The tendency to become right-handed or left-handed is not only due to the brain

Why do we become right-handed or left-handed? For decades, this tendency was attributed solely to the brain, but a new study suggests that it is not solely responsible for this decision.

In the 1980s, the tendency to become right- or left-handed was discovered to take place in the fetal stage after scans revealed that by the 13th week of pregnancy, fetuses already seemed to prefer sucking their right or left thumb. For decades, the scientific community agreed to attribute this “decision” to be right-handed or left-handed solely to the brain.

But a new study published in the journal eLife suggests that the brain is not the only one involved since the spinal cord, another crucial part of the central nervous system, could also have its share of responsibility.

The movements of our limbs are initiated via the motor cortex in the brain. To trigger a movement, it sends the corresponding signal to the spinal cord which transmits the signal to convert the command into movement. However, the motor cortex is not connected to the spinal cord from the outset and the asymmetry in arm movements notably seems to appear before this connection.

Thus, for the scientists of the Ruhr-Universität Bochum (RUB) in Germany, the fact of becoming right-handed or left-handed would rather find its origin in the spinal cord than in the brain and would result more precisely from the genetic activity inside the nerve structure.

To verify their theory, these German researchers studied the genetic activity inside the spinal cord between the 8th and 12th weeks of pregnancy. It was soon noted that differences in gene expression between the right and left parts of the spinal cord, which appeared in particular in segments controlling the movements of the arms and legs. After studying the causes of this asymmetry, they came to the conclusion that environmental factors would control the strength of genetic activity in the right or left part of the spinal cord. A process known as epigenetics.

 » Epigenetic factors appear to be at the root [du phénomène]reflecting environmental influences explains Dr. Sebastian Ocklenburg, in charge of the study.  » As this process occurs with different magnitude in the right and left spinal cord, there is a difference in gene activity on both sides. Our results suggest that the molecular mechanisms for epigenetic regulation inside the spinal cord are the starting point for laterality. « .

A conclusion that changes fundamentally our understanding of the cause of hemispheric asymmetries “, explain the researchers in a press release published by the German University.

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