In my first neuroscience course at Columbia University, I learned about homunculus. This “little man” is the part of the cerebral cortex that controls movement and is depicted as an upside-down representation of the human body moving from toe to head. Wilder, a pioneering Canadian-American neurosurgeon whose Penfield created the homunculus metaphor after mapping regions of the human brain using direct electrical stimulation in awake patients in the 1930s Did.
From this work, a nurse working with Penfield created one of science’s most iconic illustrations. It shows that the homunculus extends over the surface of the brain. It is a small body with an enlarged mouth, hands, and feet, each exaggerated in proportion to the amount of nerve area occupied. Later three-dimensional representations of the homunculus depict it as a grotesque, hairless goblin with giant lips, hands, and feet. There are also Dungeons & Dragons characters, with quite a few bat wings.
As a student, and later as a practitioner in the field, I had accepted the homunculus as an indisputable fact. taught faithfully. The evidence seemed overwhelming: Stimulation of brain regions corresponding to relevant regions of the motor cortex that produce signals directing body movements induced the expected muscle spasms in the legs, hands, and face. Neuroimaging techniques similarly map movements of fingers and toes to corresponding brain regions, and stroke damage interferes with movement of expected body parts. Given all this, new research into this brain region seemed like a novice. .
Ultimately, our team returned to replicate Penfield’s original work with a series of unusual findings using a special type of MRI that identified brain networks specialized for specific functions. bottom. Such “resting state” functional neuroimaging looks for spontaneous activity occurring simultaneously in different regions while a person is lying at rest in the scanner.
About three years ago, the results of these recent imaging methods collided violently with the classical neuroanatomy of Penfield’s time. The problem started when I was trying to decide how much to trust a new method for cleaning up image data, and by checking the connectivity of the regions corresponding to the homunculus, I was able to routinely test its effectiveness and I asked them to do what they thought they would. hand.
The homunculus hand showed expected connections to hand regions in the other half of the brain, as well as to relevant regions of the motor cortex that control foot and mouth movements. But when we moved this part of the brain that makes up the homunculus up and down, we were amazed.
A seemingly absurd pattern emerged in the images, showing three interconnected, previously undocumented regions in the homunculus section of the motor cortex. Homunculus motor regions were thought to be interconnected only across hemispheres of the brain: left and right, left and right. There should have been no connections to different locations within the same hemisphere. I tried to dismiss this odd-looking “three-point” image of his motif as an error, but in discussing it, I realized that what we saw was classical neuroscience in favor of homunculus and somehow I also acknowledged the slight possibility of inconsistency in form. I continued my research, trying different methods.
I couldn’t understand the result. Why is there only one integrated homunculus, but right in the middle of this section of the motor cortex, this separate and radically different profile of brain connections? This distinct brain in the middle of the homunculus What was the network of? I filed my broken homunculus discovery into a “do not compute” folder in my brain and tried to work on other projects. But our findings continued to haunt me.
At one point, my collaborator Evan Gordon and I were looking through the data and realized that a mysterious set of three spots on homunculus was important for planning future actions, something I had been studying since my graduate school days. It became clear that it is connected to a unique cognitive control network. These connections to higher control regions seeded the idea that three-spot networks may be important for integrating motor command signals from homunculus with neural activity for more abstract planning. .
If so, it implies that the classic “little man” figure of a homunculus may need to be redrawn. We made a pact to pursue this contradictory discovery to the end, even if it seemed to be advocating. Going back to the early 1900s, we review the published evidence for and against the homunculus story, incorporate all potentially relevant data sets available to us, and revisit previous research. Considered.
The quest to unlock the mystery of the three spots was full of surprises. Early on Evan excitedly announced that a previous publication of our own had already revealed the location of the Spotted Triad I can’t believe these three images of him hurt was sticking to I spent so much time with data that they permeated my dreams.
As part of our quest, we noticed contradictory findings in monkeys and other non-human primates that at the time were not enough to challenge the homunculus trope. Digging into Penfield’s original brain stimulation data from about 90 years ago, these conflicting results agree equally, if not more, with other models of motor cortical organization that don’t fit the homunculus metaphor. It became clear.
Upon further investigation, we found that our findings may be consistent with other relatively recent studies. In 2002, Princeton University neuroscientist Michael Graziano and colleagues found that the same regions of the nonhuman primate motor cortex control feeding, defensive behavior, and other movements that are more complex than those of the feet and lips. I discovered that This and other studies have begun to move us toward the conclusion that, 90 years later, the homunculus model of homunculus is ready to retire. The idea of the motor cortex area could no longer prevail.
We have provided evidence of our recent views Nature article. Instead of representing a single homunculus body movement continuously from head to toe, our findings show that this neural representation of the body is sliced into three sections: one one for the feet, one for the hands and one for the mouth. Separating or adjoining these areas are the locations of three spots of mystery that have caused us much frustration. , and even become active when someone is thinking of making a move. and body.
The network, named the Somato-Cognitive Action Network (SCAN), implements a plan to move the whole body. Integrate mind and body by linking to other brain regions that control breathing heart rate, muscle tone, and even stomach butterflies. All of these provide feedback for planning future actions. Here’s what you need to do to avoid stomach upsets and lacerations: SCAN also connects to areas important for drive and motivation, and may induce apathy through injury. It helps explain why they interact so often.
The positive effects of exercise or electrical stimulation of the motor cortex for relieving chronic pain begin to make sense when we realize that pain and whole-body movement are controlled by the same brain networks. If arousal and the initiation of some physical behaviors are part of the same network, then treating difficulty in staying at work and hyperactivity with stimulants does not seem paradoxical either.
Overall, our findings imply that homunculus don’t wear clothes. How could I have missed something so obvious for so long? It is amazing to see how strongly my thinking is shaped by previous assumptions. I was blinded by the data in front of me. Over time, our trust in our data grew, and we eventually came to trust data more than common neuroscience dogma.
A good story is powerful, even in the nominally objective realm of science. The homunculus says he likely lived to be 90 because everyone likes a good story. The image of a distorted figure of a homunculus with oversized lips and hands was so compelling that it brought life to itself. In his book, Penfield emphasized that homunculus is primarily a model for teaching medical students. It should not be over-interpreted. That warning never made it into textbooks.
This is an opinion and analysis article and the views expressed by the author or authors are not necessarily Scientific American.
