How Our Staff Overturned the 90-Yr-Previous Metaphor of a ‘Little Man’ within the Mind Who Controls Motion
In my first neuroscience course at Columbia College, I realized in regards to the homunculus. This “little man” is depicted as an upside-down illustration of the human physique shifting from toe to move in a portion of the cerebral cortex that controls motion. Wilder Penfield, the trailblazing Canadian-American neurosurgeon, created the homunculus metaphor after mapping areas of the human mind by utilizing direct electrical stimulation in awake sufferers within the Thirties.
From this work, a nurse working with Penfield created considered one of science’s most iconic illustrations. It reveals the homunculus stretched out over the brain’s surface, a small physique with enlarged mouth, arms and toes, every half exaggerated in proportion to the amount of neural real estate occupied. Later three-dimensional representations of the homunculus depict it as a grotesque, hairless goblin with monumental lips, arms and toes. The considerably horrifying creature is distinguished in each neuroscience textbook and even has its personal Dungeons & Dragons character, with bat wings for good measure.
As a scholar and later as a practitioner within the subject, I had accepted the homunculus as unquestioned truth. As a professor, I’d dutifully educate college students that the distorted determine represented a key facet of mind group. The proof appeared overwhelming: stimulation of mind areas that corresponded to the related areas of the motor cortex, which generates indicators to direct physique motion, elicited the anticipated foot, hand and face muscle twitches. The neuroimaging methods equally map motion of a hand or toe to the matching mind space, and harm from a stroke impedes motion within the anticipated physique half. Given all this, any new analysis of this mind space, appeared like a nonstarter. It had been definitively mapped 90 years in the past, So, I targeted my very own analysis elsewhere.
What finally made our crew return to copy Penfield’s unique work was a set of anomalous findings utilizing a specialised kind of MRI that identifies mind networks devoted to a specific perform. Such “resting state” purposeful neuroimaging seems for spontaneous exercise that happens concurrently in numerous areas whereas an individual is mendacity at relaxation in a scanner.
About three years in the past, outcomes from these latter-day imaging strategies clashed violently with the classical neuroanatomy from Penfield’s period. The issue began once I was making an attempt to determine how a lot to belief a brand new technique for cleansing up imaging knowledge and had requested to conduct what I assumed was a routine check of its validity by checking the connections within the space of the homunculus that corresponded to the hand.
The homunculus’ hand confirmed the anticipated connections to the hand area within the different half of the mind, as did the related areas of the motor cortex that management motion of the foot and mouth. However once we moved up or down this mind part that makes up the homunculus, we have been in for a shock.
A seemingly absurd sample popped out within the imaging that confirmed three interconnected and beforehand undocumented areas proper within the homunculus part of the motor cortex. Motor homunculus areas have been thought to solely be linked to one another throughout the mind’s hemispheres, the left hand with the appropriate hand, and the left foot with the appropriate foot. Connections to various places throughout the similar hemisphere weren’t alleged to exist. I attempted to dismiss this bizarre trying “three-spot” imaging motif as an error, however in discussing it we additionally acknowledged the distant risk that what we had seen would possibly one way or the other contradict the classical neuroscience supporting a homunculus. We continued to discover by making an attempt totally different strategies.
I couldn’t make sense of the consequence. Why would there be this separate essentially totally different profile of mind connections proper in the midst of this part of motor cortex if there was solely a single integral homunculus? What was this distinct mind community proper in the midst of the homunculus? I filed the damaged homunculus discovery within the “doesn’t compute” folder of my mind and tried to work on different tasks. However our findings stored bugging me.
At one level, my collaborator Evan Gordon and I have been inspecting knowledge that exposed that the mystifying set of three spots within the homunculus have been linked to a cognitive management community essential for planning future actions, which I had been learning since graduate college. These connections to higher-order management areas planted the concept the three-spot community is perhaps essential for merging motor command indicators from the homunculus with neural exercise for extra summary planning.
If that have been the case, it implied that perhaps, simply perhaps, the traditional “little man” determine of the homunculus wanted to be redrawn. Evan and I made a pact to chase this conflicting discovering wherever it would lead, even when it appeared to problem the reasons present in neuroscience textbooks. We went on to reexamine our personal prior work, pulled in all doubtlessly related knowledge units we may get our arms on, performed new experiments and reexamined the revealed proof for and towards the homunculus story, going again to the early 1900s.
The search to unravel the thriller of the three spots was stuffed with many surprises. Early on, Evan excitedly introduced that our personal prior publications had already revealed the placement of the spot triad. I used to be incredulous and fixated on these three imaging blemishes, I had spent a lot time with the info that they infiltrated my goals.
As a part of our quest, we grew to become conscious of contradictory findings in macaques and different nonhuman primates, which had not on the time sufficed to problem the homunculus metaphor. Digging by means of Penfield’s unique mind stimulation knowledge from about 90 years in the past revealed that these conflicting outcomes have been equally, if no more, according to different fashions of motor cortex group that didn’t conform to the homunculus metaphor.
Wanting nonetheless additional, we found that our findings could possibly be reconciled with different comparatively current analysis. In 2002, Michael Graziano, a Princeton College neuroscientist, and colleagues had discovered that the identical space of the motor cortex is accountable in nonhuman primates for controlling feeding, defensive behaviors and other actions extra advanced than motion of, say, the legs or lips. This and different analysis started to nudge us towards the conclusion that, after 90 years, the homunculus mannequin of the homunculus was prepared for retirement. The thought of a area of motor cortex that roughly mirrored the head-to-toe alignment of the human physique may now not prevail.
We put ahead evidence for our views in a current Nature article. As an alternative of a steady head-to-toe illustration of physique actions in a single homunculus, our findings present this neural illustration of the physique is sliced into three sections, one for the toes, one other for the arms and a 3rd for the mouth. Separating or adjoining these areas are the places of the mysterious three spots that produced a lot frustration for us. All three develop into interconnected, and we discovered that they’re liable for a spread of duties, together with planning, regulating inner organs and even turning into lively when somebody is simply occupied with a making some motion—in essence, forming a hyperlink between thoughts and physique.
This community, which we named the Somato-Cognitive Motion Community (SCAN), executes a plan to maneuver the entire physique. It integrates thoughts and physique by linking to different mind areas controlling respiratory coronary heart fee, muscle stress, even butterflies within the abdomen, all of which give suggestions for planning future actions—right here’s what is required to keep away from that upset abdomen or laceration subsequent time. SCAN additionally connects to areas essential for drive and motivation, harm to which may induce apathy. The popularity that physique management and motor exercise are represented by a standard mind circuit helps clarify why thoughts and physique states so typically work together.
The optimistic results of train or electrical stimulation of the motor cortex to alleviate power ache begin to make sense upon the belief that ache and whole-body actions are managed by the identical mind community. If arousal and initiating some bodily actions are a part of the identical community, it additionally appears much less paradoxical to deal with difficulties of staying on process and hyperactivity with stimulants.
General, our findings imply that the homunculus wears no garments. How may I’ve missed for therefore lengthy what was so apparent? It has been eye-opening to appreciate how powerfully my pondering is formed by prior assumptions. My belief in textbook teachings blinded me to knowledge that have been proper there in entrance of us. As time went on, our confidence in our knowledge grew to the purpose that we finally trusted it greater than the prevailing neuroscience doctrine.
Good tales are highly effective, even within the nominally goal realm of science. The homunculus probably lived to the age of 90 as a result of everybody loves a superb story. The imagery of the distorted determine of the homunculus, with outsized lips and arms, was so compelling that it took on a lifetime of its personal. In his e book Penfield highlighted that the homunculus was primarily a mannequin for educating medical college students; that it should not be overinterpreted. That warning by no means made it into the textbooks.
That is an opinion and evaluation article, and the views expressed by the writer or authors should not essentially these of Scientific American.