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).
What's going on?
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.
Lose this sense up the proverbial smelly creek and you're in even bigger trouble than the old saying goes because, even if you found the infamously lost paddle, you couldn't use it anyway.
This was tragically discovered by Ian Waterman, the Englishman who 'lost his body', and the neuroscientists who treated him.
After contracting a rare virus, Waterman lost his sense of proprioception while the ability to move his muscles remained intact. He eventually learned to walk and speak again but without the sense of proprioception we all take for granted.
Even the simplest movement (which clearly are not 'simple' at all) still require incredible focus and attention. He relies heavily on his sense of vision to coordinate his movements and still falls 'like a rag doll' in the dark.
Proprioceptors are the specialised sensory organs that give you this indispensable sense of 'yourself'.
One type of proprioceptor, called muscle spindles, tells your brain the length of that muscle (the degree to which the muscle is stretched).
These sensory structures are coiled around special fibres found in nearly all of your muscles. They send signals whenever you stretch or contract that muscle so your brain can keep track of its length.
The muscles in your fingers, neck, mouth and eyes have a higher density of muscle spindles than larger muscles involved in less precise movements, reflecting the demand for very accurate feedback about movements associated with fine motor skills, speech and vision.
The muscles in your arms and legs that maintain your posture against gravity also have larger numbers of muscle spindles.
Another type of proprioceptor, called the Golgi tendon organ, is found in the tendons that attach the ends of muscles to bones. They provide sensory information about the amount of tension in the muscle.
If you hold a heavy object still, your muscle length doesn't change, so the muscle spindles fall silent. But the Golgi tendon organs fires constant messages telling the brain that the muscle pulling the bone that is supporting the object is under tension.
Yet another kind of proprioceptor, appropriately called joint receptors, is found in joints and gathers information about the limb's position, orientation and movement.
With hundreds of muscles and tendons, and dozens of joints, each with numerous muscle spindles, Golgi tendon organs and joint receptors, the volume of data streaming up your spinal column and into your noggin at any time is mind-boggling.
Think about how much you struggle with comparatively simple tasks, like remembering a name or phone number, and suddenly your ability to walk while carrying on a conversation, or sip water from a flimsy plastic cup while riding a bike, or to tie your shoelaces without looking all start to look more like astonishing feats of coordination than simple daily routines.
So back to the curved surface after-effect. With so much information to contend with, it's not surprising your brain has developed ways to ignore signals that remain constant for extended periods of time.
Just as the silence of an empty room can be deafening after listening to loud music, the 'silence' of a nerve after an extended period of 'noise' can cause perceptions that aren't quite right.
We call them illusions and they are still far from perfectly understood. A truly bizarre result from a recent experiment demonstrates just how complicated and interconnected the brain and its multitude of inputs, outputs and functions are.
After their left hand had adapted, the experimenters discovered that their subjects' right hands also adapted to the curved surface. Amazing!
Feeling a flat surface as curved clearly doesn't mean the system is faulty or broken. If anything, it's quite the contrary and probably one of the best ways to notice the secret sixth sense you normally take for granted.