Rethink the gnomes

Summary.

When we discovered that the brain contained a map of the body, it revolutionized neuroscience. But now it's time for an update.

Not only did the image have a lasting impact on neurosurgical practice and basic brain research, but it also entered the public imagination, with a giant head and oversized hands attached to a tiny torso in a three-dimensional clay model on display at the Natural History Museum in London and elsewhere.

This groundbreaking work has led to significant advances in our understanding of the structure and function of the brain, and the gnomes themselves have revolutionized the art of medical illustration. However, modern research has shown that gnomes are much more complex than initially thought, and some believe that this is incorrect and needs to be completely revised.

homunculus was the brainchild of Canadian neurosurgeon Wilder Penfield (1891-1976), who co-founded the Montreal Neurological Institute at McGill University in 1934 and became its first director. There, he developed a groundbreaking technique that could identify and surgically remove abnormal brain tissue that caused seizures. Over the course of his career, he and his colleagues have used this method to create early, detailed maps of the function of different areas of the cerebral cortex.

Most people with epilepsy respond well to anticonvulsant medications, but for those who do not, as well as those with frequent, severe, and debilitating seizures, brain surgery is a last resort. Penfield's technique uses electrodes to electrically stimulate the surface of a patient's brain; It is critical that they remain fully awake on the operating table during the procedure so that the patient can describe the effects of the stimulation. This allows Penfield to remove or excision the tissue that caused the seizure without damaging adjacent tissues related to nerve function, such as:

Penfield first anesthetizes the patient's scalp, opens their skull, and then applies a small electrical current to the surface of the patient's exposed brain. Since the patient remains fully awake, Penfield can not only observe the movements caused by the stimulation of a particular area, but also ask about the sensations and perceptions they are experiencing.

Stimulation of the top of the brain causes movement or sensation in the hips and torso.

However, his most important discovery was the organizational structure of the sensory cortex and the motor cortex. The sensory and motor cortex are two narrow, adjacent strips of tissue located on either side of the brain sulcus, a deep fissure separating the frontal and parietal lobes that extends from the top to the bottom of the brain.

Here, the stimulus in front of the crack causes small movements or muscle twitches in a specific part of the body, while the stimulus behind the crack causes sensations. It is important that the body appears to map these two areas in a highly organized way, in this way, the stimulation of adjacent plaques causes movement or sensation in adjacent body parts on the other side of the body.

Thus, stimulation to the top of the brain causes movement or sensation in the hips and trunk, while stimulation that progresses down the outer surface first causes a response in the shoulders, arms, elbows, forearms, and then wrists. Finally, a large piece on both tissues is used for the hands, with each finger representing each separately, and another large piece for the face, tongue, and throat. Crucially, the sequence of responses elicited by progressive stimuli is different, although the precise size and location of the tissues in each patient's body part is different.

During each procedure, Penfield places a small, numbered sticker on the patient's brain and records the response to electrical stimulation of that particular tissue (see figure below).

Excerpt from Wilder Penfield and Edwin Bordry's 1937**. American Neurological Association.

There was a tingling sensation from the knee to the right foot, and there was no numbness. The right leg is completely numb, not including the foot. Numbness of the wrist, nether, right. Numbness in the right shoulder. Numbness in the hands and forearms up to the top of the forearms. A tingling sensation in the fifth or little finger. Tingling in the first three fingers. All 4 fingers were shocked and numb, but the thumb was not. the sensation of thumb movement; No signs of movement are visible. and 85 same. Numbness on the right side of the tongue. Tingling sensations, these findings, known in visual form as gnomes, first appeared in Penfield's book with Edwin Boldrey, Somatic Motor and Sensory Representations of the Human Cerebral Cortex in the Study of Electrical Stimulation (1937). The results of the study suggest that the motor and sensory cortex is organized in such a way that body parts have point-to-point correspondence with specific areas of brain tissue, and adjacent body parts are represented by adjacent tissue blocks.

This tissue is known as somatic anatomy, and it is widely considered to be the basic principle of brain structure and function. In addition, the technique of Penfield, later known as the Montreal procedure, is still in use today. For example, a few years ago, violinist Dagmar Turner played her instrument during neurosurgery, and the team that did so was able to remove a brain tumor without damaging the motor cortex.

Something called hermonculus is also worth discussing. The nuggets are a synthesis of mapping data obtained by Penfield from preoperative evaluations of about 400 patients. However, while the dwarfs clearly show cortical representations of male ***, the female anatomical part is clearly missing. The reason for this is unknown. This may be because when Penfield was working, it was thought inappropriate to ask or report certain feelings from female patients; Because female patients are embarrassed to report *** feelings to men.

Therefore, Penfield and his colleagues hypothesized that the female *** and the breast were in the same area as the male ***: close to the representatives of the feet, on the inner wall of the cortex, deep in the longitudinal fissure separating the left and right cerebral hemispheres.

We need to investigate Hermunculus further and populate the rest of the female map.

In the latest study, published in 2022, Andrea Knop and his colleagues at Charit Universit Tsmedizin in Berlin used functional magnetic resonance imaging (FMRI) to scan the brains of 20 women, while placing an air-controlled vibrating membrane on disposable underwear underneath to stimulate their brains, and the results showed that the performance in the brain was adjacent to the buttocks and thighs. The results provide independent confirmation of the correction of the original hillock.

The sensory and motor bands of the cerebral cortex work together to control and coordinate limb movements. The sensory cortex contains cells that process tactile and pain information, and the motor cortex contains cells that perform movement by sending signals to the spinal cord to secondary cells that activate specific muscles.

But these two regions also contain neurons associated with spatial navigation. These navigation cells are called localization cells and are located in the hippocampus, deep in the brain. They were first discovered in experiments conducted in mice in the 70s of the 20th century, which showed that individual location cells are activated only when the animal enters a specific location in the environment. Since then, researchers have found several other navigation cells in and around the hippocampus: head-oriented cells and grid cells, which discharge when the animal moves in a specific direction.

Two monkeys sit in a wheelchair controlled by a brain-computer interface and search for food in a small room.

In a study published in 2018, researchers at Duke University in North Carolina trained two rhesus macaques to navigate a small room in a wheelchair controlled by a brain-computer interface for food, while recording the activity of hundreds of cells through microelectrode arrays implanted in the animal's sensory and motor cortex. Surprisingly, they found that a large number of these cells exhibited activity similar to that of positional cells, which only activated when the wheelchair was moved to a specific position.

Researchers from Xinqiao Hospital in China confirmed these findings in a 2021 study where they recorded the sensory cortex of foraging rats and found neurons with positional, grid, and head-directed properties.

Although unexpected, the discovery of navigation cells in the sensory and motor cortex is not entirely surprising. In the hippocampus, their function is to generate maps and aid navigation, while in the hippocampus, they may encode the position and orientation of the body in its surroundings.

Finding navigation cells in the sensory and motor cortex allows us to expand our thinking about the functions of these parts of the brain. Studies of the somatic tissue of the female body have shown that gnomes need to be renewed. At the same time, a team of researchers at Washington University School of Medicine in St. Louis now believes that gnomes are completely wrong and need to be completely repainted.

Evan Gordon, Nico Dorson** and colleagues set out to replicate Penfield's findings by using functional magnetic resonance imaging to scan the brains of seven volunteers at rest and generate high-resolution brain maps for each of them as they performed a variety of motor tasks. They then validated their results with data from three large publicly available datasets, including brain scans collected from about 50,000 people.

They found that the movement of the feet, hands, and face was related to the parts of the motor cortex identified by Penfield, but other areas scattered between these discrete areas did not appear to be involved in the movement at all. These other areas are thinner than the flanking areas associated with various parts of the body and are interconnected within and between the two hemispheres of the brain, forming a chain that runs along the motor zone.

The classic gnomes of Penfield, they argue, are wrong, or at least grossly incomplete.

The researchers propose that these areas form a network that integrates whole-body movements and ** these movements through appropriate changes in arousal, posture, breathing, and heart function.

Doson said in an interview that all of these connections make sense if you think about what the brain really does. The brain is used to behave successfully in the environment so that you can achieve your goals without harming or killing yourself. You move your body for a reason. Of course, the area of movement must be related to executive function and control of basic bodily processes such as blood pressure and pain.

Based on their findings, Gordon, Dotson**, and colleagues argue that Penfield's classic dwarf model is wrong, or at least very incomplete, and needs to be radically revised to include the network they discovered, which they named the Somatic Cognitive Behavioral Network (SCAN).

Penfield was smart, and his views had been dominant for 90 years, [but] once we started researching, we found that many of the published data didn't quite agree with his views, as well as other overlooked explanations," says Doson. In addition to our own observations, we collected a lot of different data, narrowed them down and synthesized them, and came up with a new way of thinking about how the body and mind are connected.

What does this mean for neurosurgeons who use dwarf-guided scalpels? Surgery for epilepsy is extremely challenging due to the high risk of impairing sensory or motor bands. In general, seizures produced by the motor cortex are confined to certain parts of the body but may spread to adjacent areas, while the non-motor areas identified by Gordon, Dothan** and colleagues could theoretically produce seizures that spread in unusual ways.

David Steven, a professor of neurosurgery at the University of Western London, Ontario, told me that the likelihood of a seizure staying in this area without spreading to adjacent motor areas seems to be low, which I think is typical for most cases [symptoms]. Since brain regions are mixed, surgery can be high-risk, except for facial areas, which are generally safe because both sides of the brain are represented.

In practice, the little man in the brain still appears to be large. This is still crucial and very relevant for preoperative testing and intraoperative decision-making, Steven said. It may be overly simplistic, but in reality, it's still essential.

Drawing a finer map will allow the prosthesis to provide more realistic sensory feedback.

Early versions of these prostheses were bulky, but they are becoming more sophisticated, and some newer devices can simultaneously stimulate the sensory cortex to provide sensory feedback. In addition to restoring some of the sense of touch, users have more control over the device and can also reduce the phantom limb pain that most amputees feel. There is no doubt that mapping the sensory dwarfs in a finer way will allow the prosthesis to provide increasingly realistic sensory feedback to the user.

In the near future, this knowledge, combined with a better understanding of the brain activity behind different types of touch, could also be used to develop the next generation of haptic devices, including headphones, that can precisely target the sensory cortex with small electrical or magnetic pulses, eliciting a variety of real sensations in any part of the user's body.

From the prosthetics of the future to the games of the future, the little men (and women) in the brain may have just begun, even if we are still fully understanding how they work.

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