The map of where and how the brain processes speech has been modified again. Researchers from Northwestern University just published a paper in the journal Brain wherein they demonstrate that the famous Wernicke's area, thought to be the center of language comprehension, in fact is not.
In 1874, the scientist Carl Wernicke found that stroke victims whose brains were damaged in the left hemisphere region could suffer from language impairment—their speech might be fluent, but nonsensical; they may refer to every noun as "thing." Such patients, diagnosed with what became known as Wernicke aphasia, might also make up words and have difficulty understanding language.
"People who had strokes that affected Wernicke's region couldn't explain what a word such as 'umbrella' meant," the study's lead author, Marek-Marsel Mesulam said in a press release. "Secondly, they had difficulty understanding sentence construction. If you said, 'Put the apple on top of the book,' even if they understood the meaning of apple and book, they wouldn't be able to carry out the command because they can't understand the construction of the sentence."
This led Wernicke and other researchers to conclude that words and sentences were parsed in the left hemisphere. However, new brain imaging technology is now undermining that outdated finding.
Instead of working with stroke victims, the Northwestern researchers studied 72 people with a rare form of dementia called primary progressive aphasia (PPA), which affects language abilities.
The researchers looked closely at the patients' brains, particularly their Wernicke's area, to look for cortex thinning, which indicates neuron damage. They also tested the patients' linguistic capabilities by having the patients match words to pictures, or quizzing them on the names of things. Surprise: PPA patients with damage in Wernicke's area didn't seem to have trouble understanding individual words like stroke victims did.
"Half of the 10 patients with the most severely impaired sentence comprehension abilities had no significant atrophy in Wernicke's area whereas some others had intact sentence comprehension despite considerable atrophy in this region," the study stated.
This new research suggests that word comprehension isn't located in Wernicke's area, but forward in the brain, at the left anterior temporal lobe.
It looks as though Wernicke's area has more to do with phonological processing, and transforming auditory input into words and word like pattern, "for associative elaboration elsewhere in the brain," Mesulam said in an email.
While the study is more evidence that the left anterior temporal lobe deserves to be "inserted into the canonical language network with a critical role in single word comprehension and object naming," Mesulam stressed that the left ATL isn't the new "center for language."
"There is no center but a network of interconnected areas, each with a slightly different specialization," he said.