jueves, 29 de julio de 2010

Of two minds: Listener brain patterns mirror those of the speaker

Of two minds: Listener brain patterns mirror those of the speaker
By R. Douglas Fields

A new study from Princeton University reports that a female student of lead investigator, Uri Hasson, can project her own brain activity onto another person, forcing the person's neural activity to closely mirror that in her own brain. The process is otherwise known as speech.

There have been many functional brain-imaging studies involving language, but never before have researchers examined both the speaker's and the listener's brains while they communicate to see what is happening inside each brain. The researchers found that when the two people communicate, neural activity over wide regions of their brains becomes almost synchronous, with the listener's brain activity patterns mirroring those sweeping through the speaker's brain, albeit with a short lag of about one second. If the listener, however, fails to comprehend what the speaker is trying to communicate, their brain patterns decouple.

Previously, most brain-imaging studies of language used repetition of simple sounds to stimulate a listener's brain to locate regions mediating listening or they involved a speaker repeating simple words to examine cerebral areas involved in speech production. This disjointed approach was necessary because analyzing fMRI (functional magnetic resonance imaging) data requires repeating a stimulus many times during successive brain scans to average the responses and find regions exhibiting heightened or depressed activity. Also, the imaging machines are noisy, which makes it difficult to have a normal conversation. These past approaches, however, are not adequate studies of communication, which requires that the recipient is attentive and comprehends what the speaker is saying. If, for example, a teacher is lecturing and a student who is listening intently has become lost, there is a failure of communication.

In order to find out what happens in the brain when the speaker and listener communicate or fail to connect, Hasson, an assistant professor in Princeton's Department of Psychology, and his team had to first overcome both technical problems using new analytical methods as well as special nonmagnetic noise-canceling microphones. He asked his student to tell an unrehearsed simple story while imaging her brain. Then they played back that story to several listeners and found that the listener's brain patterns closely matched what was happening inside the speaker's head as she told the story.

The better matched the listener's brain patterns were with the speaker's, the better the listener's comprehension, as shown by a test given afterward. There was no mirroring of the speaker’s brain activity patterns if the listeners instead heard a different story recorded previously by the same speaker and played to them as a control experiment. English speakers listening to a story told in Russian did not show higher-level brain coupling. In other words, there is no mirroring of brain activity between two people's brains when there is no effective communication (except for some regions where elementary aspects of sound are detected. When there is communication, large areas of brain activity become coupled between speaker and listener, including cortical areas involved in understanding the meaning and social aspects of the story.).

Interestingly, in part of the prefrontal cortex in the listener's brain, the researchers found that neural activity preceded the activity that was about to occur in the speaker's brain. This only happened when the speaker was fully comprehending the story and anticipating what the speaker would say next.

"Communication is a joint action, by which two brains become coupled," Hasson explained in an e-mail. "It tells us that such coupling is extensive, [a property of the network seen across many brain areas]."

The team is interested in determining if nonverbal communication similarly causes mirrored brain activity in the recipient's brain, and whether communication in the animal world may have similar properties. "We are thinking about fly courtship song and bird songs. In a fly courtship song, only the male can sing. It was discovered however, that females have the capacity to sing, but it is inhibited," Hasson says. This fits with the new findings, because if the female's brain could not mirror activity in the male fly's brain they would not be able to communicate. Language binds brains together and in this melding of minds forms societies.

The results are detailed in the July 26 issue of Proceedings of the National Academy of Sciences.

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