How the brain strings words into sentences

While it has long been recognized that certain areas in the brain’s left hemisphere enable us to understand and produce language, scientists are still figuring out exactly how those areas divvy up the highly complex processes necessary to comprehend and produce language.

Two brain areas called Broca’s region and Wernicke’s region serve as the main computing hubs underlying language processing, with dense bundles of nerve fibers linking the two, much like fiber optic cables connecting computer servers. But while it was known that Broca’s and Wernicke’s region are connected by upper and a lower white matter pathways, most research had focused on the nerve cells clustered inside the two language-processing regions themselves. 

Working with patients suffering from language impairments because of a variety of neurodegenerative diseases, Wilsons’ team used brain imaging and language tests to disentangle the roles played by the two pathways. Their findings arepublished in a recent issue of the scientific journal Neuron.

“If you have damage to the lower pathway, you have damage to the lexicon and semantics,” Wilson said. “You forget the name of things, you forget the meaning of words. But surprisingly, you’re extremely good at constructing sentences.”

“With damage to the upper pathway, the opposite is true; patients name things quite well, they know the words, they can understand them, they can remember them, but when it comes to figuring out the meaning of a complex sentence, they are going to fail.”

The study marks the first time it has been shown that upper and lower tracts play distinct functional roles in language processing, the authors write. Only the upper pathway plays a critical role in syntactic processing.

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