How does your brain perceive your body in space?
- You and a friend
What to Do
Tell your friend you would like to test some experimental illusions on them. These work best if your friend does not know “the point” of the experiment or what to expect.
- Have your friend close their eyes and stick one arm out to the side with their fingers pointed straight out.
- Stand behind them and use one or two fingers to taps them four times at the wrist, three times just above the elbow, and twice at the upper arm. This works best if you keep a consistent rhythm during the taps.
- When you ask your friend to explain what they felt, they should say it felt like the taps were spaced equally across the whole arm and it was not isolated to three areas.
Tell your friend what you did and have them try it on you! Now that you know what is going on, is your brain still deceived?
- Have your friend lie on the floor with their eyes closed and with their arms straight out above their head.
- Hold your friends wrists and lift them until their chest is off the floor. Try to be as gentle as possible as this can get uncomfortable.
- Hold this position for one minute then slowly lower your friend to the floor.
- When describing what this felt like, your friend should say that their arms felt like they were sinking through the floor.
Does this experiment work with the lower half of your body?
- Have your friend stick their arms straight out in front of them, then twist their wrists so that their palms are facing outward, thumbs facing down.
- Have your friend cross their arms and fold their hands.
- Keeping their fingers interlocked, have them pull their hands down and back up toward their body, twisting them even further.
- At this point, you can point to a finger on your friend’s hand (without touching the finger!!) and ask them to lift that finger.
- Because of the irregular orientation, the brain will get confused and either lift another finger or take a few seconds to lift the correct finger.
What happens if your friend’s eyes are shut and you tapped the finger you want them to lift?
What is Happening?
Illusions challenging our brain’s expectations by presenting sensory stimuli that don’t match what our brain is used to experiencing.
Proprioception is our ability to know where our body is located in space – including our movements and orientation – even when it is not using sight, smell, hearing, taste or touch to do so. Just like there are common illusions that can trick your sense of sight and sound, these illusions trick your sense of proprioception.
Illusions work differently on everyone. Some people will not experience the effects of each activity. Try these illusions on multiple people (stick with people you live with for now to practice safe social distancing!) and think about why they might work better on some people than on others.