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In my initially neuroscience course at Columbia University, I learned about the homunculus. This “little man” is depicted as an upside-down representation of the human system relocating from toe to head in a portion of the cerebral cortex that controls motion. Wilder Penfield, the trailblazing Canadian-American neurosurgeon, created the homunculus metaphor following mapping places of the human brain by employing immediate electrical stimulation in awake patients in the 1930s.
From this do the job, a nurse doing the job with Penfield established a single of science’s most legendary illustrations. It exhibits the homunculus stretched out around the brain’s surface area, a tiny human body with enlarged mouth, palms and feet, each section exaggerated in proportion to the volume of neural serious estate occupied. Later three-dimensional representations of the homunculus depict it as a grotesque, hairless goblin with monumental lips, hands and feet. The rather terrifying creature is outstanding in just about every neuroscience textbook and even has its own Dungeons & Dragons character, with bat wings for superior evaluate.
As a scholar and later on as a practitioner in the discipline, I experienced acknowledged the homunculus as unquestioned simple fact. As a professor, I would dutifully instruct students that the distorted determine represented a essential element of brain corporation. The evidence seemed frustrating: stimulation of brain areas that corresponded to the related areas of the motor cortex, which generates indicators to direct overall body movement, elicited the predicted foot, hand and experience muscle twitches. The neuroimaging procedures equally map motion of a hand or toe to the matching mind location, and destruction from a stroke impedes movement in the anticipated overall body component. Offered all this, any new analysis of this mind location, appeared like a nonstarter. It had been definitively mapped 90 yrs ago, So, I focused my individual study elsewhere.
What eventually manufactured our team go back again to replicate Penfield’s authentic get the job done was a set of anomalous results applying a specialized variety of MRI that identifies mind networks focused to a particular operate. This sort of “resting state” purposeful neuroimaging seems to be for spontaneous activity that happens concurrently in different areas although a man or woman is lying at relaxation in a scanner.
About three decades ago, effects from these latter-day imaging approaches clashed violently with the classical neuroanatomy from Penfield’s period. The problem started when I was hoping to make your mind up how considerably to have confidence in a new process for cleaning up imaging information and experienced asked to perform what I thought was a regime exam of its validity by examining the connections in the space of the homunculus that corresponded to the hand.
The homunculus’ hand confirmed the predicted connections to the hand location in the other 50 percent of the brain, as did the applicable areas of the motor cortex that handle motion of the foot and mouth. But when we moved up or down this brain portion that will make up the homunculus, we have been in for a surprise.
A seemingly absurd sample popped out in the imaging that showed 3 interconnected and earlier undocumented spots right in the homunculus segment of the motor cortex. Motor homunculus locations have been assumed to only be linked to each and every other across the brain’s hemispheres, the still left hand with the proper hand, and the remaining foot with the right foot. Connections to different areas within the same hemisphere were being not meant to exist. I tried using to dismiss this odd seeking “three-spot” imaging motif as an mistake, but in discussing it we also acknowledged the distant chance that what we experienced noticed might someway contradict the classical neuroscience supporting a homunculus. We ongoing to explore by seeking various procedures.

I could not make sense of the final result. Why would there be this individual basically distinct profile of brain connections right in the middle of this part of motor cortex if there was only a one integral homunculus? What was this unique brain community suitable in the middle of the homunculus? I submitted the broken homunculus discovery in the “does not compute” folder of my brain and tried to operate on other projects. But our results retained bugging me.
At one particular issue, my collaborator Evan Gordon and I were analyzing information that disclosed that the mystifying established of a few places in the homunculus were being linked to a cognitive control community vital for setting up long run steps, which I had been finding out given that graduate college. These connections to better-order command regions planted the strategy that the a few-spot community may be vital for merging motor command alerts from the homunculus with neural action for far more abstract organizing.
If that have been the scenario, it implied that possibly, just it’s possible, the basic “little man” figure of the homunculus desired to be redrawn. Evan and I built a pact to chase this conflicting finding where ever it could direct, even if it appeared to obstacle the explanations discovered in neuroscience textbooks. We went on to reexamine our personal prior work, pulled in all probably pertinent information sets we could get our palms on, done new experiments and reexamined the revealed proof for and versus the homunculus story, likely back to the early 1900s.
The quest to remedy the thriller of the 3 spots was stuffed with several surprises. Early on, Evan excitedly introduced that our have prior publications had already exposed the site of the location triad. I was incredulous and fixated on these three imaging blemishes, I had spent so much time with the knowledge that they infiltrated my desires.
As part of our quest, we became aware of contradictory conclusions in macaques and other nonhuman primates, which experienced not at the time sufficed to problem the homunculus metaphor. Digging by way of Penfield’s unique mind stimulation info from about 90 yrs in the past discovered that these conflicting success ended up similarly, if not much more, dependable with other products of motor cortex organization that did not conform to the homunculus metaphor.
Looking still additional, we uncovered that our results could be reconciled with other comparatively the latest study. In 2002, Michael Graziano, a Princeton College neuroscientist, and colleagues experienced discovered that the very same place of the motor cortex is accountable in nonhuman primates for managing feeding, defensive behaviors and other actions additional complex than movement of, say, the legs or lips. This and other investigate began to nudge us towards the conclusion that, following 90 yrs, the homunculus product of the homunculus was ready for retirement. The thought of a location of motor cortex that approximately mirrored the head-to-toe alignment of the human physique could no longer prevail.
We put forward proof for our views in a the latest Nature post. In its place of a steady head-to-toe representation of system actions in a solitary homunculus, our conclusions show this neural representation of the physique is sliced into three sections, a person for the toes, a further for the palms and a third for the mouth. Separating or adjoining these spots are the areas of the mysterious three places that produced so considerably disappointment for us. All a few turn out to be interconnected, and we identified that they are responsible for a range of duties, together with organizing, regulating interior organs and even starting to be active when somebody is just pondering about a creating some movement—in essence, forming a website link in between brain and physique.
This network, which we named the Somato-Cognitive Action Community (SCAN), executes a strategy to move the entire entire body. It integrates intellect and system by linking to other brain areas managing respiratory heart level, muscle mass rigidity, even butterflies in the stomach, all of which offer feed-back for setting up potential actions—here’s what is necessary to steer clear of that upset belly or laceration future time. SCAN also connects to areas essential for generate and determination, damage to which can induce apathy. The recognition that human body manage and motor activity are represented by a widespread brain circuit assists describe why intellect and physique states so often interact.
The optimistic effects of work out or electrical stimulation of the motor cortex to relieve long-term suffering commence to make sense upon the realization that soreness and full-body actions are controlled by the exact mind network. If arousal and initiating some actual physical actions are portion of the very same network, it also would seem less paradoxical to address problems of remaining on job and hyperactivity with stimulants.
Total, our results mean that the homunculus wears no clothing. How could I have skipped for so extensive what was so apparent? It has been eye-opening to realize how powerfully my contemplating is formed by prior assumptions. My rely on in textbook teachings blinded me to facts that had been proper there in entrance of us. As time went on, our self-confidence in our knowledge grew to the stage that we at some point trusted it more than the prevailing neuroscience doctrine.
Very good tales are powerful, even in the nominally aim realm of science. The homunculus likely lived to the age of 90 simply because everybody loves a very good tale. The imagery of the distorted figure of the homunculus, with outsized lips and arms, was so powerful that it took on a existence of its have. In his e book Penfield highlighted that the homunculus was mainly a design for instructing professional medical learners that it should not be overinterpreted. That warning in no way manufactured it into the textbooks.
This is an view and analysis write-up, and the views expressed by the creator or authors are not necessarily those people of Scientific American.
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