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Fake Hand

Which is our dominant sense?

If we get conflicting signals from different senses, which one do we tend to believe? Students investigate the answers to these questions in the following activity.

Can you touch your finger to your chin with your eyes closed? Our brain has an internal "map" of our entire body that allows us to do things like this. Even without the visual guidance of our sense of sight, our fingers still know where to go. Our sense of the relative positioning of the different parts of our body is referred to as proprioception.

This internal map is so ingrained in us, that people who have lost limbs due to accident or amputation will often report still being able to feel sensations from their (missing) limb–a phenomenon known as the Phantom Limb Syndrome. Many people will continue to experience this sensation for months after their accident, until their brain develops a new map of their body.

Our proprioception can also be manipulated the other way around, such that we come to think that something that looks similar to one of our body parts actually IS one of our body parts.

When we receive conflicting inputs from different senses, we tend to believe what our eyes are telling us above all else, as vision is the dominant sense in humans. In this activity, when we see a fake hand being stroked while also feeling our real-but-hidden hand being stroked, we can be induced into believing that our real hand has somehow been transposed into the fake one.

Teacher's Tip: This activity is best performed as a station so that all of the students get a chance to give it a try.

Fun Fact: Anecdotal evidence from Science World testing seems to indicate that males are more susceptible to this illusion than females.

Objectives

  • Understand that tactile information is processed in the brain.

Materials

  • Per Demo or Group:
    rubber fake hand (child-size if possible – available in Halloween/costume stores or Amazon)
    2 paint brushes
    2 black cloths (big enough to cover a student’s entire forearm)
    standing barrier (e.g. a hard-cover book)
    hammer (optional)

Key Questions

  • Why did you jump, even though you knew it wasn’t your hand that was hit?
  • In this activity we got conflicting signals from different senses. Which sense did you believe?

What To Do

  1. Have a volunteer sit at the table, with their left hand resting on the table a little off to the side.
  2. Cover their entire forearm with one black cloth, leaving only their hand and wrist exposed.
  3. Place the fake rubber hand directly in front of the volunteer, and cover it with a second black cloth, exposing the same amount of wrist/hand surface as the volunteer’s own hand.
  4. Set up the barrier between the fake hand and the real one, such that the volunteer cannot see their real hand.
  5. Take two paint brushes and stroke the fake hand and real hand simultaneously (i.e. 1 paintbrush for each hand), in long slow strokes from wrist to fingertip. It is CRUCIAL to have the exact same brush placement on both hands: if you are stroking the index finger on the fake hand, be sure you are also stroking the index finger on the volunteer’s real hand.
  6. After approximately one minute of stroking, take out the hammer (or use your fist) and pound hard on the fake hand. See if your volunteer jumps!