Background

Aristotle was a keen observer and is the first to record two very interesting sensory phenomena, one of which will be described here. This phenomenon is an illusion of touch. This illusion still bears his name (Benedetti, 1985).

The question to be discussed here is how do we know if we are touching one or more than one object? When you touch something with your hand, it touches several parts of your hand. Look at the photograph below.

A person is holding a pen but it touches the tips of each finger. Still the pen will be felt as a single pen and not five separate objects. Aristotle's illusion gives some insight into this question. Follow the instructions on the next page.

Instructions

What you will need:
   • A pen or pencil
   • Two people, preferably
   • Your hand (and the hand of your partner if you are using one)

This illusion works best if you try it on a partner and your partner tries it on you. These instructions will be written assuming you have two people performing this task, but you can try it just on your hand.

First, try the non-illusory condition.
. Have your partner put out their hand and hold their two first two fingers out and together, like in the photograph below. It helps if they either close their eyes or look away. After they are ready, place your pen or pencil to touch the insides there the two finger meet as shown in the photograph. Try to make the pen or pencil touch both fingers as the same time. Ask your partner what they feel? Clicking on the photograph will show you a larger version of the photograph.

Now, try the illusory condition.
. Again, have your partner put out their hand, but this time have them cross their first two fingers. It still helps if they either close their eyes or look away. As before, place your pen or pencil to touch the insides there the two finger meet as shown in the photograph below. In this condition, it is even more important to touch both fingers as the same time. Ask your partner what they feel?

Now switch roles and have your partner try the illusion on you.

More Information

In the first condition, your partner and you should feel one pen or pencil. That makes sense. The oddity is that when your cross your fingers, most of you will feel two touches at the same time. Without going into too much informtion which will be covered later in the text, notice what parts of the fingers are touched in each case.

In the first condition, the parts of the skin touched usually face each other. You touched the sides of the fingers that are near each other and often will both be touched when we reach out and grab a single object. When you crossed your fingers, you are touching the outsides of each finger. These sides face away from each other. Usually when they are touched, it is by two different objects. It is worth considing how our nervouse system might be organized to take advantage of this fact and represent it in our brain.

In addition, it is thought that the mechanoreceptors that are being used in this illusion are the slow-adapting I (SAI) receptors. These receptors have small receptive fields and sustained response, both of which help them have high spatial resolution. This resolution is the basis of the two-point threshold which can be thought of as the touch analog to acuity.

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