| Are your lips more sensitive than your fingertips? Is the skin on your legs as sensitive as the skin on your arms and have our chimp-ancestors left us with feet as sensitive as our hands? To find out, try our two-point discrimination experiment. All over your body you have tiny pressure sensors in your skin. Some areas have many pressure sensors and other areas have relatively few. Areas where the pressure sensors are packed in are very sensitive to touch. This two-point discrimination experiment lets you discover which parts of your skin have many pressure sensors, and where they are relatively sparse. The experiment involves gently pressing two sharp points onto a friend’s skin and asking them whether they can feel one or two pressure points. |
| In sensitive areas, the pressure sensors are closer together, so the two tips of the paper clip will activate two separate pressure points. Messages from both sensors are sent to your brain and you feel both tips. In less sensitive areas, the pressure sensors are further apart, so both tips activate only a single pressure sensor. A message from this sensor is sent to your brain, and you only feel one tip on your skin. For this experiment you will need: A paperclip A pen and paper A ruler A friend You can also try this on your own, with your eyes shut, concentrating on what you feel. Instructions: 1. Straighten out a paper clip and bend it into the shape of a U. Make sure the tips are level with each other. 2. Now, ask a friend to close their eyes. Arrange the ends of the paper clip so they are about 1 cm apart. 3. Touch both ends of the paper clip gently (and at the same time) onto the back of your friend’s hand and ask if he or she felt one pressure point or two? 4. If your friend felt only one pressure point, spread the tips of the paper clip and try again. Write down the distance where your friend goes from feeling one pressure point, to feeling two pressure points. 5. To make it difficult for your friend to guess the answer, vary both the distance between the tips, and whether you actually place one or both tips on their skin. Try this test on the palm and back of the hand, fingertips, forearm, upper arm, shoulder, back, neck, cheek, forehead, lips, nose, legs, tips of the toes, soles and upper parts of your feet. Which part of your body can two points be detected with the smallest tip separation? This is the most sensitive part of your body. Below are some approximate values for this two-point-discrimination test (from The Skin Senses, edited by D. R. Kenshalo) |
| Fingers | 2-3 mm |
| Upper lip | 4-5 mm |
| Cheek | 6 mm |
| Nose | 7 mm |
| Palm | 10 mm |
| Forehead | 35 mm |
| Belly | 30 mm |
| Forearm | 35 mm |
| Upper arm | 39 mm |
| Shoulder | 41 mm |
| Thigh | 42 mm |
| Calf | 45 mm |
| Foot | 20 mm |