The volume of sensory information which the brain receives is enormous: it comes from the muscles, the joints, the tendons, and touch. To avoid overloading, the brain copes with all this hierarchically. It has learnt to ignore the signals it has come to expect, such as the stretching of our skin when we walk, or the sensation of the soles of our feet on the ground. These signals are dealt with in the unconscious parts of our brain, ‘lower down’ in the system. Information only reaches the ‘higher’ parts of the system – the conscious parts – when the experience is new or unexpected.
Every movement we make starts in our brain. Once we’ve decided to make a movement, the motor cortex in the brain sends out a command to the appropriate muscles to make them move. But it doesn’t stop there. Within 60 milliseconds, a message is sent back from the body’s sensors to report back on how the movement went. Was it right? Did it succeed? Based on this information, the brain responds by sending an updated command to improve the movement which generates yet more feedback. In small children and in people learning a new movement skill, you can actually see the results of this, as their ankles wobble and their balance sways. This ‘loop’ system – message out, message in and so on, is how we control movement, make it more accurate, more precise, smoother and more elegant.
Experts in movement, like dancers, learn to short cut this loop system by sending more accurate commands to their muscles and by better predicting the consequences of their movement commands. They learn, through practice, that when you lift your left leg, for instance, you need to shift your body to the right to compensate and they learn to make fast and accurate corrections without conscious thought. While strong muscles and flexibility may appear to make a good dancer, it is more likely their enhanced ability to deal with feedback that makes them unique.
By far the most dominant of all the feedback our brains receive is the information we get from our eyes but visual information is processed far slower than proprioceptive information, reaching the brain approximately 70 milliseconds after the information from the body. So, although dancers traditionally rehearse in front of mirrors, a dancer relying on information about the state of her body from her reflection is always going to be less controlled than a dancer listening to her body.
Most people don’t even know they have this extraordinary and vital sense. One way of understanding proprioception is by looking at someone who has lost it and see the problems he finds in doing things. His witness statement allows you to understand something about how you use proprioception.