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Stroke patients shed light on metaphors

US team finds key part of the brain that decodes figurative speech.

The left angular gyrus is vital for understanding metaphor. Credit: © Center for Brain and Cognition, University of California, San Diego

Most people understand that the proverb 'The grass is always greener on the other side' has a deeper, more general meaning: someone else's situation always looks more attractive than your own.

But people with defects in a small area of the cerebral cortex called the left angular gyrus take such sentences literally, US researchers have found. They say that the walnut-sized area, located above and behind the left ear, is needed to understand the deeper meaning behind proverbs and metaphors.

Although the area is not the only part of the brain involved in processing metaphors, "it's the main player of the orchestra", says Vilayanur Ramachandran of the University of California, San Diego, who presented the results at this week's annual convention of the American Psychological Society in Los Angeles.

Any monkey can reach for a peanut, but only humans can reach for the stars or even understand what that means. Vilayanur Ramachandran , University of California, San Diego

The researchers studied four patients who had a defect in the left angular gyrus as a result of a stroke or tumour, but were otherwise normal. They asked them to explain the deeper meaning behind 20 proverbs and metaphors. The patients took most of the phrases literally, whereas three patients with lesions in different brain areas correctly interpreted the deeper meaning.

When asked, for example, about the meaning behind 'All that glitters is not gold', one person with a deficient left angular gyrus said, "The deep meaning is that we have to be very careful when buying jewellery."

Sensible abstracts

Ramachandran says it makes sense that the gyrus is involved in understanding metaphors. It is located next to areas of the cortex that process hearing, vision and touch, making it a good candidate for integrating completely different sensations and extracting a common, abstract meaning from them.

To test this prediction, Ramachandran's team tested the patients' ability to translate one kind of sensory information into another, such as shapes into sounds. They found that the patients couldn't associate an image of sawtooths with the sound 'rrrrrrrr' or an image of speckled dots with the sound 'shhhhhh'. Normal people can make the connection between the sensations, even though they have nothing more in common than the abstract concept of jaggedness or fuzziness.

This suggests that the left angular gyrus helps humans to think in abstract terms. The area is much larger in humans than in other primates, says Ramachandran. "Any monkey or ape can reach for a peanut," he says, "but only humans can reach for the stars, or even understand what that means."

Turn the other cheek

Neuroimaging studies have implicated several areas, including the left angular gyrus, in the processing of metaphors, says Mark Jung-Beeman, a neuroscientist at Northwestern University in Evanston, Illinois. But activation in an area does not prove it is a necessary part of the process, he says, which is what makes Ramachandran's study so important: it shows the left angular gyrus is indispensable.

"Studies of a very specific lesion are rare," Jung-Beeman says of the field.

Ramachandran now wants to study the role of the right angular gyrus, which could be more involved in the interpretation of spatial metaphors such as 'having a big heart'. This would be consistent with the notion that the right hemisphere of the brain tends to process spatial information, he says.

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University of California, San Diego

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von Bubnoff, A. Stroke patients shed light on metaphors. Nature (2005). https://doi.org/10.1038/news050523-9

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