The motor homunculus goes to Guatemala

Whether you’re a linguist like me and you focus on the organs of speech, or you’re a physician who focuses on the hand, the amount of brain “real estate” that is devoted to each of them reflects the fact that both of them are central to being a human being.

The odd guy in this picture?  He’s “the motor homunculus.”  The picture represents the proportions of the motor cortex that are dedicated to controlling the movements of the parts of our body whose movements we can control.  The motor cortex is a part of the outer layer of the brain that is used for controlling movement.  Note that not all parts of the body get equal amounts of brain dedicated to them.  Some get more than others, and the relative sizes of the body parts in the picture reflect those unequal amounts.  Which parts get the most?

  1. The organs of speech
  2. The hand

For decades, people like me have been showing this figure to our Linguistics 101 students and saying: you can tell how important the organs of speech are because as much of the motor cortex is devoted to them as to the control of our hands.  Neurologist Frank Wilson sees it the other way around, though.  His take on it: you can tell how important the hands are because as much of the motor cortex is devoted to them as to the control of our organs of speech.  I like that–it’s always interesting when people see things the opposite of the way that do.

How do we know how much of the brain is devoted to any organ?  It all goes back to a Canadian-American neurosurgeon by the name of Wilder Penfield.  Penfield was a pioneer of modern brain surgery.  He developed a procedure for treating epilepsy by finding the region of the patient’s brain from which the unfortunate electrical storms originate, and destroying it.  When you’re doing this, you don’t want to destroy a part of the brain that carries out some irreplaceable function, so Penfield developed a procedure for stimulating parts of a patient’s brain and watching what happened.

The oropharynx–just the beginning of the sophisticated and complicated process of swallowing. Spelling error: that should be PHARYNGEAL wall. Source:

Now, I know what you’re thinking: but, you can’t see everything that might happen–it’s not like you can watch someone’s pharynx and see what part of the brain we use to control the incredibly complicated process of swallowing a bite of pizza.  That’s a good point.  As Dr. Peter Pressman, a neurologist at the University of Colorado School of Medicine, told me:

You’re right, Zipf–he didn’t exactly pull up each individual muscle.  More like “hand, arm, throat, etc.”  His was a rough map, though revolutionary at the time.  It turns out that more recent work using functional magnetic resonance imaging has led to essentially the same findings–Penfield’s work was amazing.

The take-home point: whether you’re a linguist like me and want to focus on the organs of speech, or a physician who wants to focus on the hand, the amount of brain “real estate” that is devoted to each of them reflects the fact that both of them are central to being a human being.

Dr. Courtney Retzer-Vargo making a splint.

Once a year I travel to Guatemala with Surgicorps, a group of surgeons, therapists, nurses, and anesthesiologists who spend a week donating free surgical services for people for whom the almost-free national health care system is too expensive.  We bring with us specialists who can perform techniques that are beyond the skills of the local surgeons.  The team includes Dr. David Kim, who specializes in hand surgery, and Dr. Courtney Retzer-Vargo, an occupational therapist who specializes in rehabilitation of the hand.  These are both exceptionally rare skill sets–Dr. Kim did two separate four-year fellowships (in plastic surgery and in orthopedics) to learn his trade, and Dr. Retzer-Vargo is one of a very small number of people in the world with her specialized skills.  Their work is an important part of what we do because giving someone back the ability to use their hands can mean keeping them alive in this country where most work is manual labor, and if you don’t work, you starve–as do your children.

Surgicorps members pay for their own travel, lodging, and food on these missions–and donate a week of vacation time (that’s a lot in the United States), as well as their professional services.  Donations from generous people like you go entirely to covering the costs of the surgeries and pre- and post-operative care.  This includes supplies, oxygen and anesthetic gases, medications, lab work, and lodging for the family members who accompany them on the long trip to the facility out of which we work.  To give you some perspective: the cost of surgery for one patient works out to $250.  $100 pays for four surgical packs.  $10 pays for all of the pain medications that we will send our patients home with this week.  Want to help?  Follow this link to make a donation–you’ll be surprised at how good it will make you feel.

English notes

homunculusa small man.  The concept of tiny little people was an important but wrong idea about how exactly our physical bodies get made: before we actually knew anything about embryology, the idea that we start out as so-tiny-that-we’re-invisible fully formed humans whose development consists simply of getting bigger seemed to make about as much sense as anything else.  (This idea is known as preformation–see the Wikipedia article about it for its history.)

Later conceptions of the homunculus have focussed on the extent to which we can think of it as a “representation” of the human–something that lets us think logically about people by simplifying them down to the elements that are essential to whatever it is that we’re trying to figure out about them.  For example, the motor homunculus simplifies the human to a set of purposeful movements.  Every representation has its benefits–in this case, the ability to have a 1200-word discussion about what the brain can tell us about the parts of humans that are most important to making them…human.  Representations also have their costs.  For example, representing an entire human being as a motor homunculus doesn’t let us say anything about why a human might want to move something.  Life always has its trade-offs–how about trading a few of your spare dollars/euros/quetzales for the warm feeling of contributing to Surgicorps making it possible for a woman to cook her child’s tortillas in the morning, or for a man to earn the money to send that child to school?  Click here to donate.



3 thoughts on “The motor homunculus goes to Guatemala”

  1. Pioneers in any field are extraordinary, breathtakingly so, if one takes the time to actually assess what they achieved which has been shrugged into our lives over time. The brain is as mysterious as the depths of the ocean but this Penfield chap paved the way through his amazing work for so many people who follow behind. That his work is proven through modern methods is wonderful but more wonderful are the countless numbers of people who were helped in the meantime by his endeavours. I think more of my brain is devoted to speech than hands, by the way …. I’m not at all dexterous but quite articulate 😉

    Liked by 2 people

  2. I am using my fat, completely non-dextrous fingers to type this comment on my phone so my reply will be far less articulate than Osyth’s but — what an amazing group of people and an entirely worthy cause! 😊

    Liked by 2 people

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