- This study made use of intrinsic functional connectivity to explore the anatomy of the language network in 18 volunteers and contextualize it alongside neighboring functional networks
- A language network was defined that was distinct but spatially adjacent to the default and frontoparietal control networks throughout the cortex and had a distributed spatial organization similar to other association networks
- The network responded to language tasks in a manner specific to the functional anatomy of each volunteer
- A distinct intermediate distributed network occupied regions between the language network and the orofacial motor and auditory regions, suggesting a network hierarchy that links language to functionally related sensorimotor regions
- Patients undergoing interventions such as intracranial neuromodulation or resection of brain tumors might someday benefit from precise functional mapping of their language network
For years, research at Massachusetts General Hospital and elsewhere has focused on large-scale networks within association zones of the human brain. Studying their functional properties can advance the understanding of cognition, including the ability to use language.
Regarding the anatomy of language function, contemporary findings suggest a specialized, left-lateralized network that is active during speech reception and production. To build on that research, Rodrigo M. Braga, PhD, postdoctoral fellow, and Randy L. Buckner, PhD, director of the Psychiatric Neuroimaging Research Program at Martinos Center for Biomedical Imaging at Massachusetts General Hospital, and colleagues recently used neuroimaging to characterize the spatial organization of the language network in relation to nearby, functionally distinct networks.
In the Journal of Neurophysiology, they present evidence that the language network is spatially distinct from similarly organized but differentially specialized association networks. Moreover, regions within the language network are particularly close to a network of motor and sensory regions that are hypothesized to participate in speech production and reception.
Identifying the Language Network Within Individuals
The researchers recruited 18 adults to undergo intrinsic functional connectivity, a technique for identifying activity patterns within and between specific regions of the brain during MRI. In all individuals, they observed a distributed network that contained regions in classic perisylvian language areas. However, the language network occupied regions that were juxtaposed or even interdigitated with other association networks, such as the default, frontoparietal control and salience networks.
Organization of the Language Network
The language network was found to be a characteristic association network—it had widely distributed regions located in multiple cortical zones. It extended beyond the classic left-lateralized language areas to the parietal, midcingulate and inferior temporal cortices, with a further candidate region within the ventromedial prefrontal cortex. An anterior prefrontal region that appeared to be distinct from the posterior superior frontal gyrus was also detected.
Additional regions were detected in the right hemisphere, and they displayed a distributed organization homologous in many ways to the spatial distribution of the left hemisphere.
Task Activation and the Language Network
While in the MRI scanner, each volunteer completed language localizer tasks (reading meaningful sentences as well as lists of pronounceable nonwords, such as "zoller"). Overlap between functional connectivity and task activation maps was observed throughout the cortical mantle.
In addition, the overlap between the language network and the regions showed increased task activation tracked with each individual's functional anatomy. Most notably, the task activation map for one subject was strongly right-lateralized, and in two others it was weakly right-lateralized, but correspondences with the language network remained.
An Intermediate Network
The researchers thought the location of prominent language network regions might be explained by their proximity to the orofacial motor and auditory regions involved in hearing and producing speech. While testing two individuals, they unexpectedly found a slight separation between these sensorimotor regions and the language network in both frontal and temporal cortices.
Further investigation revealed a distinct intermediate network that bordered the language network. This network was juxtaposed with the language network at dorsolateral, dorsomedial and ventrolateral locations in the frontal lobe and at the posterior superior temporal cortex. These key features of the intermediate network were confirmed in the 16 other volunteers.
These spatial relationships raise the possibility that the language and intermediate networks form a hierarchy of distributed networks that fall along a gradient from language regions to sensorimotor and possibly other association networks.
Potential Clinical Applications
Patients undergoing interventions such as intracranial neuromodulation or resection of brain tumors might someday benefit from the precise functional mapping of their language network.
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