One Point on Your Back, Two on Your Fingertip? The Mystery of Your Skin Sensors, Revealed with Toothpicks
Hi, I’m Ken Kuwako, your Science Trainer. Every day is an experiment!
Close your eyes and gently press two toothpicks against your back. Even though there are two of them, it feels like just one. Now press those same two toothpicks against your fingertip, and you can clearly feel two separate points. Same body, so why the big difference? In this article, I’ll show you an experiment that uses your own body to explore the mystery of how your skin senses touch.
Goal
Find out how touch sensitivity differs from one part of the body to another.
What You’ll Need
Toothpicks (2 per person), a ruler, and cellophane tape
Steps
1. Tape two toothpicks to a ruler, spaced 5 mm apart.
2. With your eyes closed (or without looking), lightly touch both toothpicks to your skin at the same time.
Image generated with ChatGPT
3. Note whether it felt like one point or two.
4. Try it on several body parts: your tongue, lips, fingertip, palm, arm, foot, and so on.
A few safety notes:
・Toothpicks have sharp tips, so be careful. No horsing around, and never touch them to another person.
・Use only the lightest touch, so the toothpicks never break the skin.
・Test only on your own body, and stay safe!
Results
The spots where most students felt two points were the tongue, lips, and fingertips.
The spots where they felt just one point were the palm, the back of the hand, the arm, and the foot.
On the tongue and fingertips, the two points were clearly distinct even though they were only 5 mm apart. The lips were also quite good at picking out two points. On the arm and foot, though, the two toothpicks blended into one. The palm was much harder to read than the fingertip, and most people felt just one point there.
By the way, the smallest distance at which you can still tell two points apart is called the “two-point discrimination threshold.” Here are some typical values: about 1 to 2 mm for the tip of the tongue, 2 to 3 mm for the fingertip, 4 to 5 mm for the lips, around 10 mm for the palm, 30 to 40 mm for the arm and foot, and 40 to 70 mm for the back. That’s why, with toothpicks 5 mm apart, the tongue, fingertips, and lips usually feel like two points, while everything beyond the palm tends to feel like one.
Discussion: Why Does It Differ by Body Part?
Your skin is packed with receptors (sensory points) that detect pressure and touch. Areas where you feel two points are thought to have a dense crowd of these receptors. When receptors are packed closely together, two stimuli that are a little apart get picked up separately, so you feel two points. Where receptors are sparse, both toothpicks land within the territory of the same receptor, and you feel just one.
So why does receptor density vary across the body? The likely answer is that each body part has a different job.
Your tongue and lips act as security checkpoints, closely inspecting the size, temperature, texture, and any foreign objects in your food before deciding whether it’s safe to let in. Your fingertips are like the body’s high-performance sensors, grabbing things and reading the roughness of surfaces. Parts like these probably pack in lots of sensory points so they never miss even the tiniest differences.
Your arms, legs, and back, on the other hand, are mostly there to support and move your body rather than to examine things in fine detail. They can get by just fine with fewer sensory points.
This Mechanism Is at Work All Around Us
Take Braille, for example. People who are blind read it by running their fingertips over the dots. The dots are spaced a little over 2 mm apart, which matches the distance a fingertip can tell apart as two separate points. If you tried to read Braille with your back, all the dots would blur together into one continuous surface.
The brain plays a part, too. It has dedicated regions that receive signals from each part of the body, and the more sensitive the body part, the more brain real estate it gets. A figure that draws the body according to this map is called a “homunculus,” and it looks wonderfully strange, with giant hands, lips, and tongue. So it’s not just the number of sensory points, but also the amount of processing power your brain devotes to them, that shapes how sensitive each area is.
Image generated with ChatGPT
Touch sensitivity isn’t the same all over your body. The places that need the finest detail are the most sensitive, and your body allocates its sensors accordingly. Investigating your own body like this is a great reminder of how cleverly it’s built to suit each part’s job.
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