What is a homunculus? A dwarf in the brain that appears in the mapping of the cerebral cortex


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What is a homunculus

Through research on the conditions of aphasia patients and people whose brains have been damaged in war, the functions of each part of the brain have been clarified. When we map the role of the human neocortex for each part, a picture of a dwarf in the brain emerges. It is a homunculus, a dwarf in the brain, which is famous in brain science. I will explain the fun of cerebral cortex research in an easy-to-understand manner.

Aphasia and war damage: Brain injury patients who clarified the functional localization of the cerebrum

The idea that different parts of the brain, especially the cerebral neocortex, have different functions is commonly called “functional localization theory. As a result, the functional localization theory of the cerebrum has progressed at once.

Through various case observations, we can gradually clarify the role of the cerebral neocortex by linking strokes that occur in very limited areas of the brain and the peculiar symptoms caused by them. It became

There have also been many reports of people receiving bullets to the head during the war. Unlike disease, the bullet injury was localized, which made it easier to match the location of the brain injury to the loss of function. Ironically, the more the war progressed, the more we learned about localization of brain function.

Furthermore, in the first half of the 20th century, treatment using lobotomy was attempted, mainly for schizophrenia patients, and the relationship between the frontal lobe and higher mental functions began to be discussed. A lobotomy is a surgical procedure that disconnects the neural circuits of the cerebrum from the rest of the brain. It was first used in 1935 by Portuguese neurologist Antonio Egas Moniz to cut the neural circuits in the frontal lobe that were thought to be responsible for the repetitive thought patterns seen in mentally ill patients. It is said that. By the way, I think that there are many people who imagine “robot” from the sound, but that is a misunderstanding. The English spelling of lobotomy is “lobotomy”, a word created by combining “lobe”, which corresponds to “leaf” such as the frontal lobe of the cerebral neocortex, and “-tomy”, which means “amputation”. Ethical issues have also been pointed out, and lobotomy is not done in modern times.

In any case, I have mixed feelings about the fact that the records of war victims and lobotomized patients have contributed to the progress of brain science.

Research in brain-injured patients continues today. However, there is a limit to what we can learn by just waiting for someone with a disability to emerge. Therefore, methods of more active intervention and research have been considered.

A new idea of ​​​​”stimulating a limited area of ​​​​the brain”

In 1870, the Germans G. T. Fritsch and E. Hitchch, who were interested in Broca’s discovery of the motor language area, conducted an experiment in which the cerebral neocortex of dogs was stimulated with a very weak electric current, and the results were published as ” On the electrical excitation of the cerebrum (Ueber die elektrische Erregbarkeit des Grosshirns)”. This included the following important findings:

  1. Muscles such as hands and feet move when the front part of the cerebral neocortex is stimulated.
  2. Electrical stimulation of the part involved in movement causes muscle contraction in the opposite side of the body.
  3. When very weak currents are applied and localized stimulation occurs only in limited muscle groups. Other muscle groups and ipsilateral muscle groups respond when stimulated with more intense currents to excite more brain regions.

1) indicates that the “motor cortex”, which issues motor commands for moving muscles such as the limbs, is located in the frontal lobe, and 2) indicates that outputs from the cerebral neocortex to the periphery are bilaterally crossed. I think many people know that when motor paralysis appears in the left side of the body due to cerebral infarction, it is presumed that the infarction site is in the motor cortex of the right hemisphere. The first time I went was Fritsch and Hitchhi. The result of 3) is the discovery that there are differences in function depending on the location even in the motor cortex, and that the corresponding relationship of which part of the nerve controls which part of the body is determined. was further supported.

After that, the British neuroscientist D. Ferrier conducted a detailed analysis of the motor cortex through stimulation experiments and destruction experiments using dogs and monkeys. The idea has taken root.

Read Also: What is a CT scan of the head? Differences, advantages, and disadvantages from X-rays

Doesn’t the brain itself feel pain? Surgery for epilepsy that preserves function

Some researchers have attempted to further clarify functional localization in the human neocortex using a similar technique. I’m Wilder G. Penfield, a Canadian neurosurgeon.

Penfield was born in the United States, but in 1928 he was invited to work at McGill University’s Royal Victoria Hospital in Montreal, Canada, where he worked on the pioneering surgical treatment of epilepsy. Today’s neurosurgery is generally performed under general anesthesia, giving priority to safety. This surgery was possible because the brain itself has no pain receptors. Since there is no general anesthesia, the patient is conscious. Then, by applying a thin metal electrode to a limited area of ​​the cerebral neocortex and stimulating it with a weak electric current, we were able to see the patient’s various reactions, and we were able to proceed with the surgery while confirming this. .

In the surgical treatment of epilepsy, the expected lesion in the brain is removed, but the actual situation is that it is impossible to know whether it is really okay to remove the site until it is actually done. As a result, not a few patients suffered from severe aftereffects due to unnecessary excision. In order to solve this problem, Penfield confirmed during surgery what function the area of ​​the brain he was focusing on was related to, and so as not to interfere with the patient’s life after surgery. I tried to carefully determine the resection site while preserving function as much as possible.

This technique not only brought about progress in the surgical treatment of epilepsy, but also increased our knowledge of brain function localization.

A dwarf in the brain, a homunculus… A dwarf made by mapping the human brain

Because the patient was conscious, Penfield could hear what the patient felt in response to the location of the brain stimulated. For example, one patient who was stimulated in the posterior neocortex complained of seeing flashes of light. Patients whose lateral neocortex was stimulated complained of hearing buzzing sounds. Through extensive research, it is now clear that the centers responsible for vision and hearing are located in the occipital and temporal lobes, respectively.

Penfield also focused on the area around the central sulcus, which is the boundary between the frontal and parietal lobes of the cerebral neocortex, and conducted a fairly detailed analysis. When the area immediately behind the central sulcus is stimulated, a somatosensation of something touching the skin is induced, and when the area immediately in front of the central sulcus is stimulated, the muscles of each part of the body, such as limbs, are stimulated. It was confirmed that the “movement” was induced. These areas corresponded to the somatosensory area and the motor area, respectively. Furthermore, when I investigated which part of the brain and which part of the body were related, I found that there was a fixed correspondence. Penfield then succeeded in making the map shown below.

Movement and sensation in each part of the body are assigned to defined regions of the motor and somatosensory cortices anterior and posterior to the central sulcus of the neocortex (created by Guide with reference to Penfield’s paper). original drawing)
The data on which this figure is based was obtained by recording in detail which part of the body reacted (motion or sensation) when each region was electrically stimulated. Penfield drew the corresponding body parts side by side on the surface of the brain. This drawing came to be called a ‘homunculus’ in the sense that it seemed as if a dwarf was living inside his head, and made Penfield famous all at once.

This map is really well done. You can clearly see that the areas of the brain regions are quite different for each corresponding body part. The fact that the brain area in charge is wide means that “a lot of nerve cells are involved”.

For example, we can see that much of the motor cortex is dedicated to moving the face and hands. In communication with people, it is very important to move the mouth to talk and to move the muscles of the face to convey emotions. It is said that we humans have evolved to be able to use our hands freely by choosing to walk on two legs. There is none.

The face of a dwarf in the somatosensory area has very thick lips and looks strange. This indicates that the lips are sensitive. The fact that the index finger is particularly large among the fingers is also related to the sensitivity of the index finger.

The fact that dwarves have almost no torso means that they don’t need to move it precisely or feel it sensitively.

Penfield is a great pioneer of brain science

Penfield founded the Montreal Institute of Neurology in 1934 and contributed to the surgical treatment of epilepsy, and continued to vigorously conduct research to clarify the functional localization of the cerebral neocortex.

For example, we found that there is a “supplementary motor area” in the front part of the motor area mentioned above. Damage to the supplementary motor cortex does not imply a loss of motor function itself, but it does impede the initiation of movement, sequenced tasks, and coordination of the hands. If you give a book and tell them to read it aloud, they can read it without any problems, so it’s not that they can’t speak, but if they say, ‘You can speak whatever you want,’ they will start to speak on their own. can not. It is difficult to perform multiple movements in order, such as “opening the lid of cup noodles, pouring hot water and closing it,” and pouring hot water with the lid closed. To open the cap of a PET bottle, it is necessary to hold the bottle firmly with one hand and twist the cap with the other hand. becomes difficult. With the discovery of these supplementary motor areas, the above-mentioned areas that were simply called “motor areas” are now called “primary motor areas” to distinguish them.

Penfield’s research also yielded important findings regarding the mechanisms of memory. Electrical stimulation of a portion of the temporal lobe of a patient undergoing epilepsy surgery brought back specific visual images. The location may be a storage location for visual memories or may play a role in triggering recall of memories. Regarding the mechanism of memory, ” Working and functioning of the hippocampus in the brain… It is deeply related to memory and spatial cognition. “”, the hippocampus in the limbic system plays a role in “making memories”, but it is still unclear where the information is sent from the hippocampus and ultimately stored as memories that will not be forgotten. Not.

Penfield passed away in 1976 at the age of 85, but his achievements continue to inspire many researchers and have a great impact on the progress of brain science.