What makes a sound without a mouth?
This question arises all the time. The answer, it turns out, is something very unusual.
One might wonder if the fact we can’t move our mouths doesn’t show that the brain is working more slowly than it usually does. Indeed, if moving our mouths did show signs of a reduced cerebral cortex, for instance, brain scans conducted in the 1980s might have revealed some kind of neuronal network in the temporal lobe that were engaged, as they might in our daily speech acts.
And in fact it turns out that the same kind of network was indeed revealed in the brain of two monkeys who had both developed a congenital condition in which their motor cortex was greatly reduced.
But there are also other, even more striking examples that suggest the brain can work much more slowly. It turns out that speech, like all complex behaviors, is made possible by an elaborate system of “reciprocal connections” that run from the central nervous system to every limb of the body. These connections have a basic physical meaning: the connection between parts of the brain is not simply a matter of making things go, but a form of mutual reinforcement, or signaling.
Every movement our bodies make has an equivalent reciprocal effect on our brains, says R. J. Leakey, a neurobiologist at the University of Pennsylvania. Each motor movement has its own feedback signal from the motor cortex to the brain, which then interprets these signals to make the muscles perform their intended function.
The same thing occurs when the nervous system moves the lips and tongue, for instance. As the brain makes the tongue move into position, the neural network also signals that it should move the lips in the same way. Both of these signals are interpreted by the brain; the brain then uses its motor and respiratory systems to make the lips move. Even though the tongue moves independently of the lips, it is nevertheless driven by the same motor system, because there are reciprocal signals from the lips to the tongue.
For that reason, it seems probable that the brain’s response to speech is also slow. After all, the signals from the mouth and tongue are so closely linked that the brain must not only interpret these signals, but must also transmit them into the muscles when they are needed, or inhibit them for a longer period if those are not needed. That’s why it takes so much work to make a language sound. There have to be lots of reciprocal signals between words and body parts.