A week ago a commenter challenged my remark that, “We have direct links between the brain and our vocal apparatus.” The commenter asked:
Animals also have vocal apparati. Aren't the links between their brains and their vocal apparati direct? If not, what is the difference between in the linkages? How does one determine neurologically whether there is voluntary control or not?
My remark had been a quick response another comment and this direct challenge sent me back to the books so that I might better know what the heck I was talking about. I was pretty sure I had gotten the information originally from Terrence Deacon’s The Symbolic Species and sure enough chapter 8 has the story in painful detail along with some useful diagrams.
Faces, as Deacon notes, are very old. Reptiles already had jaws, tongues, breathing passages, and structures for sorting food and air so that the air passed to the lungs and the food ended up in the stomach. The face is used for seeing, hearing, eating, and breathing. Seeing and hearing are irrelevant to our story and can be ignored in this discussion. Eating and breathing do matter. In reptiles these actions are controlled by the reticular premotor region of the brain, a portion of the brain stem, and all of it is automatic. Nobody is likely to call any of its contributions voluntary.
The same basic facial structure and brain control is found in mammals, with one change. There is now a connection running directly from the cortex to the reticular premotor region, perhaps giving the mammal some contextual sensitivity. They can look around a bit before they start chewing.
Primates have taken even more control, adding direct cortical links to the tongue and facial muscles. At a blow, this piece of information explains why primate facial expressions are so much richer than that of most mammals. They can use it to signal. And again, because of the cortical links, they can take context into account. A few weeks ago I posted a report on changes in ape vocalizations according to context. (See: Apes Are Shrewd Listeners) Very likely it is this alertness to context that gives them some control over their output.
When we look at the human brain we have even more direct cortical control over the output. In fact, there is now a direct cortical link bypassing the reticular premotor area for every one of the controls. We have added direct links to the systems regulating breathing and the operation of the larynx. These links do not mean that we have gotten rid of the automatic operation of the reticular premotor neurons, but they can be overridden. Although direct control of the larynx and breathing has obvious benefits in shaping vocalizations, it also has a serious cost. We no longer keep food out of our windpipe as automatically as the other species do. And food that is small enough to pass through the windpipe can end up in the lungs. This cost in choking and seeds germinating in the lungs is serious enough that the benefit had to be quite strong before evolution would tolerate it.
Looking back at the clumsy sentence that started this discussion, I want to revise it: We have direct links between the brain cortex and our vocal larynx and breathing apparatus.
And how about “voluntary control”? That’s a subject for books, but as more and more control becomes dependent on context the individual becomes increasingly unpredictable. I’ll save the discussion about free will for another time.