(Pronounced Pull-Freesh)
Note: this illusion will work best in real life (i.e. not from the video). It will only work for people with reasonably good vision in both eyes.
How can you experience this yourself?
You will need:
- A pendulum (e.g. a wired computer mouse OR lanyard with keys OR any weight on a string).
- A pair of sunglasses
- A pen or pencil (optional)
- Another person
How to do it:
- Person A needs to have the wired computer mouse (or alternative) and Person B needs the sunglasses.
- Person A holds the pendulum up and swings it from side to side over a pen or their thumb.
- Person B stands at least two metres away from Person A, facing them, and notes how the pendulum swings from side to side.
- Person B then holds one lens of the sunglasses over ONE of their eyes, whilst keeping BOTH EYES OPEN so that they are looking through the lens.
- Person B watches the pendulum swing, noticing the direction it now moves in. Then, they move the sunglasses to the other eye and watch the pendulum again. Repeat step 5, checking the direction of the pendulum without the lens over the eye in between.
What should happen?
Without the lens, Person B should see that the pendulum swings in a straight line from side to side. When they put the lens over one eye (with both eyes open) the pendulum should look like it is swinging in a slight ellipse or circle! When they swap which eye is looking through the sunglasses, it should look like the pendulum swings in the opposite direction (clockwise/anticlockwise) around the pen. If you have the lens over your right eye, the pendulum should look like it swings towards you when it is moving to the right and away from you when swinging left (i.e., anticlockwise). If you have the lens over your left eye, the pendulum should look like it swings clockwise. This illusion varies depending on a variety of things, such as eyesight, darkness of the lens, speed of the pendulum and your distance from the pendulum. If it does not work for you at first, try standing further back or swinging the pendulum faster. If you can’t see it at all, don’t worry and try some of the other illusions instead.
How does this relate to pain?
Our brains do a lot of calculations without us knowing. They use assumptions about the timing of the light to both eyes to work out where things are. This is really helpful, because it means that we know which things around us are closer than others. However, the sunglasses lens slows down how the light from one eye is processed, so these assumptions are no longer correct, and what we see (perceive) is different to reality. This illusion demonstrates that the brain interprets the world around us before we become aware of it, so our perceptions can seem different to what is really going on. In the case of persistent pain, the brain often interprets signals from the body as threatening when there is no real danger (i.e., the brain has become overprotective) and so we feel pain. Importantly, we can use a variety of techniques to reduce this protectivity back to a more helpful level.
How does this illusion work?
The brain uses what you can see through both eyes to work out where things are. One eye looking through the sunglasses means that the brain gets the information from one eye a bit slower than the other eye, so it looks like the pendulum is in a different place to where it really is.
More info…
Objects reflect light into our eyes so that we can see them. The brain uses the timing of the information from both our eyes to work out where things are. As the sunglasses lens dims the light to one of the eyes, the eye and brain process this light slightly slower than the other eye (there is some debate about exactly how this happens in the scientific community); this means that the brain gets the location of the pendulum (mouse/keys) wrong. The pendulum seems closer or further away than it really is, depending on whether it is moving towards the side of the eye covered by the lens or away from it. The faster the pendulum swings, the more of a difference there is between where it looks like the object is and where it really is. So, the faster you swing it, the bigger the ellipse seems. When the object gets to the end of one swing, it stops momentarily before swinging back – at this point, the object looks like it is in the same place as it really is, creating the elliptical shape.
Fun Fact: The person who invented this illusion, Carl Pulfrich, could not see it! He was blind in one eye, so he worked out that it should happen, but he could never experience it for himself.