How Your Brain Sees You

YOU HAVE A PRETTY GOOD IDEA OF what you look like from staring into the mirror each morning. But your body's appearance is radically different from the way your brain perceives it, because the sensory nerves in your skin that send messages to the brain are more densely packed in some areas than in others. For example, the touch sensors in a single fingertip are 15 times more numerous than those on the legs, so the area of the cerebrum--the somatosensory cortex--representing one finger is much larger than that representing a leg. This means that the brain's skin-sensation map, called the homunculus, or "little person," looks weird. Try the two experiments on this page to find out more about how your body looks to your brain.


EXPERIMENT 1

Place the tip of your tongue under your front teeth and feel the small ridges of their cutting surfaces. Now place the back of your hand against the same teeth, observing how much less sensory detail is perceptible. That's because there are more than 15 times as many touch receptors per square inch on your tongue than there are on the back of your hand. Your homunculus thus shows considerably more space devoted to the tip of your tongue than it does to the skin on the back of your hand.


EXPERIMENT 2

To draw your own homunculus you'll need a notepad, two toothpicks and a metric ruler to gather data. Persuade a trusting friend to sit with his eyes closed while you gently stick him with the pointy ends of the two toothpicks at these 10 body locations: middle of the forehead; right temple; right cheek; meaty part of the upper lip; middle of the right bicep; tip of the right index finger; Adam's apple; stomach just above the belly button; upper thigh; and tip of the right big toe. For each point, start by positioning the toothpicks close together. Then gradually move the toothpicks farther apart, making sure they both touch the skin at the same time. Keep asking if your subject feels one point or two as you poke. Measure and jot down the distance between the toothpicks--in centimeters--the first time your friend says "two," then move on to the next location and repeat the entire process. When I tried this, I first made a "normal" sketch of myself using the measurements from 10 different patches of my skin, which I then used to produce the homunculer self-portrait. For example, I took the two-point distance on my stomach, 4 centimeters, and multiplied its reciprocal (0.25) by the size of the normal torso in my drawing to come up with this minuscule torso. Make a rough sketch of yourself, then distort it with the scale factors for a homunculus collected from your own experiment. Then sit back and look your own brain in the eye. View more issues: Eric Haseltine "How Your Brain Sees You". Discover. . FindArticles.com. 05 Dec. 2008. http://findarticles.com/p/articles/mi_m1511/is_9_21/ai_64698214





What Does Your "Homunculus" Look Like? Mapping Your Brain Alice Kagi, Judith Kemlitz, Warren Marchioni, and Patricia Seybert 1991 Woodrow Wilson Biology Institute

http://www.accessexcellence.org/AE/AEPC/WWC/1991/homunculus.php