Jimmy Hendrix well known for being the greatest guitarist of all times is also a member of a group that makes only about 10% of the world population which are left-handed. How does this peculiarity manifest itself in the brain? Researchers have analyzed several thousand brain scans to determine the differences between left-handed and right-handed people.
Jimi Hendrix
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About 10% of people the left-handed prefer to use their left hand for writing and all other usual tasks. Scientists have long been interested in the differences between the brains of right-handed and left-handed people. So far, the observations have been contradictory and no clear differences emerged. Until this study the samples tested were too small. An international team of researchers, led by the Max-Planck Institute for Psycholinguistics, compared the brains of 31,864 people, including 3,062 left-handed people. On brain scans, they compared 8,681 points across the cortex to compare asymmetries between the brains of right-handed and left-handed people.
Left-handed brain asymmetries
The human brain is asymmetrical, with each hemisphere specializing in different functions. For example, the left hemisphere controls the hand of right-handers and the right hemisphere controls the hand of left-handers. But this is also true for other functions, such as language. In 95% of right-handers, the right hemisphere dominates language, but only in 70% of left-handers, according to the study authors. To what extent is left-handedness related to brain asymmetry? That’s what the researchers wanted to know. They also looked at the role of some genes that were already known to be linked to left-handedness.
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Analyzing all this brain was no easy task. “It took about three months to process it on 12 computers running in parallel,” recalls Zhiqiang Sha, a postdoctoral fellow who worked on the project. From all those hours of computer work, the scientists extracted interesting information. They identified ten regions where there is a difference in asymmetry between right- and left-handed people. These ten regions are scattered throughout the cerebral cortex and are characterized by a thicker layer of gray matter, and therefore more connections between neurons, in the right hemisphere of the brain in left-handed people than in right-handed people. This is consistent with the fact that in left-handers, it is the right hemisphere that controls the movements of the dominant hand. “This is the first time that specific areas of brain anatomy have been linked with certainty to left-handedness,” explains Clyde Francks, director of this study.
Distinguishing a left-handed person’s brain from a right-handed person’s?
In the language regions of the brain, structural asymmetries have been associated with genes involved in left-handedness. Specifically, six genes have been identified, including NME7, which is also responsible for the position of organs on the right or left side of the body. Despite the large number of images analyzed, the scientists were unable to find a clear marker with which to distinguish a right-handed person’s brain from a left-handed one at a glance.
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