The brain’s response to another person’s body language may help explain why humans are quite efficient at fleeing from danger, according to researchers.

In a series of experiments, the investigators found that when they showed men and women still images of a person taking a fearful stance, it activated areas of the brain involved in movement. The same was not true of images of a “happy” body posture, which, compared with emotionally neutral stances, enhanced activity only in visual centers of the brain.

In all cases, faces were blocked out of the images in order to get at the specific effect of body language. In the past, research has focused almost exclusively on the face as the conveyor of human emotion, said lead study author Dr. Beatrice de Gelder, of Harvard Medical School in Boston.

Her team’s findings highlight the importance of the whole body in expressing emotion, particularly when it comes to fear. This makes evolutionary sense, de Gelder explained in an interview, since it is in the best interest of a person to quickly read, and act upon, the bodily expression of fear in another person.

For the study, which is published in the Proceedings of the National Academy of Sciences, de Gelder and her colleagues used the brain-imaging technique functional MRI to monitor brain activity in seven adults while they were shown still images of actors in various body postures - making either happy, fearful or emotionally neutral gestures.

Compared with the neutral postures - of someone combing his hair, for instance - fearful stances triggered greater activity in a number of brain areas involved in processing emotion, and in regions associated with movement.

In other words, a fearful body posture primes others to flee. “Due to this mechanism, people are very efficient at running away,” de Gelder said. Survival is better served, according to the researcher, when a person immediately “copies the body posture” of others who are expressing fear, rather than standing around trying to assess the situation.

Beyond their relevance to evolution, the study findings may also prove useful in the understanding of brain disorders such as schizophrenia and autism, de Gelder said.

Both of those disorders, she noted, involve problems in expressing and reading emotion. Knowing how emotional body language activates the brain could aid in understanding or even treating these conditions, according to the researcher.

SOURCE: Proceedings of the National Academy of Sciences, November 23, 2004.