Thanks to unconscious processing, most of us instinctively know where our limbs are and what they are doing. This ability, called proprioception, results from a constant conversation between the body and brain. This adds up to an unerring sense of a unified, physical “me”.
This much-underrated ability is thought to be the result of the brain predicting the causes of the various sensory inputs it receives – from nerves and muscles inside the body, and from the senses detecting what’s going on outside the body. “What we become aware of is the brain’s ‘best guess’ of where the body ends and where the external environment begins,” says Arvid Guterstam of the Karolinska Institute in Stockholm, Sweden.
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The famous rubber-hand illusion is a good example of this. In this experiment, a volunteer puts one hand on the table in front of them. Their hand is hidden, and a rubber hand is put in front of the participant. A second person then strokes the real and rubber hands simultaneously with a paintbrush. Within minutes, many people start to feel the strokes on the rubber hand, and even claim it as part of their body. The brain is making its best guess as to where the sensation is coming from and the most obvious option is the rubber hand.
“The brain unconsciously keeps track of an invisible ‘force field’ around the body“
Recent research suggests this sixth sense extends to the space immediately surrounding the body. Guterstam and his colleagues repeated the experiment, stroking the real hand but keeping the brush 30 centimetres above the rubber hand. Participants still sensed the brush strokes above the rubber hand, implying that as well as unconsciously monitoring our body, we keep track of an invisible ‘force field’ around us. Guterstam suggests this might have evolved to help us pick up objects and move through the environment without injury.
Move to improve
A lack of proprioception is rare but can happen with nerve or brain damage. The case of Ian Waterman, who lost proprioception after nerve damage caused by a flu-like virus in 1971, demonstrates just how much we rely on this ability. After being told he would never walk again, he slowly learned to consciously control his muscles to move his body. Decades later, it is still far from easy and he only has full control over his movements if he is looking at the relevant body part and concentrating. “Because his proprioceptive system is shot, these things are not automatic for him. It requires constant conscious effort,” says Anil Seth, a neuroscientist at the University of Sussex in Brighton, UK.
Even if the system is working fine, there is some evidence that it might be worth consciously trying to improve it. A recent study in which volunteers trained in MovNat exercise – a programme designed to tax the body’s natural balancing, jumping and vaulting abilities – improved more on measures of working memory than a control group who did yoga or no exercise.
This article appeared in print under the headline “4. Keep track of the body in space”
