Why are some people left-handed?
About 10.6% of people are left-handed (Papadatou-Pastou et al., 2020) but what causes awkwardness is still not fully understood. One thing is clear, is that laterality is not caused by the hands. It is totally impossible to tell if a person is left or right handed just by looking at their hands. There are no obvious differences between the bones, muscles, tendons and other parts that make up the hands of left-handed and right-handed people.
Instead, the predominance is caused by the central nervous system, for example the brain and spinal cord. Laterality is one of the many left-right functional differences in the brain. Specifically, in lefties, the motor cortex on the right side of the brain (the left side of the body is controlled by the right side of the brain, and vice versa) is dominant for fine motor behavior like writing with a pen. In contrast, in right-handed people, the left motor cortex is better for such tasks.
Investigate the link between awkwardness and asymmetries in brain structure
This discovery led researchers on laterality to an intriguing question: Could these asymmetries in brain structure be relevant to asymmetries in brain function, such as laterality?
A new study, now uploaded to the bioRxiv preprint server, focused on answering this question by examining both the structural differences in the brain between left-handed and right-handed people and the genetic link between laterality and body structure. brain (Sha et al., 2021). In the study, researchers analyzed brain imaging data from 28,802 right-handed and 3,062 left-handed people that were obtained from UK Biobank. The UK Biobank is a large body of psychological and neuroimaging data obtained from volunteers in the UK and made available to trained scientists around the world. The UK Biobank has the huge advantage of containing data from far more participants than any scientist can get in a reasonable amount of time when acquiring data from volunteers themselves. Because of this large sample size, researchers may be more confident in the results obtained from the UK Biobank data than if they collected a few dozen datasets themselves. This is because the results of larger data sets are less influenced by individual participants who exhibit unusual outcome patterns and therefore replicate better in other samples.
In the study, the researchers analyzed asymmetries in brain structure across the brain for their dataset, as well as genetic variation related to workability and brain structure and the overlap between the two.
What did the researchers find?
The researchers found that left-handers exhibited a right-shifting hemispherical structural cortical surface asymmetries in eight areas of the brain and a right-shifting hemispherical structural asymmetries in cortical thickness in two areas of the brain. This right-handed shift in asymmetries in left-handers suggests that left-handedness is associated with a shift in neural resources to the motor-dominant right hemisphere (the right side of the brain controls the left side of the body and vice versa). Functionally, these brain areas were linked to motor functions, but also to so-called executive functions, for example a range of higher cognitive abilities such as decision-making, vision and language.
Second, the researchers studied which genetic factors were linked to asymmetries in brain structure in areas of the brain that showed differences between left-handed and right-handed people. They found that for two regions of the brain, an increased genetic disposition for left-handedness was significantly associated with a right-shift in brain asymmetries, suggesting a genetic link between left-handedness and asymmetries in brain structure. In addition, 18 different locations in the genome that had previously been associated with awkwardness were associated with asymmetries in the structure of the brain. Interestingly, six of the genes (called TUBB, TUBA1B, TUBB3, TUBB4A, MAP2, and NME7) associated with the identified locations in the genome were functionally relevant for microtubules. Microtubules play a role in early brain development because they are important for the cytoskeleton, which provides structure and shape to nerve cells and other cells. It is important to note that microtubules have also been associated with determining the left-right axis in the brain during early development. This suggests that the early developmental processes that determine the left and right sides of the brain link left-handedness and asymmetries in the structure of the brain.
In conclusion, the study showed that awkwardness and asymmetries in the structure of the brain are related. Left-handed people show a shift in cerebral asymmetries towards their motor-dominant right side. Genetic analyzes suggest that the developmental processes that determine the left and right sides of the nervous system represent the functional link between left-handedness and asymmetries in the structure of the brain.