Developmental biology - Brain Function|
Tracking Brain Signals That Drive Language
Understanding how our brain makes word-choices...
Scientists have discovered where in the brain neuronal signals become distinguished into language. Their discovery paves the way for treating aphasia (lost language comprehension) due to stroke or other disorders.
Effective verbal communication depends an ability to retrieve and select appropriate words that convey our intended meaning. For many, this process is instinctive, but for someone who suffers a stroke or has another type of brain damage, communicating even the most basic message can be difficult.
Scientists know a brain region called the left inferior frontal gyrus (LIFG) is critical for language production and word processing. However, it remains unclear exactly how the LIFG region interacts with other brain regions.
Using magnetic brain stimulation - a method sometimes used to treat depression - along with network control theory, researchers at Drexel University and the University of Pennsylvania took a novel approach to how brain networks interact when making word-choices. Their results are published this month in the Journal of Neuroscience.
"Our ability to understand neural systems is fundamentally related to... our brain's ability to integrate and segregate signals across major brain networks."
John Medaglia PhD, Assistant Professor, Psychology, Drexel University; first author.
Medaglia, along with study co-author Danielle Bassett PhD, is using brain stimulation to create a map of all brain interactions and identify how one network might connect to and affect another. "Network neuroscience provides computational methods to uncover structure in brain imaging data," explains Bassett.
To see how the LIFG brain region is affected by language tasks, the team sent transcranial magnetic stimulation (TMS) to parts of the brain of twenty-eight study subjects. This was while each subject was asked to complete two different language tasks. In the first task, study participants completed open-ended sentences such as, "They left the dirty dishes in the..." and were asked to say a single word to complete that sentence. In the second task, study participants were asked to name specific images or numbers when presented.
During each task, researchers measured a participants' response time while administering TMS. Then researchers used mathematical formulas to determine controllability amongst the brain's network systems. The research team focused on how each language task affected two distinct brain networks: (1) modal control, which is the ability of a brain region to drive a network into "difficult to reach" states and (2) boundary control, a theoretical region guiding brain networks to intercommunicate.
Researchers found that boundary control is needed to retrieve and select a single word in the face of competing, alternative responses. By contrast, modal control is related to closed-ended language tasks. This suggests LIFG's ability to integrate and segregate communication between brain networks is used when choosing among several possibilities.
Medaglia's group was surprised to find a very clear distinction in brain response to these two similar language tasks.
"I thought our results would be more muddied. There are debates about how unique these processes truly are, and now we have evidence that you can make a clear distinction between them," Medaglia said. "It was also surprising to me that you could find this effect when studying the whole brain, whereas a lot of traditional views on language would have you focus on a much more specific area."
The research team will next use the same technique with stroke patients to see if TMS on damaged brains can help improve a patient's speech.
In language production, humans are confronted with considerable word selection demands. Often, we must select a word from among similar, acceptable, and competing alternative words in order to construct a sentence that conveys an intended meaning. In recent years, the left inferior frontal gyrus (LIFG) has been identified as critical to this ability. Despite a recent emphasis on network approaches to understanding language, how the LIFG interacts with the brain's complex networks to facilitate controlled language performance remains unknown. Here, we take a novel approach to understand word selection as a network control process in the brain. Using an anatomical brain network derived from high-resolution diffusion spectrum imaging (DSI), we computed network controllability underlying the site of transcranial magnetic stimulation in the LIFG between administrations of language tasks that vary in response (cognitive control) demands: open-response (word generation) vs. closed-response (number naming) tasks. We find that a statistic that quantifies the LIFG's theoretically predicted control of communication across modules in the human connectome explains TMS-induced changes in open-response language task performance only. Moreover, we find that a statistic that quantifies the LIFG's theoretically predicted control of difficult-to-reach states explains vulnerability to TMS in the closed-ended (but not open-ended) response task. These findings establish a link between network controllability, cognitive function, and TMS effects.
This work illustrates that network control statistics applied to anatomical connectivity data demonstrate relationships with cognitive variability during controlled language tasks and TMS effects.
Authors: John D. Medaglia, Denise Y. Harvey, Nicole White, Apoorva Kelkar, Jared Zimmerman, Danielle S. Bassett and Roy H. Hamilton.
The authors declare no competing financial interests.
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Jun 27, 2018 Fetal Timeline Maternal Timeline News News Archive
The brain region called left inferior frontal gyrus or LIFG
is critical to language production and word processing. Image credit: Wikipedia