When Brain Asymmetries Develop

When looking at the human, its visually most striking aspect is that it consists of two halves, often called brain hemispheres. While the two halves of the brain look roughly similar on the outside, they are not completely equal regarding their structure and function.

Many so-called brain asymmetries, e.g., differences between the left and the right half of the brain, exist (Ocklenburg, 2024). For example, in most people, the left half of the brain controls language function, and if they have a stroke that affects the left half of the brain, they often cannot speak anymore or at least have a lot of language problems. These functional brain asymmetries are also reflected in structural asymmetries. For example, some brain areas involved in processing language are bigger in the left half of the brain. While brain asymmetries have been investigated very thoroughly in adults, studies in children only have been conducted in smaller groups of volunteers. There is generally less research in this group than in adults. One of the major questions in this field is whether children and adolescents already show the same patterns of brain asymmetries that adults do. If no asymmetries are present in children, this would suggest that asymmetries are a result of development. If asymmetries are already present in children, they may reflect an early principle of brain organization.

A new study on the development of brain asymmetries

To investigate the development of asymmetries in a large and representative group of people, a new study was conducted by the international ENIGMA consortium (Enhancing NeuroImaging Genetics through Meta-Analysis). In the study, first-authored by scientist Florian Kurth from the University of Auckland in New Zealand, a large, international team of more than 90 scientists from all around the world analyzed brain imaging data on the development of brain asymmetries (Kurth and co-workers, 2024). Overall, brain imaging data showing the brains of more than 4,000 children and adolescents aged between one and 18 years were included in the study. The data were collected at 69 different study centers around the world, making the study very representative.

Brain asymmetries start early in life

What did the scientists find out?

Overall, four main analyses were conducted.

First, the scientists wanted to know whether brain asymmetries that exist in the adult brain can also be observed in the brains of children and adolescents. The answer to this question was a clear yes. The left hemisphere was significantly thicker than the right, but the right had a larger surface than the left. Also, many specific brain areas showed significant asymmetries. Thus, brain asymmetry seems to be a major principle of organization in the human brain already present early in life.

Second, the scientists wanted to know whether brain asymmetries changed with age. Overall, 15 regions showed changes in brain asymmetries related to age, with some showing increases and others decreasing. As asymmetries in 34 regions were assessed, this shows that there are moderate changes in brain asymmetries related to age, but there was no clear pattern of increase or decrease.

Third, the scientists wanted to know whether biological sex affected brain asymmetries. Again, some effects were found, but overall, they were not very strong. Male children and adolescents showed stronger asymmetries than female children and adolescents in two brain areas, while the reverse was true for four brain areas.

Fourth, the scientists also assessed whether the effects of biological sex on brain asymmetries differed with age. However, no effects were found.

Taken together, these effects clearly show that brain asymmetries can be observed early in life and are not something that only adults show. Overall, they seem to be stable, with some changes during aging and some differences between males and females. This makes it likely that they are a fundamental principle of how our brains are organized from the very beginning and not a result of development.