Language is created in the same areas of the brain, regardless of whether a person speaks English or uses American Sign Language to communicate, new research found. The discovery suggests that something about language is universal and doesn't depend on whether people use their voices or their hands to talk.
Two centers in the brain – Broca's area, which is thought to be related to speech production, and Wernicke's area, which is associated with comprehending speech – have long been associated with verbal communication. But now scientists have found the brain areas might be tied to language, no matter whether it's spoken or signed.
Scientists suspected these areas might be particular to speaking, because they are located spatially near areas that are connected to moving the vocal chords, and to the auditory cortex, which is used to hear sounds. In that case, it stood to reason that deaf people who use American Sign Language (ASL) to communicate should use other brain areas to create language, such as parts located near the visual cortex, used for seeing.
But when researchers tested 29 deaf native ASL signers and 64 hearing native English speakers, they found no difference in the brain. They showed both groups pictures of objects, such as a cup or a parrot, and asked the subjects to either sign or speak the word, while a PET (Positron Emission Tomography) scanner measured changes in blood flow in the brain.
In both groups, Broca's and Wernicke's areas were equally active.
"It's the same whether the language is spoken or signed," said Karen Emmorey, a professor of speech language at San Diego State University. Emmorey described the work last week at the annual meeting of the American Association for the Advancement of Science in San Diego, Calif. The research was also detailed in a 2007 issue of the journal Neuroimage.
In a more recent study, which has not yet been published in a scientific journal, the scientists tested whether sign language taps into the same parts of the brain as charades. They wanted to figure out whether the brain regards sign language as more similar to spoken language, or more similar to making pantomime gestures to mimic an action.
The scientists showed both deaf people and hearing people pictures of objects, such as a broom or a bottle of syrup, and asked the subjects to "show how you would use this object." The charade gestures for pouring syrup and for sweeping with a broom are different from the signs for syrup and sweep, so the researchers could be sure the deaf participants were pantomiming and not signing.
Then they asked the deaf subjects to sign the verbs associated with particular objects, such as syrup or broom. The researchers found that the signers activated different parts of their brains when pantomiming versus when signing. Even when the sign is basically indistinguishable from the pantomime – when similar hand gestures are used – the brain treats it like language.
"The brain doesn't make a distinction," Emmorey said. "The fact that many signs are iconic doesn't change the neural underpinnings of language."
And the scans showed that the brain areas signers used when pantomiming were similar to the brain areas hearing participants used when pantomiming – both groups activated the superior parietal cortex, which is associated with grasping, rather than brain areas connected to language.
"It suggests the brain is organized for language, not for speech," Emmorey said.