How is the brain asymmetric?

At first glance, the human body appears symmetrical: arms, legs, eyes, ears, and even the nose and mouth look like a reflection on an imaginary axis that divides most people’s faces. And finally, the brain: it splits into two halves of roughly the same size, and the grooves and bulges also follow a similar pattern. But the first impression is deceptive: different brain regions have subtle but functionally related differences between the right and left sides. The two hemispheres specialize in different functions. Spatial attention, for example, is processed mostly in the right hemisphere in most people, while language is processed largely in the left hemisphere. In this way, the work can be distributed more effectively to both halves and thus the scope of tasks is broadened in general.

But this so-called lateral coordination, which is the tendency of brain regions to process certain functions more in the left or right hemisphere, varies from person to person. And it’s not just in the minority whose brains specialize in mirroring in comparison to the majority. Even people with classically arranged brains differ in how pronounced their asymmetry is. Previous studies have shown that this, in turn, can also affect the functions themselves. For example, a lack of left asymmetry in certain language areas is observed in dyslexia. Lateral degeneration of the brain also appears to play a role in diseases such as schizophrenia and autism spectrum disorders or in childhood hyperactivity.

Until now, however, it has not been clear to what extent brain asymmetry variance can be inherited between individuals and how much is due to different requirements? Moreover, are similar traits of brain asymmetry present in monkeys as well?

Scientists at the Max Planck Institute for Cognitive and Brain Sciences (MPI CBS) and Forschungszentrum Juelich (FZJ) have now investigated the asymmetry of functional gradients, describing the axes of smooth differences of brain function across the cerebral cortex. They found that there were slight differences in the functional organization of brain regions on the left and right side of the brain. On the left side, the areas involved in language processing are farthest from those involved in vision and sensation. While on the right side the so-called frontal-parietal network, responsible for attention and working memory, for example, is farthest from those sensory areas. The researchers also found that individual differences in these functional arrangements were heritable, meaning that they are partly influenced by genetic factors. At the same time, much of the asymmetry in human brains cannot be explained by genetic factors. This means that some of the variance is influenced, at least in part, by a person’s experience.

In addition, the team found that the brains of humans are more asymmetric than those of monkeys. “The observed functional variance likely reflects the interaction between genetic and non-genetic influences derived from personal experiences,” explains Ben Wan, an MPI CBS doctoral student and lead author of the study now published in the journal eLife. Indeed, in the elderly, they observed a decrease in right wing asymmetry, indicating a slight variance across the lifespan.

“We want to understand why subtle differences between the right and left hemispheres are important for language and attention, and are implicated in various developmental disorders,” explains Sofie Valk, study leader and Cognitive Neurogenetics at MPI CBS. “If we understand the genetics of asymmetry, it would be an initial step toward 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 left-right divergence is dysfunctional.”

The researchers investigated these links using two databases, one containing brain scans of humans, including twins, and the other containing brain scans of 19 macaques. By comparing monozygotic twins, dizygotic twins and unrelated individuals, they were able to determine how siblings differ from each other and, therefore, what is determined not genetically but due to environmental influences. In contrast, the comparison with macaques made clear where the differences between humans and apes lie, which arose through evolution. The scientists calculated these differences with the help of the so-called low-dimensional functional brain connectivity organization. This reveals the degree to which separate brain regions can work together. The researchers calculated this regulatory advantage in each hemisphere, and then calculated the asymmetry index by subtracting right from left.

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