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Comb Dot

In this illusion, students take advantage of a (typically) unfamiliar sensation for our fingertips' touch receptors: having the skin on our fingertips stretched.

When we place our fingertips against a brush-like surface with semi-rigid bristles, and then move the bristles back and forth, the skin on our fingertips becomes alternately stretched and compressed by the undulating motions. Because of the novelty of this particular sensation, our touch receptors have difficulty interpreting it properly. Instead, we reinterpret the stretch/compress motion as the more familiar sensation of running our finger across a raised surface, like a dot.

Objectives

  • Understand that tactile information is processed in the brain.

Materials

  • Per Student Pair:
    a pencil
    a plastic comb (flat comb with flexible bristles)

Key Questions

  • What do you feel?
  • Do you feel the bristles move from side to side?
  • Why do you think we feel something different to what is actually happening?
  • In this case, which sense has taken over: sight or touch?

What To Do

  1. Pair up with a partner.
  2. Take the comb and hold it sideways (as it would naturally lie on a flat surface).
  3. Hold the comb in one hand, and place the index finger of your other hand on top of a section of bristles.
  4. Have your partner hold the pencil vertically–perpendicular to the horizontal comb–and run the pencil continuously along the bristles of the comb horizontally.
  5. Swap roles with your partner.