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One of the most recognizable hallmarks of neurology and neuroscience is the cortical homunculus (also known as the dwarf kinesiography). This model visualizes how the body is systematically mapped to the sensory and motor cortex with its disproportionate representation of body parts in specific areas of the brain, revealing the relative proportions of the brain allocated to individual body parts.
▷Figure 1.Schematic diagram of the two-dimensional villain model, left: sensory villain model, right: movement villain. Source: musicians-focal-dystonia
Not only did this image have had a lasting impact on neurosurgical practice and basic brain research [1], but it was also deeply rooted in people's minds, as evidenced by the three-dimensional clay models on display at the Natural History Museum in London and elsewhere, in which the giant head and hand, attached to tiny bodies, vividly illustrate the characteristics of the small human model.
Figure 2: A three-dimensional model of a sensory figure and a motor figure on display at the Natural History Museum in London shows what the body would look like if it grew according to the distribution of the cerebral cortex that controls movement. Source: Dr. Joe Kiff/wiki
The introduction of the villain model is considered a giant leap in our understanding of the structure and function of the brain, and it has brought innovation to the art of medical illustration, but as modern research has deepened, we have found that the villain model is much more complex than we first recognized. Some experts even consider it to be incorrect in some respects, calling for a radical correction of it.
01 Classic cortical villain model
This unique concept of the "little man" was first proposed by Canadian neurosurgeon Wilder Penfield (1891-1976). In 1934, he co-founded the Montreal Institute of Neurology at McGill University and served as its first director. During this time, he developed an innovative technique for identifying and removing abnormal brain tissue that causes seizures. With this approach, he and his colleagues mapped the detailed functioning of various regions of the cerebral cortex.
For most people with epilepsy, anticonvulsant drugs are effective in controlling symptoms [1], but for those who do not respond to drug therapy, and for those whose seizures are frequent and severe enough to affect their quality of life, brain surgery is the last hope. The technique employed by Penfield involves the use of electrodes to stimulate the surface of the patient's brain during surgery, and the unique feature is that the patient remains awake throughout the procedure and is able to describe the sensations of the stimulation. This made it possible for Penfield to remove the diseased tissue that causes epilepsy without damaging the tissues responsible for vital functions such as movement and language.
After a local anesthetic is applied to the patient's scalp and a craniotomy, Penfield applies a weak electrical current to the exposed surface of the brain. Because the patient remains awake, Penfield is not only able to observe the motor response to a specific area stimulus, but also to ask the patient about the sensations and perceptions experienced.
Penfield treated more than 1,000 patients in the 30s and 40s of the 20th century, thus comprehensively "mapping" the function of each area of the cerebral cortex. He found that stimulation of certain areas of the brain can evoke memories buried deep in the patient's mind, while stimulation of other areas may trigger music or olfactory hallucinations. One of the most well-known examples was when a patient reported, "I smell burnt bread!"
However, Pengfield's most outstanding achievement is his discovery of the structural organization of the sensory and motor cortex. These two long, narrow bands of tissue stretch from the top of the brain down on either side of the central furrow that separates the frontal lobe from the parietal lobe.
In these areas, stimuli in front of the furrow can cause slight movements or muscle twitches in specific parts of the body, while stimuli behind the furrow can trigger a sensory response. It is important to note that parts of the body are mapped with a high degree of accuracy in these two areas, so stimulation of tiny areas adjacent to either area will produce movement or sensation in the corresponding part of the body on the opposite side of the body.
When the top area of the brain is stimulated, it triggers a motor or sensory response in the hip and trunk, and as the stimulus moves down the surface of the brain, the shoulder, arm, elbow, forearm, and wrist respond in turn. In particular, the hands, face, tongue, and throat occupy a relatively large area in these two tissue bands, and each finger has its own regional representation. The point is that although the exact size and location of each body part in a dedicated area of the brain may vary from person to person, the sequence of responses elicited by stimuli from the top to the bottom of the brain is consistent.
During each procedure, Penfield places small, numbered labels on the patient's brain and details the specific responses triggered by electrical stimulation of specific areas (see image below):
▷ Figure 3.Penfield Dyer's experiment with brain electrode stimulation in an epilepsy patient, with each number in the graph corresponding to a specific brain function and sensation mapped by Penfield. The details are described below. Source: American Neurological Association
> 14. Tingling sensation from knee to right foot, no numbness.
> 13. The whole right leg feels numb, not including the foot.
> 12. Feeling numbness on the lower edge of the right wrist.
> 11. 右肩感到麻木。
> 3. The hand and forearm feel numb, reaching up to the top of the forearm.
> 10. 第五指或小指有刺痛感。
> 9. 前三个手指有刺痛感。
> 4. Four fingers (excluding the thumb) feel like numbness after an electric shock.
> 8. The thumb feels a sensation of movement, but no evidence of movement is seen.
> 7. 同8。
> 5. 舌头右侧感到麻木。
> 6. There is a tingling sensation on the right side of the tongue, especially at the tip.
> 15. A tingling sensation on the tongue accompanied by an up-and-down vibrational movement.
> 16. Numbness is felt in the posterior midline part of the base of the tongue.
> Anterior central gyrus from top to bottom:
> (G) 膝盖的屈曲。
> 18. Slight twitching of the arms and hands is like an electric shock and a feeling of wanting to move them.
> 2. Shoulders shrugged upward;
> (H) Clonic movements of the right arm, shoulder, forearm, no movement of the trunk.
> (A) 手腕、肘部和手的极端屈曲。
> (D) Closure of the hand and flexion of the wrist, like seizures.
> 17. Feeling like you're about to have a seizure, your arms and forearms flexed, and your wrists extended.
> (E) Slight closure of the hand, local flushing of the brain after stimulation, repeated at an intensity of 24. Redness is followed by paleness for a few seconds.
> (B) The patient states that he closes his right eye uncontrollably, but in fact he closes both eyes.
> (C) made a little sound; This action is repeated twice. The patient said he couldn't control it. This is related to the movement of the upper and lower lips, which is equal on both sides......
These findings were first permanently documented in visual form, through a villain model, in the paper "Somatosensory Motor and Sensory Representations of the Human Cortex by Electrical Stimulation" (1937), co-authored by Penfield and Edwin Bouldley. [4] These important findings shed light on the way the motor and sensory cortex are organized: there is a one-to-one correspondence between the various parts of the body and specific areas of the brain, and adjacent body parts are represented by adjacent brain regions.
This type of organization is widely regarded as a fundamental principle for understanding the structure and function of the brain, and is known as the "somatic atlas". The technique developed by Penfield, later named the "Montreal Procedure", is still used today. A famous example of this is the scene of violinist Dagmar Turner playing the violin during a neurosurgery operation a few years ago, which helped the surgical team successfully remove a brain tumor without damaging his motor cortex.
02 Female villain model
It is worth mentioning the "hermonculus" problem. Penfield synthesized data from preoperative evaluations of about 400 patients to arrive at a villain model. However, although the model clearly depicts the representation of male genitalia in the cerebral cortex, it does not represent the corresponding parts of the female, and there are different theories behind the reasons. This may have been due to the conservative attitude of society at the time regarding the questioning or reporting of female genital sensations, or the fact that female patients did not feel comfortable reporting such sensations to male doctors, or the reluctance of Hortense Cantlie, the medical illustrator responsible for modeling the villain, to include female genitalia in her illustrations.
Another possible reason is that Penfield lacks sufficient data on female patients—in the study based on the small person model, there were only nine female patients, and only one reported genital sensation at preoperative evaluation. The 27-year-old female patient "EC", who had spontaneous seizures caused by a tumor before surgery, experienced a tingling sensation from her left hip to her breast. During the procedure, the electrical stimulation felt in her left hip and her left foot twitched.
Thus, Penfield and his team speculated that the representation of female genitalia and breasts in the cerebral cortex may be located in the same area as the male genitalia: adjacent to the foot, in the inner wall of the cortex, and deep in the longitudinal fissure separating the left and right hemispheres.
As for the neural representation of the female body, our understanding is still limited. During the Pengfield era, the only other study looked at a woman with epilepsy who was diagnosed as a "nymphomaniac" who experienced a peculiar vaginal sensation during a seizure. When the tumor that triggered the attack was removed, her symptoms were significantly relieved. [5]
From then until 2011, only 10 studies explored the positional tissue structure of female anatomical sites. [6] These studies show conflicting results, suggesting differences in the localization of female anatomy: some scientists believe that the sensory areas associated with female anatomy are mapped to the inner wall of the cerebral cortex, consistent with Penfield's view, while others believe that these regions map to a higher apical position in the brain. Some researchers have suggested that more active research should be carried out to clarify this issue, explore the "female villain model", and further improve the understanding of the female physiological map. "Pregnancy, menopause...... Or how will the body feel after an ovariectomy, for example?," neuroscientist Paula Di Noto and her team asked in the journal Cerebral Cortex in 2012. [7]
In a study published in 2022, Andrea Knop and colleagues at the Charité Medical School in Berlin scanned the brains of 20 women with the help of functional magnetic resonance imaging (fMRI) and simultaneously stimulated their clitoris with a pneuo-controlled diaphragm placed on disposable underwear below the pubic eminence, showing that the representation of the clitoris in the brain is located in the adjacent areas above the buttocks and thighs, a finding that "independently validates a revision of the original villain model". [8]
The study also found that the frequency of sexual intercourse in participants in the 12 months prior to the scan was positively correlated with the thickness of specific areas of the sensory cortex in the brain, with participants who were sexually active exhibiting thicker cortical tissue. In contrast, the different phases of the menstrual cycle did not significantly affect the thickness of the cortex in the "reproductive domain".
▷ 图4：Haven Wright, Preston Foerder绘制的3D女性皮质小人模型。 论文：Wright, Haven, and Preston Foerder. "The missing female homunculus." Leonardo 54.6 (2021): 653-656. 图源：Anne Urai。
03 Fix the villain model
The sensory and motor areas of the cerebral cortex are responsible for coordinating and controlling limb movements. The sensory zone is responsible for processing tactile and pain signals from the body, while the motor zone includes neurons that can activate specific muscles for movement by sending downward signals.
These two regions also contain neurons involved in spatial navigation. These navigation cells, called "location cells," are located in the hippocampus, deep in the brain. [9] They were first discovered in the study of rats in the 70s of the 20th century, and they were only activated when the animal was in a specific location in its environment. Since then, researchers have discovered more types of navigation cells in and around the hippocampus: head direction cells, which activate when the animal moves in a specific direction, and grid cells, which periodically discharge as the animal travels through open space.
These cells make up the brain's global positioning system, and they work synergistically to generate a map of the environment and help form the spatial memory we use to navigate our surroundings. Recently, two groups of researchers independently demonstrated that this spatial navigation system is also present in the sensory and motor areas of the brain.
In 2018, a team of researchers from Duke University in North Carolina released a study [10] in which they trained two rhesus macaques to move around a small room through a wheelchair controlled by a brain-computer interface to obtain food. At the same time, they recorded the sensations of these monkeys with the activity of hundreds of cells in the motor cortex. Surprisingly, they found that many cells emit electrical signals when the wheelchair is moved to a specific position, exhibiting activity characteristics similar to those of the location cells.
This finding was further validated by researchers at Xinqiao Hospital in China in 2021. [11] When they observed rats foraging, they recorded the activity of neurons in their sensory cortex and identified neurons with the characteristics of positional, grid, and cephalic cells.
While this result was unexpected, the discovery of navigation cells in the sensory and motor cortex was not entirely surprising. The navigation cells of the hippocampus are responsible for generating maps and aiding navigation, and here, they are likely responsible for encoding the position and orientation of the body in its surroundings.
These navigation cells, found in the sensory and motor cortex, provide us with an opportunity to expand our understanding of the functions of these areas of the brain. Research on the spatial organization of women's bodies points to the need for an update to the traditional model of the villain. In addition, the research team at Washington University School of Medicine in St. Louis now believes that the traditional villain model is fundamentally wrong and must be completely redrawn.
Evan Gordon, Nico Dosenbach and their team attempted to replicate the results of Penfield's research and generate high-resolution brain maps for each individual by using fMRI to scan the brains of seven volunteers while at rest and performing a variety of motor tasks. They then validated using data from three large publicly available datasets that collectively included brain scans from about 50,000 people.
They found that movement of the feet, hands, and face was associated with the parts of the motor cortex identified by Penfield, but between these specific areas, there were other areas that seemed unrelated to movement. These other areas, thinner than the areas on both sides that are directly related to the parts of the body, are connected to each other, forming a chain that runs down the motor zone not only within the same hemisphere, but also between the two hemispheres of the brain.
After further research, the researchers found that certain brain regions are not only closely connected to each other, but also form strong connections with distant brain regions. These distal areas are involved in "executive" functions such as thinking and planning, visual processing, and the processing of touch, pain, and signals within the body. These areas become more active when participants consider moving.
The research team found that these brain regions together form a network that not only integrates whole-body movements, but also predicts them by regulating changes in alertness, posture, breathing, and heart function.
Figure 5: Left: Penfield's villain model, right: Dossenbach's team drawing a corrected villain model. The parts marked in red are the parts that cannot be explained by the classic villain model. Source: Nature
In an interview, Dossenbach noted: "These interconnections seem very plausible given the real functioning of the brain. The brain exists to enable an individual to take effective action in the environment to achieve a goal without being harmed. The areas of the brain that control movement are necessarily interrelated with those that manage executive function and underlying physiological responses, such as blood pressure and pain sensing. ”[12]
In light of these findings, Gordon, Dossenbach, and colleagues argue that the classic Penfield model of the villain is outdated, at least largely incomplete. They felt that it was necessary to fundamentally revise the model in order to include the network system they had discovered. They named the network the Somatic-Cognitive Action Network (SCAN). [13]
Dosenbach added: "Penfield's contribution is outstanding, and his theories have dominated for 90 years...... But when we began to dig deeper, we found that a large amount of published data did not fully support his view, and there were overlooked alternative explanations. "We've taken a lot of different data and combed through and synthesized it to come up with a whole new way of thinking about how the body and mind are connected. ”
04 Significance
What does this mean for physicians who use the villain model to guide neurosurgery? When performing epilepsy surgery, doctors face a huge challenge because of the potential for damage to sensory or motor areas. Typically, seizures caused by the motor cortex may be confined to a specific part of the body, but they can also spread to surrounding areas. According to Gordon, Dossenbach, and their team, these non-motor areas may contribute to the spread of epilepsy in an atypical way.
"There is a small chance that epilepsy will be confined to this area and will not spread to adjacent areas of movement, and I expect typical symptoms to occur in most cases," explains David Steven, a professor of neurosurgery at Western University of London, Ontario. The risks of surgery are relatively high due to the blurring of boundaries between brain regions, although "the facial area is generally safer because it is represented on both sides of the brain".
In fact, the villain model in the brain still plays a key role. "It's still critical and extremely relevant for preoperative preparation and intraoperative decision-making," says Steven. "While this model may be simplified, it is essential from a practical point of view. ”
Beyond the operating table, a better understanding of how to map the body to the motor cortex is critical to the development of brain-computer interfaces for controlling prosthetic limbs,[14] which can help restore function to paralyzed patients and amputees. These devices typically include arrays of microelectrodes implanted in the motor cortex that read brain activity when planning and executing actions and translate them into instructions to control a wheelchair or robotic arm. [15, 16]
While early prosthetic devices were cumbersome, as technology advanced, newer devices became more sophisticated and capable of simultaneously stimulating the sensory cortex and providing sensory feedback. Not only does this restore some tactile perception, but it also gives the user more control over the device and may alleviate phantom limb pain experienced by most amputees. Drawing a more accurate model of the sensory villain will undoubtedly allow the prosthesis to provide users with more realistic sensory feedback. [17]
In the future, this knowledge, along with a deeper understanding of the brain activity behind different types of haptic sensations, may lead to the development of the next generation of haptic devices, including those that can precisely act on the sensory cortex with tiny electrical or magnetic pulses to produce a variety of realistic sensations anywhere in the user's body.
From prosthetics to the future of gaming, the little man model in the brain – male or female – may be just the beginning. Even as we are still exploring how they work in full, the potential for the application of this knowledge is already beginning to emerge.
Original: https://aeon.co/essays/the-iconic-brain-map-thats-changing-neurosurgery-and-gaming
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