Last week I wrote about a crazy diagram called the “homunculus,” which shows how your body would look if each part were scaled to the amount of real estate it occupies in the brain: Your hands and face would be huge, reflecting the number of neurons that correspond to those body parts, there, while your arms and legs — which are far less sensitive — would be quite small.
The homunculus seems like the kind of thing that’s either completely abstract or only knowable through sophisticated brain imaging technology. But, a couple days after the article ran, I received an e-mail from a neuroscience researcher who explained that actually, the homunculus diagram exists on the level of a kids’ science experiment.
Rebekah Corlew of the Max Planck Florida Institute for Neuroscience pointed me to a fun experiment that allows you to create your own personalized homunculus diagram. The experiment, known as “The Homunculus Mapper,” is described on the Institute’s website. To run it, you need six toothpicks, two index cards, tape, and a ruler. You tape the toothpicks to the index cards in pairs, with the toothpicks in each pair separated by varying distances ranging from 7.5 mm to 60 mm.
Once you have the cards, you’re ready to conduct the experiment, which is based on a technique known as “two-point discrimination testing.” One person, the test subject, sits with her eyes closed. Another person, the experimenter, gently presses the 60mm pair of toothpicks to her forehead. The test subject reports whether she feels one or two points pressing on her skin. If two, the experimenter moves to the next-narrowest set of toothpicks (30mm) and asks the question again. The experimenter continues this process with progressively narrower sets of toothpick pairs until the test subject reports only feeling one point. That’s the test subject’s homunculus measurement for her head. The same process is repeated for the torso, arms, hands, legs, and feet. Overall, people will be more sensitive to two-point discrimination on the parts of their bodies with more neurons dedicated to them.
As essential as it is, our central nervous system operates out of sight, which makes it especially neat to be able to use such a simple experiment to infer something about it. Of course, the coolest thing about the homunculus is that grotesque diagram itself. The Max Planck Florida Institute has you covered there. It’s created an interactive graphic that lets you pick an avatar, enter your experimental measurements, and create a neurologically scaled, personalized picture of yourself. The end result is comical and also potentially useful: Have you been telling people your whole life that you have unusually sensitive feet? Now’s your chance to prove it.
Kevin Hartnett is a writer in South Carolina. He can be reached at firstname.lastname@example.org.