Like the musicians in an orchestra, our lips, tongue and vocal cords coordinate with one another to pronounce sounds in speech. A map of the brain regions that conduct the process shows how each is carefully controlled ? and how mistakes can slip into our speech.
It's long been thought that the brain coordinates our speech by simultaneously controlling the movement of these "articulators". In the 1860s, Alexander Melville Bell proposed that speech could be broken down in this way and designed a writing system for deaf people based on the principle. But brain imaging had not had the resolution to see how neurons control these movements ? until now.
Using electrodes implanted in the brains of three people to treat their epilepsy, Edward Chang and his colleagues at the University of California mapped brain activity in each volunteer's motor cortex as they pronounced words in American English.
The team had expected that each speech sound would be controlled by a unique collection of neurons, and so each would map to a different part of the brain. Instead, they found that the same groups of neurons were activated for all sounds. Each group controls muscles in the tongue, lips, jaw and larynx.
The neurons ? in the sensorimotor cortex ? coordinated with one another to fire in different combinations. Each combination resulted in a very precise placing of the articulators to generate a given sound. Surprisingly, although each articulator can theoretically take on an almost limitless range of shapes, the neurons imposed strict limits on the range of possibilities.
Speech quirks explained
This map of language could explain some speech quirks, the team say. The areas responsible for pronouncing consonants and vowels, for instance, were relatively far from each other in the brain. This might explain why "slips of the tongue" make us mistake vowels for other vowels, but not for consonants.
The team also found that while the lips and tongue are each controlled by one set of neurons, the larynx is controlled by two. It's unclear what this second larynx neural map is doing, but it is thought that non-human primates lack it. It will be very interesting if this turns out to mean that the two sets of neurons allow humans to use the larynx in two different ways, says William Idsardi of the University of Maryland in College Park, who was not involved in the study.
The authors say that their next step will be to look at people who speak a different first language and see if their speech is controlled by the same neural combinations that control American English. For instance, the French and English languages put different stresses on different consonants and speakers might make use of the larynx and other organs in different ways, says Idsardi.
Gregory Iverson, a linguist at the University of Wisconsin-Madison, says the new study, and follow-up work, may help answer questions such as how brain injuries can cause speech loss, or even why some people learn languages more easily than others. "I feel like a kid in a candy story now that the brain has been opened," he says.
Journal reference: Nature, DOI: 10.1038/nature11911
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