By the time we reach adulthood, we take for granted many tasks that were extremely challenging to our perceptual system when we first entered the world. We had to learn how to distinguish our self from others, sense where our limbs are in space, and control our limbs. Some scientists even believe that movement is the sole purpose of the brain, as (other than making us sweat) movement is the only way we can impact the external world. If we are to consciously interact with our surroundings we must possess a very reliable sense of movement.
Early philosophers did not recognize the origins of our sense of movement. Aristotle wrote that we have only five conscious senses: sight, touch, taste, hearing, and smell. But there was a reason why Aristotle may not have made room for the sense of movement, as unless we attend to our body we are typically unaware of the position of our limbs. By the end of the 1800s, scientists had acknowledged the existence of movement as a sense, referring to it as “kinesthesis” or “proprioception.”
German scientist Hermann von Helmholtz proposed that proprioception arises from a copy of the signal from the brain to the muscles. Recent experiments found evidence for this contribution. An effort to move the hand when a nerve block is applied still results in a sensation of movement even when the hand is physically prevented from moving. Englishman Charles Sherrington took an opposing view, arguing that movement-sense originates from the sensory information from the limbs themselves. This peripheral contribution is easily demonstrated by the movement sensation arising from someone else grabbing and moving your arm.
We now believe that Sherrington and Helmholtz were both correct. By comparing our planned and actual movements, we can fine-tune our movement control. This comparison process underlies the remarkable capability for our proprioceptive senses to improve with practice, as seen in circus performers or in the many extraordinary cases where that movement-sense has been relearned in those with lesions to movement areas of the brain.
Knowing where our limbs are in space is inherently useful, but how do we know what actions we can take? In the 1970s, James Gibson postulated that the range of actions we believe to be available is based on our perception of our selfhood and our surroundings. You have experienced this yourself when you intuitively knew whether you could fit your hand into a cookie jar, or if you had to fetch the tongs. But how do you know how big your hand is?
Fortunately, our perceptual system is very proficient at mapping the body, with recent experiments showing how complex neural networks learn to map the body based on the statistics of sensory inputs. This process maintains stable perception even as our body shape changes with age.
Another challenge remains: How do you know what is self and what is other? This is a very difficult topic to study, as philosopher William James pointed out that unlike the traditional five senses, the body is always there.
Fast-forward to a Halloween party before the turn of the millennium, when psychologist Matthew Botvinick noticed that a rubber hand aligned with his real hand could be made to feel as if it were his own hand. The illusion worked best if his real hand was out of view and the fake and real hands were touched simultaneously. In that situation the visual and touch inputs were co-incident — he could “see” what he thought was his real hand right there on the table.
Later studies showed that if the experimenter simultaneously moved the rubber limb to match voluntary movements of the real limb, the illusory embodiment was further strengthened. If the experimenter also moved the real limb, the participant maintained embodiment of the fake hand but lost any sense of agency they had over it.
As technology races forward, we might extend our agency and embodiment to multiple selves, such that we might control machines in other environments. If these are virtual environments, does this change anything?
If we believe that perception is only our reconstruction of the physical world, perhaps not. At the very least, society will have to reconsider its concept of self, to incorporate new technologies and make people accountable for their actions.