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You're Getting On My Nerves! What Is The Two-Point Threshold?

Here's a fun experiment that tells you something about your nervous system. You can do it on yourself, but it works best if you do it on someone else, and vice versa. All you need is two pointed objects, such as pencils.

Try This!

Have your friend close her eyes, and tell her you're going to touch the inside of her forearm with the two pencil points simultaneously about eight inches apart. Then you're going to lift the points and bring them down again and again, each time moving the points closer together. She's to tell you when you're touching her with only one pencil.

It's surprising that when she thinks she's being touched by only one point both are still touching her arm, and they are one to two inches apart!

Now do the same thing on the pad at the end of her index finger, but start with the points about an inch apart. This time when she thinks she feels only one point the two will be about an eighth of an inch apart.

Why Do This Work?

Why does she feel only one point when she's being touched by two? And why on the forearm are the points as much as two inches apart when one point is felt, but only an eighth of an inch apart on the fingertip?

It's because some areas of the body, such as the fingertips have more nerves going to them. And, the areas of the brain which receive information from these sensitive areas of the body have a greater density of nerves. So, since more nerves are present to detect sensations these areas are more discriminating.


This is an interesting experiment, but there is a useful side to the information. This psychological phenomenon is known as the two-point threshold, and it can be used to test for nerve damage.

Also, Braille dots were designed so that they were farther apart than the two-point threshold.

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