At first glance, the human body appears to be symmetrical: two arms, two legs, two eyes, two ears, and even the nose and mouth appear to be mirrored in an imaginary axis that divides most people's faces. Finally, the brain divides into two halves of nearly equal size, with the grooves and bulges following a similar pattern. However, the initial impression is misleading, as there are small, functionally relevant differences between the left and right sides of different brain regions.
The two hemispheres have different functional specializations. For example, most people process language primarily in their left hemisphere, while spatial attention is processed primarily in their right hemisphere. In this way, the work can be distributed more efficiently to both sides and the overall range of tasks is expanded.
However, this so-called lateralization, or the tendency for brain regions to process certain functions more in the left or right hemisphere, differs between people. And not just in the minority whose brains are reversed compared to the majority. Even people with classically arranged brains have varying degrees of asymmetry. Previous research has indicated that this, in turn, may have an effect on the functions themselves.
For example, dyslexia is characterized by a lack of left asymmetry in certain areas of language. Inadequate brain lateralization can also contribute to conditions such as schizophrenia, autism spectrum disorders, and hyperactivity in children.
Until now, however, it has not been clear how much variation in brain asymmetry, between individuals, is heritable and how much is due to different demands. Also, are similar features of brain asymmetry also present in monkeys?
Scientists from the Max Planck Institute for Human Cognitive and Brain Sciences (MPI CBS) and the Forschungszentrum Juelich (FZJ) have now investigated the asymmetry of functional gradients, describing axes of smooth variations of brain function along the cortex. They discovered that there are subtle variations in the functional organization of brain regions on the left and right sides of the brain. On the left side, the regions involved in language processing are the most distant from those involved in vision and sensation.
While on the right side, the so-called frontoparietal network, responsible for attention and working memory, for example, is the furthest from those sensory regions. The researchers also found that individual differences in these functional arrangements were heritable, meaning they are influenced in part by genetic factors. At the same time, much of this asymmetry in human brains cannot be explained by genetic factors. This means that some asymmetry is influenced, at least in part, by the person's experience.
Furthermore, the team found that human brains are more asymmetrical than monkey brains. Bin Wan, Ph.D. MPI CBS student and lead author of the recently published study writes:
"It is likely that the observed functional asymmetry reflects the interaction of genetic and non-genetic effects derived from personal experiences."
In fact, in older people, they observed reduced asymmetry to the right, suggesting subtle variation across life.
"We want to understand why subtle differences between the left and right hemispheres are relevant to language and attention and are implicated in various developmental disorders," explains Sofie Valk, head of the study and the MPI CBS Cognitive Neurogenetics research group. . “If we understand the heritability of asymmetry, this would be an initial step towards understanding the role of genetic and environmental factors in shaping this trait. We may eventually be able to figure out where something goes wrong when the difference between left and right is altered."
The researchers investigated these connections using two databases, one containing human brain scans, including twins, and the other containing brain scans from 19 macaque monkeys. By comparing monozygotic twins, dizygotic twins, and unrelated individuals, they were able to determine how siblings differ from each other, and therefore what is not genetically determined but rather due to environmental influences.
In turn, the comparison with macaques made it clear where the differences between humans and monkeys lie and which ones have arisen through evolution. The scientists calculated these differences with the help of the so-called functional organization of low-dimensional brain connectivity. This reveals the degree to which separate brain regions can work together. The researchers calculated this organizational characteristic in each hemisphere and then calculated the asymmetry index by subtracting right from left.
