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Brainiac

Why are my feet so tiny? Your body, according to your brain

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You may have seen that deliberately annoying “View of the World from Ninth Avenue” map featured on the cover of the New Yorker a while back. It shows the distorted way geography appears to a Manhattanite: 9th and 10th avenues are the center of the world, New Jersey appears, barely, and everywhere else is just a blip if it registers at all.

As it turns out, a similar kind of map exists for the human body — with at least some basis in neuroscience. In August I wrote a story for Ideas on the rise of face transplants and spoke to Michael Sims, author of the book, “Adam’s Navel: A Natural and Cultural History of the Human Form.” During our conversation Sims mentioned an odd diagram published in 1951 by a neurosurgeon named Wilder Penfield. The diagram is known as “Homunculus” (a name taken from a weird and longstanding art form that depicts small human beings); it shows the human body scaled according to the amount of brain tissue dedicated to each part, and arranged according to the locations in the brain that control them.

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In the diagram, the eyes, lips, nose, and tongue appear grotesquely large, indicating that we devote an outsized amount of brain tissue to operating and receiving sensation from these parts of the body. (Sims’s point was that we devote a lot of processing power to the face, and for that reason find it biologically disorienting that faces could be changeable.) The hand is quite large, too, while the toes, legs, trunks, shoulders, and arms are tiny, the equivalents of Kansas City and Russia on the New Yorker map.

“Homunculus” seems like the kind of thing that would have long since been superseded by modern brain science, but it actually continues to have a surprising amount of authority, and often appears in neuroscience textbooks. When I looked up recent efforts to revise the diagram, I found mainly small adjustments rather than wholesale revision. For example, a 2005 paper in the Journal of Neuroscience used fMRI to show that while Penfield placed the genitals below the toes on his diagram of the cortex, really they fall between the toes and the abdomen (see the image below). It’s the kind of error we’d never make when studying the actual human corpus, but which crops up easily when trying to pin things down in the murky geography of the brain.

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Kell C A et al. J. Neurosci. 2005;25:5984-5987

Kevin Hartnett is a writer in South Carolina. He can be reached at kshartnett18@gmail.com.