From thought to speech in a fraction of a second
The human brain has to do quite a lot just to utter a single word. In the case of verbs, it must select the word that best expresses the idea it wants to convey (for instance, “to walk”); decide on the proper tense (“walks,” “walked,” “walking,” etc.); figure out whether adding an “-ed” also adds another syllable; determine whether the word should end with a “d” or a “t” sound; and devise a plan for maneuvering the mouth muscles to make the appropriate sounds.
How does this happen? Neuroscientists, linguists and others have been debating whether the brain gets all this done by splitting up the jobs and completing them simultaneously or by finishing this litany of tasks one at a time.
Now, a trio of epilepsy patients has provided the answer. A small section of the cortex called Broca’s area completes all these tasks sequentially, and all within about half a second, according to a study published Friday in the journal Science.
Scientists can study all kinds of diseases in animals, but when it comes to investigating language, only human brains will do. So researchers at UC San Diego and Harvard piggybacked on a rare procedure called intracranial electrophysiology, in which epilepsy patients allow doctors to implant dozens of electrodes directly into their brains. While they are awake, the patients answer questions so that doctors can determine which parts of the brain are necessary to maintain language and which parts can be safely removed to treat epileptic seizures.
The researchers showed the patients a series of 240 words and asked them to pronounce them in their minds. In some cases, the words had to be converted to another tense first. The probes recorded the electrical activity in the brains as the patients churned through these tasks.
The researchers found remarkably consistent patterns among all three patients. Electrical activity spiked 200 milliseconds, 320 milliseconds and 450 milliseconds after being presented with a new word. The researchers concluded that those peaks corresponded to the times when the brain decided on the appropriate word to use, picked the proper grammatical form, and figured out how to pronounce it.
Not only did those tasks occur in sequence, they all took place in Broca’s area. That should dispel the long-held notion that Broca’s area is involved in speaking, but another area of the brain called Wernicke’s area is responsible for reading and hearing, said lead author Ned Sahin, a postdoctoral researcher at UC San Diego and Harvard. The finding will make many textbooks obsolete.
-- Karen Kaplan
Photo: This X-ray shows the electrodes used to identify the part of the brain that's responsible for seizures in patients with epilepsy. Scientists used the setup to study how the brain forms words.
Credit: Ned T. Sahin