Testing Your Sense of Touch

In this experiment one lab partner will gently touch the other with two pencils, close together. You will examine how different parts of the body are better are detecting the difference between one and two pencil touches. This is because different body parts (hands vs. shoulder for example) have a different density of nerve endings.

Only poke gently!

• 2 people

• 2 pencils

• ruler (optional)

1. Have one partner look away or close their eyes.
2. The second partner should gently touch the first person’s finger with the tips of two pencils at the same time. Have the tips close together, but not touching.
3. The first person should see if they can feel two distinct tips or not. If they can, try again, with the tips closer. If they can’t, try again with the tips farther apart. Remember or record how far the pencil tips had to be from each other for the person to detect two tips.
4. Now repeat 1-3, only try the back of your partner’s hand, shoulder, or calf.

1. Note how different body parts have different abilities to resolve the two tips.

Notes

• You can take turns if this is more interesting.

• One interesting variation is to sometimes only touch the person with one tip. If someone “knows” they are supposed to feel two tips, they might be inclined to say they do if they are uncertain. You can instead try to have them guess one or two tips, and do 10 guesses at each distance on each body part. Record your results. (Remember that random guessing would have you guess correctly 5 times out of 10 on average).

Q: Did you notice some body parts were better at discriminating between one and two touches, or were they all the same.

A: Some body parts should be better than others.

Q: Why do you think this is true?

A: See below.

Some body parts are specialized or designed to be good at sensing things. Fingers, for example, are used to manipulate objects, and we use touch as a guide. Even complex tasks, such as tying your shoes, can easily be done with your eyes closed by using the great sense of touch in your fingers. Because other body parts, like shoulders, aren’t used for fine manipulation or sensation, they do not have as dense a network of nerves. A shoulder still has some nerves – you need them to tell if you’ve been injured, for example – but you don’t need as many.

Your ability to discriminate between one and two touches can be called resolution. Because your fingers can detect two touches, even when they are very close together, we say they are good at resolving physical stimuli.

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