Ask yourself: Where's your left hand?
I bet you don't need to look around to answer this. Or try this: Shut your eyes and reach down and touch your right heel. Not that hard for most of us.
Proprioception is the name of this ability we have that lets us keep track of, locate and make use of our own bodies. Proprioception works thanks to sensors in our muscles (muscle spindles), but it also depends on our sensitivity to stretching and pressure on the skin.
What happens if you lose proprioception?
Obviously, you won't be able to perceive or intuit the position of your limbs and your own posture.
Well, we have known for some time now that the loss of proprioception has even more devastating effects. This was illustrated by the famous case of IW (now long since identified as Ian Waterman). Ian Waterman worked closely with his neurologist Jonathan Cole — and his story is told with great beauty and insight in Cole's book Pride and the Daily Marathon.
The bottom line is this: Without proprioception, you are as good as paralyzed. Lacking feedback from your limbs as to where they are or what they are doing, you can't move them, or rather, you can't even really try to move them. Although Waterman's motor systems were fully intact — the virus that destroyed his proprioception left his motor nerves unhurt — he couldn't sit up or walk or reach.
Eventually, Ian came to learn to use vision to compensate for the absence of proprioception. He was able to achieve a high level of functioning thanks to his use of sight as a way of keeping track of his body. Vision came to be the means by which, in a way, his different body parts were joined together in a whole so that they were his, so that they were under his control.
This is useful background for appreciating the significance of new work on "haptic feedback" and prostheses published by teams working jointly at Rice University in Houston and the University of Pisa in Italy. Their study, published this month, reports that healthy, intact subjects using a prosthesis together with a device attached to the upper arm that stretches the skin of the arm in coordination to the movements of the prosthetic hand were better and more precise at using the artificial hand.
The hand and the "rocker" skin-stretching device are experimental technologies.
Simply by setting up a link so that what you do produces an effect on what you feel — rather than just an effect out there in the world — changes your ability to control what you are doing. And it takes us closer to the ultimate goal, which is that of designing prostheses that can be more fully integrated into a person's self.
Prostheses aren't easy to use. They usually require a fair degree of consciousness visual monitoring and control, and frustrated users often abandon them. The current research on haptic feedbacks holds open the prospect of developing more fully integrated artificial limbs.
Alva Noë is a philosopher at the University of California, Berkeley, where he writes and teaches about perception, consciousness and art. He is the author of several books, including his latest, Strange Tools: Art and Human Nature (Farrar, Straus and Giroux, 2015). You can keep up with more of what Alva is thinking on Facebook and on Twitter: @alvanoe