All Resources

Finger-To-Elbow

In this activity, students discover how the physical distribution of sensory receptors in our bodies can create a tactile illusion.

Sensory receptors are not distributed evenly across our bodies. Some parts of our body, like our fingertips, are very densely packed with nerve endings and touch receptors. They are highly sensitive to touch, and can differentiate between even very fine, minute touch sensations. Other parts of our body, like our forearms, are less packed with touch receptors, so they are not as good at sensory discrimination. If you move from an area of higher sensory discrimination to a lower one, you can experience something very odd – two distinct sensations can suddenly feel like just one!

Objectives

  • Understand that tactile information is processed in the brain.

Materials

  • Per Student Pair:
    paper clip

Key Questions

  • Why does it feel like your partner has reached your elbow when they haven’t?
  • Where can you tell there are two "clip-tips" and where can you only distinguish one?
  • Which part(s) of your arm have more sensory receptors? How do you know?

What To Do

Part 1: Finger

  1. Pair up with a partner and sit facing each other.
  2. Have your partner hold out their left arm and close their eyes.
  3. Lightly place your finger on their index fingertip, and SLOWLY move your finger up their forearm towards their elbow (keep your finger on the inside of their forearm and move towards the ‘crook’ of their elbow).
  4. Let your partner know to tell you when they think you’ve reached their elbow. They can open their eyes to see how accurate they were!
  5. Swap roles with your partner.

Part 2: Paper Clip

  1. Unfold a paper clip into one straight line, then bend it in half so it looks like a horseshoe, with the clip-tips about 1 cm apart.
  2. Repeat the activity of part one, but gently use the two tips of the paper clip instead of your finger.
  3. Have your partner tell you when they no longer feel two distinct clip-tips, but one.
  4. Swap roles with your partner.

Extensions

  • Try Parts 1 and 2 going the opposite way (i.e. from crook-of-the-elbow to fingertip).