections, so they are concentrated in certain regions”, Mousley comments.

Understanding these changes could help explain why mental disorders usually appear before the age of 25 and why the risk of dementia increases after 65, she adds. “Understanding the natural turning points would make it possible to study what changes in the case of mental disorders or neurodegenerative diseases”, says Rubia. “It would be possible to investigate which environmental factors or chemicals cause anomalies, and to intervene with therapies or drugs”.

However, further studies are needed to understand whether these results also apply to populations of different ethnicities and geographic origins, says Rubia.

Source: New Scientist

edited: Nicolas F. E.