Published online 12 November 1999 | Nature | doi:10.1038/news991118-2


Making sense of sentences

"The cat chased the mouse." "The mouse chased the cat." These two sentences contain exactly the same words, but obviously don't mean the same thing. When your brain works out the meaning of a sentence, it can't just rely on the meanings of the individual words: it must also take into account their context and order.

Or to put it another way, there are two aspects to processing a sentence: meaning and structure - or, more technically, semantics and syntax. New evidence published in Neuron1suggests that these two properties are processed separately, in different areas of the brain. The results also hint that the way we understand and use language is more complicated than was previously thought.

Mirella Dapretto and Susan Bookheimer, of the University of California at Los Angeles, have used brain scans to find out which areas of the brain are most active when people are working out what speech means. They asked study subjects to listen to pairs of sentences and decide whether or not they had the same literal meaning.

Some of the test sentences differed in syntax - for example, "The policeman arrested the thief" and "The thief was arrested by the policeman" - while others had the same structure but different words. In the latter set of examples, the words could either be synonyms of each other, in which case the overall meanings of the sentences were the same, or not. For example, "The car is in the garage" and "The automobile is in the garage" mean the same thing, "The bike is in the garage" does not.

Dapretto and Bookheimer suspected that the two groups of sentence pairs would require the brain to use different types of processing. The former set of sentences should, they reasoned, tap into the brain's syntax-analysis machinery, whereas the latter set would require the brain to use its semantics processors. The researchers used functional magnetic resonance imaging, which measures increases in blood flow, to look at which areas of the brain were particularly active during each of the two tasks.

Their results indicate that these two aspects of language processing are carried out by two different parts of the brain. Both are on the left side, in an area called the 'inferior frontal gyrus', which is known to be important in language use.

Dapretto and Bookheimer found that when the brain was processing semantic information, an area called the 'pars orbitalis' was active. Syntax, in contrast, appears to be processed in part of the brain called 'Broca's area'.

Damage to 'Broca's area' normally causes difficulty in producing speech, rather than in understanding it. Neuroscientists had therefore thought that it was important only for speaking, not for comprehension - yet another part of the brain, 'Wernicke's area', was thought to take care of this. The new results suggest, however, that Broca's area has more than one string to its bow. 

  • References

    1. Dapretto,M. & Bookheimer, S. Y. Form and Content: Dissociating Syntax and Semantics in Sentence Comprehension Neuron 2, 427 1999