Right-handed and left-handed: everything you need to know about brain asymmetry

2024 Author: Malcolm Clapton | [email protected]. Last modified: 2023-12-17 03:44

Why people (and animals!) Are divided into right-handers and left-handers, does this affect creativity and is it worth retraining left-handed children.

Are they really born left-handed and right-handed, or is it still something acquired?

Laterality is inherent in our brain, that is, a functional division into the left and right sides. Because of this, a person has a leading arm as well as a leading leg, a leading ear, and a leading eye. Of course, the predominant use of the right or left hand is the most prominent manifestation of brain laterality.

On average, 90% of the people on Earth are right-handed.

At the same time, the work of the right hand is controlled by the left hemisphere - the one in which the speech center is located.

Ultrasound examinations of the fetus in the womb show that already from the ninth week, three quarters of the embryos begin to move with the right hand, and by the 15th week, they also begin to suck the finger of the right hand. By the 38th week, the fetus turns its head to the right.

Perhaps the preference for the right side was somewhat accidental, due to the peculiarities of the anatomy. Let's show how this can happen using the example of birds. When hatching eggs, birds place the eggs in the nest in such a way that the right eye of the embryo is periodically illuminated through a semi-permeable shell. This feature of offspring care is key in the development of brain laterality in chicks: if eggs are incubated in the dark, chicks will not be "asymmetrical." Correct illumination of the embryo is ensured by its specific position within the egg.

And in mammals, the location of the embryo in the womb is determined, which contributes to the specific development of the left hemisphere. It is possible that the peculiarity of the development of the circulatory system is also responsible for this. One way or another, the human right hand turns out to be the leading anatomically.

However, 10% of people still use their left hand predominantly. Where do lefties come from? In the eighties, American researchers Geshwind and Galaburda put forward the assumption that excessive testosterone action on the fetus during fetal development leads to left-handedness. According to this hypothesis, sex hormones inhibit the development of the left hemisphere and its functions are partially transferred to the right.

The influence of hormones explains, for example, the increased percentage of left-handed children among children whose mothers experienced stress during pregnancy.

However, in addition to this, there are other hypotheses, such as a genetic one. In addition, twins, premature babies, babies born to older mothers and mothers who smoke are more likely to be left-handed. There are also few left-handers, but statistically significantly predominant among men: for 12 left-handed men, there are 10 left-handed women.

And how is this inherited?

It is known that left-handedness is inherited. In families where one of the parents is left-handed, the birth of a left-handed child is more likely than in a family that is right-handed. To date, about forty genetic loci have been associated with left hand preference. Among them, the PCSK6 gene, which is involved in the formation of the left-right axis of symmetry at the earliest stages of development, and the LRRTM1 gene, which is responsible for organizing synapses in a certain type of neurons in our brain.

Are there always more right-handers than left-handers?

Investigating hand preference in humans is not as easy as it sounds. People can use different hands for different tasks. The most difficult, complex tasks are usually performed with the leading hand. Therefore, to test manual preference, questionnaires are used that include several actions.

One of the most cited tests, the Edinburgh Questionnaire, published in 1971, consists of 20 items, including tasks such as writing, drawing, using scissors, comb, toothbrush, broomstick, throwing an object, opening a box, distributing cards, and so on. Further.

In such questionnaires, each task is assigned a +1 or −1 point, depending on whether the person performs it with his right or left hand. Further calculation of the manual preference index may vary, but in the simplest case, the points are added, and if the sum is positive, the person is considered right-handed, and if negative, left-handed.

The disadvantages of the Edinburgh questionnaire are that not all tasks are well known to different age groups: for example, children do not play cards, and older people do not know how to use a tennis racket. Other tasks are outdated: nowadays, people rarely use a broom, but more often a vacuum cleaner. And the set of tasks itself is clearly formed based on the realities of Western civilization. In some cultures, for example in China (as before in the Soviet Union), left-hand writing is unacceptable, and children are retrained from childhood. Other cultural features that distort the real picture of the spread of left-handedness include, for example, the fact that Muslims consider the left hand unclean.

In total, only two items were selected from the questionnaire to study representatives of different nationalities - throwing an object and using a hammer - actions that are least susceptible to cultural influence.

Analyzing the performance of these two actions, the researchers found that the largest number of left-handers is concentrated in Papua New Guinea, where almost a quarter of the population prefers left-handedness to the right, while in the United States and Great Britain the number of left-handers hovers around 10%. Thus, despite the fact that in some populations there are more left-handers than elsewhere, right-handers still prevail.

Are there right-handers and left-handers among animals? Who do they have more?

For a century after the publication in 1865 of the work of Paul Broca, in which he showed that speech is "encoded" in the left hemisphere of the brain, brain asymmetry was considered a form of advanced organization of the nervous system inherent in humans, because only humans have speech, and right-handed and left-handed animals have not been observed before.

However, in the 70s of the XX century, brain asymmetry was discovered in laboratory animals - rats and chickens. Moreover, the preference for one side turned out to be inherent in the most ancient creatures of Cambrian origin: among fossilized trilobites attacked by predators, the number of bites on the body on the right side three times prevailed over that on the left. To date, the asymmetry of the nervous system has been found even in the nematode Caenorhabditis elegans, a small worm, although it consists of only 302 neurons.

Thus, the laterality of the brain can be considered a fundamental property inherent in animals.

As for the preferences of this or that limb, then in different animals they diverge. By observing how chimpanzees perform a pipe test that simulates the fishing of termites from a rotten stump in nature (from a pipe where fingers can not get through, you need to get something tasty using a simple tool for this), the researchers concluded that chimpanzees - right-handed.

Toads and chickens are right-handed. But parrots prefer to take a treat with their left paw. Lefty, most likely, are dogs, however, this does not concern the dog's paws, but the lateralization of the muzzle. Cats, on the other hand, in different types of tasks preferred either the right or the left paw, however, according to the results of the study, it was concluded that cats are left-handed and cats are right-handed.

And why is this necessary from an evolutionary point of view?

If scientists cannot yet give a clear answer to the question of how the preference of one side or the other is formed, then the significance of this phenomenon is quite obvious. It is believed that the asymmetry of the hemispheres makes it possible to accommodate a larger number of functions by eliminating their duplication.

Individuals with asymmetric brains have a faster and more accurate response to external events than "symmetrical" individuals.

This hypothesis is supported by studies on fish and birds, in which the asymmetry of the brain is especially pronounced. Due to the division of functions between the hemispheres, chickens can look for grains with one eye, and with the other watch whether a hawk is flying over them. It is the same with fish: if you artificially breed "symmetrical" fish in an aquarium, then their reaction to food in the presence of a predator in a neighboring aquarium will be twice as slow as that of ordinary fish.

Returning to right-handers and left-handers, it should be noted that, perhaps, the evolutionary process contributed to an increase in asymmetry, making it possible to improve technologies, concentrating them in one hand. According to archaeological evidence, manual preference existed in the human population from the very beginning and the first tools of labor were already sharpened under the right hand.

However, a fairly high percentage of left-handed individuals is stably maintained in the population, and the so-called fighting hypothesis has been proposed to explain the success of left-handed individuals. It states that left-handers are more likely to win fights due to the surprise effect caused by the fact that the right-handed opponent does not expect an attack from the left.

Is it true that lefties are more creative and talented?

The "combat hypothesis" is confirmed by the standings in modern contact sports. Research shows that there are statistically more left-handers among successful athletes in sports such as boxing and fencing, as well as in contact games such as football and baseball. At the same time, in single sports, for example, in running and gymnastics, left-handers have no advantage.

In addition to being successful in sports, left-handers also contribute intellectually to the common cause. There are more left-handers among gifted children with an IQ above average. The same Geshwind and Galaburda suggested that due to the predominant development of the right hemisphere, left-handed people should have a penchant for architecture and mathematics, and there is some evidence in favor of the latter statement (the immortal work of Nikolai Leskov also tells about the same). In addition, there is research showing that left-handed men who graduate from college earn slightly more than their right-handed peers.

However, the bad news is that left-handedness is known to be more common among people with autism or schizophrenia. The life expectancy of left-handers is slightly lower than that of right-handers. However, the latter fact may well be explained by the increased number of accidents that occur due to the fact that left-handers are forced to live in a world adapted for the right hand. Also, there are more left-handed people among homosexuals.

Do you still need to retrain or is it harmful?

Left-handed children in Soviet and Chinese schools were considered the main enemies of discipline, so it was customary to force left-handers to retrain. Fortunately, such prejudices are a thing of the past, and now the education system is trying to give everyone equal opportunities. For example, special training programs for left-handed students who want to become surgeons are being discussed.

In addition, retraining children is not only meaningless (as we have already found out, this is not a whim, but an innate feature), but also harmful: it leads to neuroses and impairment of learning. On the contrary, if you are left-handed, you should be proud of it and try your hand at architecture, mathematics or contact sports while waiting for the world to become a little more left-handed.