The so-called “right brain” and “left brain” are popular, but what are the actual differences between the left and right hemispheres of the cerebrum? I will explain the left-right difference of the brain, including the truth of the story that “Left-handed brains have a high percentage of 〇〇” and “It is better to talk to the person you like from the left side so that it is easier to reach the emotional right side of the brain.” .
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Difference between right brain and left brain? Are there functional localizations in the left and right hemispheres of the cerebrum?
Left hemispheres,” our cerebrum is divided into left and right hemispheres. In general, sensory and motor nerves on the right side of the body are connected to the left hemisphere of the brain, and sensory and motor nerves on the left side are connected to the right hemisphere. In other words, the left and right cross each other. The reason why the cerebrum is divided into two hemispheres may have been to correspond to the right and left halves of the body.
However, some functions are localized in either the left or right hemisphere. A typical example is The expressive language area (=Broca’s area), which plays a necessary role in verbalizing what you want to express, which is found only in the frontal lobe of the left hemisphere in more than 90% of people. The same is true for the receptive language area (Wernicke’s area), which plays a necessary role in reading human speech and written text as words.
In rare cases, the language center is located in the right hemisphere, but Broca’s area and Wernicke’s area are always on the same side. For example, no one has Broca’s area in the left hemisphere and Wernicke’s area in the right hemisphere. Broca’s area and Wernicke’s area are connected to each other in the same cerebral hemisphere via a brain region called “angular gyrus” located posterior to Wernicke’s area and through a neural pathway called “arcuate fasciculus”. In order to establish smooth communication by listening to what the other person is saying and replying to it, it is probably faster for one of them to process it all at once.
If you say that there is a functional localization, it is there, if you say that it is not, it is not. This time, let’s talk in more detail about the roles of the left and right hemispheres of the cerebrum in such a delicate relationship.
Left-handers have a language center in the right hemisphere? Associating with handedness is a misinterpretation
Let’s supplement the left-right difference in the language center.
There is a theory that “Whether the language center is on the left or right is related to the handedness”. In fact, it is said that only a few percent of right-handed people have their language centers in the right hemisphere, while 30-50% of left-handed people have them. Just hearing this, you might think that there is a relationship between the dominant hand and the distribution of language centers, but please don’t jump to conclusions. That’s a big misunderstanding. Number tricks just got you hooked.
First of all, there is a big difference between the total number of right-handed and left-handed people. We don’t know how handedness is determined, and it can be corrected, so we don’t know the exact number, but it is said that about 10% of the world’s population is left-handed. increase. So, let’s assume that there are 9 right-handed people and 1 left-handed person, and they all have their language center in the left hemisphere. And we found two people with a language center in the right hemisphere, one right-handed and one left-handed, so let’s add those data and calculate the percentage. As a result, the percentage of people whose language center is in the left hemisphere is 10% right-handed (1 out of 10) and 50% left-handed (1 out of 2). Notice that even though it’s just one person’s data, the weight of that one data in the whole is completely different depending on whether you’re right-handed or left-handed.
It is the same as the problem of “difference in the weight of one vote” in the election of members of parliament. It has even been argued that it is illegal to treat results with different weights of votes equally, so comparing left-handed and right-handed data is unreasonable. Even if there is data that seems to indicate that left-handed people have a higher proportion of right-hemisphere language centers, it is a trick of numbers and does not mean that the localization of language centers is related to handedness. Let’s get it right.
After all, even if you are left-handed, more people have their language center in the left hemisphere.
The hemisphere on the side of the language center is sometimes called the “dominant hemisphere”. When removing part of the brain for the purpose of treating epilepsy, etc., care must be taken because if the side with the language center is carelessly removed, severe sequelae including aphasia may appear. It is a word with meaning. However, the term “dominant hemisphere” seems to be taking on its own, as the hemisphere where the language center is located is arbitrarily interpreted as being “superior.” And this seems to lead to the so-called “right-brain/left-brain” hypothesis, which has been broadly interpreted as if there is superiority or inferiority between the left and right sides of the brain. increase. I will discuss this issue in detail in another article.
How are the right and left eyes connected to the brain?
Now let’s think about vision. We have two eyes in many ways. For example, it is possible to see a wider range of objects at once than with one eye, widening the field of view, and to obtain depth information of what you are looking at and to obtain a sense of perspective. And the “visual cortex” responsible for vision is located in the occipital lobe of both the left and right hemispheres, so that it fits both eyes.
However, in the same way that body movements and somatic sensations are crossed, people tend to think that the visual cortex of the left eye and the right hemisphere, and the visual cortex of the right eye and the left hemisphere are connected, but this is a little different. . In fact, both the left and right eyes are connected to both the left and right visual cortex. The wiring is cleverly arranged so that the world in the left half of the center of your body is projected onto the visual cortex of the occipital lobe of the right hemisphere, regardless of whether you look at it with your left or right eye. Conversely, the state of the world in the right half is projected onto the visual cortex in the occipital lobe of the left hemisphere. The details of the wiring of the nerves will be explained in another article, but it is good to remember that “the right half of the world is seen by the left hemisphere, and the left half of the world is seen by the right hemisphere of the visual cortex.”
As evidence of this, those who have damaged the visual cortex in the right hemisphere cannot see the world in the left half, regardless of whether they see it with their right or left eye. Therefore, for example, when you apply makeup while looking in the mirror, you may not notice that you have forgotten to apply makeup to half of your face.
Are there differences between the left and right senses of hearing, smell, and taste?
Consider hearing, too. From a physical point of view, it is rare for sounds generated in the outside world to enter only one ear, so there is almost no difference in hearing between the left and right ears. Sound information received by the left ear is transmitted to both the left and right hemispheres. The same is true for the right ear. The “primary auditory cortex”, which forms the center of hearing, is located in the same way on the left and right in the upper part of the temporal lobe of the cerebral neocortex.
Have you ever heard that it is better to speak from the left side when talking to your lover or asking someone for a favor? A person who expanded the so-called “right brain/left brain” theory thought that “the right brain is an emotional brain, and since it crosses the left and right, if you put a sound in the left ear, you can directly appeal to the emotions of the right brain.” I think that’s what I’m chanting. Unfortunately, that is incorrect. As explained above, the sound that enters the ear is transmitted to both the left and right hemispheres in almost the same way, so it doesn’t matter which side you speak to. It is pointless to change your position because you think you have to speak from the left side.
However, there is a left-right difference in the process of “cognition” in which the content of the sound transmitted to the left and right primary auditory cortices is interpreted. For example, understanding language is the left hemisphere’s job. The right hemisphere is said to be better at listening to music.
The sense of smell occurs when olfactory cells in the back of the nostrils receive odor-causing chemical substances, and the information is transmitted to the olfactory cortex in the frontal lobe of the cerebral neocortex. Taste occurs when taste cells in the “taste buds” on the tongue receive chemical substances that are the source of taste, and the information is transmitted to the “gustatory cortex.” However, the human sense of taste is complex, and the brain mechanisms related to taste have not yet been fully elucidated. The place corresponding to the gustatory cortex is thought to be in the lower part of the parietal lobe of the cerebral neocortex (near the boundary between the frontal and temporal lobes). In any case, there are few reports of bilateral differences in smell and taste.
Stories such as “right brain” and “left brain” seem to attract the attention of many people, but there does not seem to be a difference between the left and right brains, and the debate continues. I would like to take up the left-right difference about other functions in another article.