Today I learned...
When animals move their sense organs provide two kinds of information. There’s exafference, signals produced by stuff happening in the world. There’s also reafference, signals produced by an animal’s own actions. […] You can think of them as other-produced and self-produced. […] Every animal, for each of its senses, has to distinguish between these two kinds of signals. But here’s the catch: These signals are the same from the point of view of the sense organs.
Consider a simple earthworm. When it burrows through the soil, the touch receptors in its head register pressure. But if you prod the worm in the head, the same touch receptors register the same kind of pressure. So how does the worm know if a given sensation comes from its own movement (reafference) or someone else’s (exafference)? How does it know if it is touching something, or if it has been touched? […] If an animal can’t tell other-produced signals from self-produced ones, its Umwelt [i.e., perception of the world] would be an unintelligible mess.
This problem is so fundamental that very different creatures have solved it in the same way. When an animal decides to move, its nervous system issues a motor command—a set of neural signals that tell its muscles what to do. But on its way to the muscles, this command is duplicated. The copy heads to the sensory systems, which use it to simulate the consequences of the intended movement. When the movement actually occurs, the senses have already predicted the self-produced signals that they are about to experience. And by comparing the prediction against reality, they can work out which signals are actually coming from the outside world and react to them appropriately. […]
[Duplicated motor commands are often called corollary discharges.] Whenever an animal moves, it unconsciously creates a mirror version of its own will, which it uses to predict the sensory consequences of its actions. With every action, the senses are forewarned about what to expect and can prepare themselves accordingly. […]
Corollary discharges explain why you can’t tickle yourself: You automatically predict the sensations that your writhing fingers would produce, which cancels out the actual sensations that you feel. They’re why your view is stable even though your eyes are constantly darting around. […]
Some scientists have suggested that schizophrenia is fundamentally a disorder of the corollary discharges. People with the condition might experience hallucinations and delusions because they can’t distinguish their own inner speech from the voices around them. A failure to sort self from other might also explain some of schizophrenia’s stranger symptoms, like the ability to tickle yourself.
--from the book An Immense World: How Animal Senses Reveal The Hidden Realms Around Us, by Ed Yong
