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Thursday, Jan. 16, 2003

NATURAL SELECTIONS

When two hemispheres of the brain work as one


The French surgeon Paul Broca had a patient in his care in 1861 who had fallen and broken his hip. Eighteen months earlier the man, called Lelong, had collapsed with a stroke that left him unable to speak. When Lelong died on Broca's ward, a hip fracture being a fatal condition in those days, an autopsy revealed damage to the left frontal lobe of his brain, a region now known as "Broca's area."

Broca was a child prodigy and graduated from medical school at 20. Soon he was appointed professor of surgical pathology at the University of Paris. He was initially interested in cartilage and bone, but turned to brain anatomy, and, after Lelong, saw many more patients with damage to the left part of their brains who had lost their ability to use language.

Eventually, Broca concluded "the two halves of the brain do not have the same attributes" and that the left hemisphere is concerned with language. The insight was startling at the time, but thanks to Broca and to the subsequent work of neurologists all over the world, it is now common knowledge. "Broca's aphasia" is a recognized disorder, aphasia meaning loss of language ability.

Most of us are also aware that while the left hemisphere is more important in controlling language, the right processes emotions. This information forms the basis of books such as Betty Edwards' "Drawing on the Right Side of the Brain," which encourages people to tap their artistic potential by exercises designed to "switch on" the brain's right hemisphere. It also underlies assertions that "left-brained people" are more logical while "right-brained people" are more creative and emotional.

But what happens when the brain is faced with emotional language? A study published in January's issue of Neuropsychology suggests a sensible answer: Both hemispheres of the brain work together. The left side works on what was said, and the right side processes how it was said; that is, the left side labels the feeling, the right side deals with its emotional content.

Scientists at Ghent University in Belgium looked at the brains of volunteers using a specialized form of ultrasound scanning, similar to that used to look at a developing fetus, but with a useful addition. Transcranial Doppler ultrasonography makes use of the Doppler effect, the phenomenon whereby the pitch of sounds rise as they approach you and fall as they go away.

Transcranial Doppler ultrasonography uses the Doppler effect to measure the speed of blood cells traveling through the arteries of the brain. This allows researchers to measure the amount of blood flow and thus the activity in different parts of the brain, because the harder the neurons in the brain are working, the more glucose and oxygen they will require from their blood supply.

Neuropsychologist Guy Vingerhoets and colleagues asked 36 participants, hooked up to ultrasound monitors, to identify the emotion conveyed in dozens of prerecorded sentences. The volunteers were asked to focus either on the actual words (semantics) of the sentences, or on the emotion conveyed by the way in which they were spoken -- that is, their prosody.

Prosody is the musical quality of speech, the way its tone rises and falls, speeds up and slows down, and how its volume increases or decreases according to the emotions expressed. Steven Hawking's computer-generated voice lacks prosody. Prosody comes from the right hemisphere, which is also responsible for metaphor, sarcasm and humor.

To keep things simple, each test sentence had just one of four basic emotional meanings (happiness, sadness, anger or fear) or a neutral semantic meaning. For example, "He really enjoys that funny cartoon" (happiness), "The little girl lost both her parents" (sadness), "Panic broke out in the dark tunnel" (fear), or "Always store disc in its protective case" (neutral). Actors spoke the sentences with either emotional or neutral prosody.

As they listened to the sentences, participants pointed to the appropriate emotion on a card listing them. Because movement on one side of the body is controlled by the opposite side of the brain, the volunteers pointed with both fingers to minimize the chance of activating only one side of the brain.

Vingerhoets and his colleagues found that when participants focused attention on what was said, that is, on the semantics of the sentence, blood flow velocity went up significantly in the left side of the brain. When participants shifted attention to how it was said, to the tone of voice, whether happy, sad, anxious, angry or neutral, velocity went up in the right side of the brain. However, it did not correspondingly decrease in the left side, probably, say the researchers, because the left hemisphere automatically processes semantic content and also helps to label the emotions.

Thus, physical evidence from ultrasound has revealed that the right hemisphere, while indeed the brain's more "emotional" side, is not solely responsible for processing the expression of emotions.

"Understanding emotional prosody," said Vingerhoets, "appears to activate right-hemispheric brain regions." However, the left brain stays active to categorize or label the emotion, as we would expect from its dominance in language processing. "Even if you pay attention to the 'how' information," says Vingerhoets, "you can't help hearing the semantic content, the 'what' of the message. We do this all the time; we are trained in it."

The finding has clinical implications, Vingerhoets says. "People with right-hemispheric lesions would have more difficulty paying attention to and discriminating emotional prosody."

This gives us an important insight into the complexity of human emotional communication. No one doubts that it is instructive to think separately about how each hemisphere works, and people like to generalize, for example, that the left brain is all about logic and even that some people are more "left-brain" than "right-brain." Presumably this means that some people are more logical than emotional. But we should remember that the brain's two halves work as a united organ and that categorizing people in this way is, at best, simplistic.

Rowan Hooper welcomes readers' comments at rowan.hooper@tcd.ie


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