Tricking the mind can often be more than meets the eye. Using nothing more than a ball or a bowl, you too will be able to discover your myterious sixth sense.
The word 'illusion' makes most people think of images that trick the brain, or a person being chopped in half on stage. But this tricky illusion will fool your sixth sense.
Grab a large ball or a salad bowl or both. A football will work too and the bowl doesn't need to be clear.
Grip the ball with both hands and squeeze hard for about 20 seconds.
Now lay one of your hands on a flat surface, for example, a table. It will feel like you are touching a concave surface, like the inside of a bowl. That is, the opposite shape of the ball.
Try it with a bowl. Press your hand down into the bowl so your fingers are stretched upwards for around 20 seconds. Now lay your hand on a flat surface and it feels like you are touching a convex surface, like the surface of a ball. Amazingly, the illusion might even work for the other hand (the one you didn't put in the bowl).
Illusions that fool your sense of touch are called haptic illusions. The word haptic is derived from the Greek word haptesthai, which means 'to touch'.
Haptics research is the study of the human sense of touch and its applications, such as patient rehabilitation and interfaces between humans and robots or computers.
This illusion is referred to as a haptic after-effect because it results from an initial adaptation to a stimulus (the curved surface of the ball or bowl), followed by the touching of another (the flat surface).
After adaptation to the curved surface, the flat surface feels like it is curved in the opposite direction. How this works exactly, is not well-understood but the term used to explain it is 'sensory adaptation'.
All your senses are capable of sensory adaptation to a stimulus. Olfactory adaptation is the brain's ability to ignore persistent smells, which is why you stop noticing the novel smell of someone else's house a few minutes after you enter. Amazingly, only the persistent smell is 'ignored' so you will still smell the onions if your host starts preparing a meal.
Another example of sensory adaptation is the famous 'waterfall illusion'. Stare at the falling water for 30 seconds, then look at the adjacent stationary rock face and you perceive a strange upward motion that isn't really there.
What's remarkable about every type of sensory adaptation is that the sensory systems being 'tricked' are incredibly sophisticated and yet easily duped by their own recent history.
The curved surface after-effect, as it is known, results from the brain's adaptation to nerve signals that combine to give you your sense of proprioception. The name for this 'secret sixth sense' was coined by Charles Sherrington in 1906. It is how you know where your limbs are, how they are oriented in space and how they are moving.