One day, while threading a needle to sew a button, I noticed that my tongue was sticking out. The same thing happened later, as I carefully cut out a photograph. Then another day, as I perched precariously on a ladder painting the window frame of my house, there it was again!
What’s going on here? I’m not deliberately protruding my tongue when I do these things, so why does it keep making appearances? After all, it’s not as if that versatile lingual muscle has anything to do with controlling my hands. Right?
Yet as I would learn, our tongue and hand movements are intimately interrelated at an unconscious level. This peculiar interaction’s deep evolutionary roots even help explain how our brain can function without conscious effort.
A common explanation for why we stick out our tongue when we perform precision hand movements is something called motor overflow. In theory, it can take so much cognitive effort to thread a needle (or perform other demanding fine motor skills) that our brain circuits get swamped and impinge on adjacent circuits, activating them inappropriately. It’s certainly true that motor overflow can happen after neural injury or in early childhood when we are learning to control our bodies. But I have too much respect for our brains to buy that “limited brain bandwidth” explanation. How, then, does this peculiar hand-mouth cross-talk really occur?
Tracing the neural anatomy of tongue and hand control to pinpoint where a short circuit might happen, we find first of all that the two are controlled by completely different nerves. This makes sense: A person who suffers a spinal cord injury that paralyzes their hands does not lose their ability to speak. That’s because the tongue is controlled by a cranial nerve, but the hands are controlled by spinal nerves.
These are fundamentally different kinds of nerves. Cranial nerves penetrate the skull through small openings, connecting directly to the brain. Each one carries out a specific sensory or motor function; the first cranial nerve, for example, conveys the sense of smell. The tongue is controlled by the 12th cranial nerve, called the hypoglossal nerve. In contrast, the muscles controlling our hand movements, like most other muscles in our body, receive instructions from nerves that reach out from our spinal cord, threading their way between our vertebrae. Sensory signals make the reverse journey. Clearly, any short-circuiting between tongue and hand-control circuitry must originate upstream of these two nerves, somewhere inside the brain itself.
Looking next at the neural wiring of the brain’s motor cortex, we see that the areas controlling the tongue are not adjacent to the region that controls fingers. The link between tongue and hands must therefore be someplace else in the brain, most likely in an area where complex neural circuits execute highly sophisticated functions. After all, one of the most sophisticated functions that humans can perform is speech — indeed, it seems to be unique to human beings. The next most sophisticated thing we can do is master the use of tools. Notably, in each of the different situations where my tongue was sticking out, I was using a tool: a needle, scissors or a paintbrush.
That connection is borne out by research showing that hand and mouth movements are tightly coordinated. In fact, that interplay often improves performance. Martial artists scream short explosive utterances, called kiai in karate, as they execute thrusting movements; tennis players often shout as they smack the ball. And research shows that coupling hand movements with specific mouth movements, often with vocalization, shortens the reaction time needed to do both. This neural coupling is so innate, we are usually oblivious to it, but we do this continually without awareness because the neural circuitry involved is in a region of the brain that operates automatically — it literally lies beneath brain regions providing conscious awareness.
Hand movements come in two general forms: Power grip movements involve opening and closing a fist, while precision hand movements involve delicate pinching between the thumb and index finger. These two types of hand movements, we’ve learned, are often accompanied by different tongue and mouth movements. Take, for instance, the movements made by the late rock singer Joe Cocker, who was famous for his wild arm and hand gestures during performances. In part, these were air guitar and piano pantomimes, but Cocker did not play either instrument, so they also likely reflected a natural connection between hand and mouth. He often displayed the power grip movement of an open fist when his tongue was retracted as he sang an open vowel like “aw.”