The human brain is one of the most complex organs in the world, and it consumes a lot of the energy that people take in by eating food (about 20 percent of the daily calorie intake). To make it more energy-efficient, the brain has so-called brain asymmetries. A brain asymmetry is essentially a division of labour between the two halves of the brain. For example, language is processed by most people in the left side of the brain, and processing faces or spatial attention is conducted by the right hemisphere in most people.
By having one half of the brain specialising in doing certain tasks, the brain needs less energy to do these things than it would if both halves were doing them at the same time. A widespread form of brain asymmetry is handedness. About 90 percent of people are right-handed, indicating a dominance of the motor cortex of the left half of the brain (the left half of the brain controls the right side of the body and vice versa) for doing complicated fine motor tasks such as writing. One large open question in psychological research on brain asymmetries is how they developed.
A new study on brain asymmetries in patients with switched organ placement asymmetries
One theory about how brain asymmetries develop is that they are affected by the same developmental factors (such as genes) that also determine asymmetries in the body. Several organs show placement asymmetries in our body. Most prominently, the heart is on the left side in almost all people, and the liver is on the right, but there are many more subtle asymmetries. Interestingly, not all people have their heart on the left side and their liver on the right. There is a very rare condition called situs inversus totalis in which all the organ placement asymmetries are switched. Thus, people with situs inversus totalis have their heart on the right side and their liver on the left side.
A new study in the neuroscientific journal Brain Structure and Function focused on comparing brain asymmetries in people with situs inversus totalis to those of control volunteers with regular organ position (Karlsson and co-workers, 2025). In the study, Swedish neuroscientist Emma M. Karlsson and her research team used advanced neuroimaging techniques to scan the brains of 21 people with situs inversus totalis and 21 control volunteers. Specifically, they used a technique called fixel-based analysis of magnetic resonance imaging diffusion data.
What did the scientists find out?
Using this technique, the scientists were able to assess many different asymmetries of the brain’s white matter. Together with the grey matter (cell bodies of neurons), the white matter makes up most of the mass of the brain. It consists of the connections between different brain areas. Both groups showed a number of pronounced asymmetries in white matter, some of them leftward, some of them rightward. For example, a part of the arcuate fasciculus, an important connection in the language system, showed a strong leftward asymmetry, mirroring the fact that in most people, language is processed by the left half of the brain.
Interestingly, there was absolutely no difference between the groups. The people with situs inversus totalis had brain asymmetries largely comparable to those of the control volunteers. This implies that the developmental processes that are relevant for the development of organ placement asymmetries do not control brain asymmetries. Instead, two different types of processes seem to control these two forms of asymmetries—an important insight from this study.
