MRI image of the human brain showing the curves of the nerve fibers. mgh-ucla Human Connectome Project
Beginning in the 1930s, American neurosurgeon Wilder Penfield opened the mind to hundreds of people. It was supposed to cure epilepsy or a tumor, but since they were open, he administered electric shocks to different parts of the body while asking patients how they were feeling and observing their reactions. This enabled him to create a map of the motor functions of the cerebral cortex. When drawing, a kind of homunculus (Latin little man) came out for each half of the brain. The drawing showed a series of continuities that still fascinate today. For example, the right index finger is moved by stimulating a point adjacent to where the middle finger is moving. Or the movements of the mouth, lips, teeth or larynx are grouped. The guy has huge hands and a huge head, reflecting the relevance of these body parts. This discovery and its graphical representation have since appeared in neuroscience textbooks. The problem is that Penfield’s homunculus is poorly drawn after extensive work that is now being done.
“This discovery was accidental,” says Evan Gordon, a neuroscientist at the University of Washington (USA) and first author of this recently published paper in Nature. Along with his lab colleagues, Gordon spent hours performing MRIs on various people while they were at rest or in motion. “This enabled us to map the organization of the brain in great detail. We expected to find many interesting features of the brain organization of the more complex parts responsible for memory, planning and abstract thinking. What we didn’t expect was to find a new brain system in the primary motor cortex!” he elaborates in an email. “We didn’t even think to look into that area because since Penfield’s work 90 years ago everyone already knows what the motor cortex is supposed to look like,” he adds. But what they found was a network within another network that circumscribes what the Canadian neurosurgeon had discovered. “It was so amazing that we did it at a time couldn’t believe it for a long time and had to really work hard to convince ourselves it was real,” adds Gordon.
In their attempt to replicate Penfield’s maps, the researchers confirmed that the foot control was in the location that Penfield had identified. Located at the outermost part of the brain, the motor cortex is a sort of strip that starts just above one ear and unfolds until it reaches the other. This band is interrupted by the division that divides the brain into two left and right parts along its length (the interhemispheric fissure). For example, the movement of the right foot activates the fringe section, which is just to the left of this glitch. They verified the same for the hands (in the middle of the brain surface) and for the face (already near the auditory pavilion).
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“Penfield wasn’t wrong, but he was limited by the technology available at the time,” Gordon clarifies. In fact, the Canadian neurosurgeon himself said that his homunculus should not be taken at the foot of the stroke. What he found wrong, however, was that the distribution of body movements on the brain map was not linear or continuous as he believed. The researchers have now discovered that the movement unfolds concentrically. Thus, the act of pointing a finger activates a point in the middle of an area where moving the hand, bending the elbow, or shrugging activate points further and further from the center activated by the phalanges.
mind-body connection
More importantly, interspersed with the key areas for movement of the hands, feet, or face, they saw the activation of other areas that don’t appear to be directly involved in motor functions, despite lying at the heart of the motor area of the brain brain. We show that Penfield’s homunculus representation is interspersed with domains of a somato-action network. [en referencia al cuerpo] cognitive [referido a la mente]Gordon explains. The various parts of this network that represent SCAN are highly interconnected and active in many types of movement, particularly when they think about performing the movement before executing it. This is reinforced by the observation that this network, now identified, is connected to other areas of the brain responsible for so-called higher functions.
Nico Hosenbach, Gordon’s colleague at the University of Washington and senior author of this research, sees this SCAN network “as an important mind-body interface, perhaps the most important one we know of to date.” According to Hosenbach, “Associating goals with all action-relevant bodily functions: skeletal muscle movement (gross motor skills), smooth muscle (organ) control, autonomic tone (e.g., adrenaline release) seems abstract plans. ) … “. “SCAN is functionally linked to the regions of the frontal cortex that make plans, decisions and initiate actions, and through the spinal cord it is also linked to the muscles of the body,” adds the researcher.
According to the authors, the existence of such an interface would explain why so many mental functions and disorders have physical manifestations. “When I think about how to solve a problem, I start running. Or before I give a lecture, I always feel like peeing. There are a million examples of how your thoughts affect your body. We believe that the SCAN is the physical structure that makes all of this possible,” concludes the neuroscientist.
The little man’s charm
Michael Graziano, a Princeton University professor and author of books such as Consciousness and the Social Brain, had begun dismantling Pienfield’s legacy earlier this century. In 2002 he published a paper on the existence of more complex action cards, such as the one activated on breathing that Penfield’s homunculus blurs.
“My work should have ended with the Penfield homunculus,” says Graziano bluntly. “But there were traditionalists who couldn’t accept our results and ignored the data. So there are still many followers of Penfield’s Homunculus and it’s still in the manuals. Now, in a way, this new study is a confirmation of what should already have been obvious and helps move forward,” he adds. Graziano realizes that the little man’s drawing has its charm. “The Penfield muscle map, while snazzy, isn’t necessary or useful. The spinal cord already has a map of the muscles. Why would the bark double it? The crust can integrate much better than separate small engineering units,” he concludes.
“The mind does not exist, there is only the brain”
Jesús Porta, Vice President of the Spanish Society of Neurology
The Vice President of the Spanish Society of Neurology, Dr. Jesús Porta, points out that both the work of Graziano and that of Gordon and Donsebach show that “the mind does not exist, there is only one brain”. Since the beginning of the century, research has accumulated to consider the mind as a function of the brain: “Music is tangible, but whoever produces it is the orchestra,” compares Porta. This is much more than a philosophical or metaphysical discussion. The various functions of the brain, but also its disorders, have their correlate or physical substrate. As Porta says, “Fear causes rapid breathing and is accompanied by tachycardia, and it’s all brain.”
Penfield’s homunculus
Wilder Penfield, a physician and neurosurgeon, in a 1967 picture. Boris Spremo/Toronto Star via Getty Images
Wilder Penfield, who came to Spain in his formative years to learn from Ramón y Cajal’s team, apparently operated on more than 400 people. Most were soldiers from both world wars whose brain injuries had left them with focal epilepsy. He also operated on dozens of brain tumors. During his interventions, he developed a system to stimulate the different parts of the brain with electrodes and small discharges. David Ezpeleta, neurosurgeon at Quirón University Hospital in Madrid, admits that “this examination can no longer be carried out during the operation”.
The disproportionate homunculus that Penfield drew to illustrate the map of movements in the motor cortex, with exaggeratedly large hands, feet, and face, has its scientific basis. As Ezpeleta recalls, “the different proportions show that these areas were involved in actions that required more dexterity and fine movements,” such as those of the fingers or the tongue.
Beyond the validity and validity of the homunculus, Ezpeleta highlights that Penfield was a pioneer in many aspects, such as epilepsy surgery or the identification and analysis of the motor cortex and the somatosensory cortex. “He was religious and looked for the foundations of emotions, including the human soul,” says Ezpeleta, executive secretary of the Spanish Society of Neurology. “In addition to healing, he tried to find and access the hard drive of our minds,” he concludes.
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