November 30, 2011
New Brain Study Illuminates Complexity of Linguistic Processing
While researchers have long known that the left hemisphere of the human brain is critical in the process of producing and decoding semantics and language, the intricacies of the ℠division of labor´ between the different areas of the brain have largely remained a mystery. Technological advances in the field of brain imaging in the last decade, however, have begun to allow researchers to get a better look into the geography of the brain in these extremely complex cognitive processes.
One of the most surprising discoveries has been the role played by white brain matter as a sort of highway connecting various regions and functions of the cerebral cortex .
Prof. Stephen Wilson, a recent addition to the University of Arizona℠s research division on speech, language and hearing, says that scientists´ understanding of the complexity of the brain is steadily expanding.
“With this new technology, scientists started to realize that in the language network, there are a lot more connecting pathways than we originally thought,” he stated.
“They are likely to have different functions because the brain is not just a homogeneous conglomerate of cells, but there hasn´t been a lot of evidence as to what kind of information is carried on the different pathways.”
The degree of specialization between different parts of the brain is one very important part of this complexity. For instance, behind the phenomenon of language processing are two critical areas of the brain called Broca´s region and Wernicke´s region which are able to communicate via the densely bundled nerve fibers that connect them to each other. While researchers were aware that these pathways were divided into an upper and lower region, most of their work had previously been devoted to trying to decipher the activities of the two processing regions rather than examining the pathways that connected them. However, as a recent publication by Wilson in the scientific journal Neuron outlined, these pathways themselves are highly important in their own right.
“If you have damage to the lower pathway, you have damage to the lexicon and semantics,” Wilson explains. “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."
Wilson´s article was the outcome of a study focusing on language impairments that resulted from various neurodegenerative diseases. His research team utilized a combination of cutting-edge brain imaging techniques and specialized language tests to examine a group of men and women patients in their mid-sixties. Subjects were able to participate in the project via the Memory and Aging Center at UCSF.
The results, Wilson says, are helping to decipher the different roles played by the two transmission pathways.
“If you have damage to the lower pathway, you have damage to the lexicon and semantics,” Wilson explained. “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.”
Wilson´s ground-breaking research represents the first neurological study to demonstrate the distinctive linguistic functions associated with the two nerve pathways.
Unique to Wilson´s study is the fact that his team was able to examine patients with various degrees of often degenerative brain damage. By contrast, the majority of previous studies have relied on patients whose acquired language disorder--known as aphasias-- was the result of single event brain traumas.
“Most aphasias are caused by strokes, and most of the strokes that affect language regions probably would affect both pathways,” says Wilson. “In contrast, the patients with progressive aphasias who we worked with had very rare and very specific neurodegenerative diseases that selectively target different brain regions, allowing us to tease apart the contributions of the two pathways.”
The result was that the research team was able to differentiate between the divergent functional roles of the upper and lower pathways. Wilson explained how his team combined the latest MRI technology with specially designed linguistic tests.
“We would give the study participants a brief scenario and ask them to complete it with what comes naturally. For example, if I said to you ℠A man was walking along the railway tracks. He didn´t hear the train coming. What happened to the man?´ Usually, you would say ℠he was hit by the train´ or something along those lines.”
“But a patient with damage to the upper pathway might say something like ℠train, man, hit.´ We found that the lower pathway has a completely different function, which is in the meaning of single words.”
The decision of Wilson´s team to focus on the white matter that connects the brain´s processing centers rather than the grey matter where all the action was assumed to take place is another factor that has set their study apart.
“Our study shows that the deficits in the ability to process sentences are above and beyond anything that could be explained by gray matter loss alone,” Wilson explained.
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