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Poster C4, Friday, August 17, 10:30 am – 12:15 pm, Room 2000AB

Cortico-striatal tractography: Structural connectivity of the left inferior frontal gyrus along the rostrocaudal length of the putamen

Simone Renée Roberts1,2,3, Anastasia Bohsali4,5, Stella Maria Tran1,2,3, Jonathan Harris Drucker1,2, Steven Hirschmann, Tricia Z. King3, Lisa C. Krishnamurthy1,2,3, Venkatagiri Krishnamurthy1,2, Thomas Mareci5, Bruce Crosson1,2,3;1Center for Visual & Neurocognitive Rehabilitation, US Department of Veterans Affairs, Atlanta, GA, 2Emory University, 3Georgia State University, Atlanta, 4Brain Rehabilitation Research Center, US Department of Veterans Affairs, Gainesville, FL, 5The University of Florida, Gainesville, FL

The left inferior frontal gyrus (IFG) is widely acknowledged for its domain-specific, functional contributions to the human language network. In contrast, the basal ganglia are not believed to maintain domain-specific language functions, but rather to provide domain-general or executive support that influences behavior and development. The putamen of the basal ganglia is primarily considered to be specialized for motor and sensory support, in consideration of extensive, rostrocaudally distributed projections from motor and sensory cortices. However, accumulating evidence implicates the putamen for substantial language network involvement as well. For instance, putaminal lesions are associated with speech articulation changes. Volumetric studies have correlated native-proficiency bilingualism with greater putaminal volume, while functional MRI research has demonstrated that non-native, bilingual speech production may activate regions along the rostrocaudal length of the left putamen. Structurally, ex vivo neuronal tracing studies in non-human primates have demonstrated that the anterior putamen receives white matter projections from cortices homologous with all three regions of the IFG. In vivo, white matter tractography from diffusion-weighted MRI has similarly demonstrated anterior putaminal connectivity in humans, for Broca’s area of the left hemisphere’s IFG (i.e. pars opercularis and pars triangularis). The present study replicates and extends previous tractography research by demonstrating structural connectivity of Broca’s area and the putamen in the left hemisphere, evaluating the presence of similar projections from the left IFG’s pars orbitalis, and visually mapping within-putamen white matter topography for all left IFG projections. Diffusion-weighted images were acquired from 11 healthy, young adult volunteers. High-angular, deterministic streamline tractography was generated using a mixture of Wisharts signal attenuation model and spherical deconvolution. Structural networking eliminated streamline vectors that did not directly connect nodes of interest in order to filter data for relevance, while retaining all fiber estimations within nodes to permit visualization of within-putamen connectivity. Resulting networks replicated previous findings of Broca’s area’s structural connectivity with the left anterior putamen and suggested the presence of similar projections originating from pars orbitalis. Across IFG cortices, projections along the rostrocaudal length of the putamen were also observed. Streamlines that entered the midsection and posterior of the left putamen passed through the external capsule and terminated medially. Comparable external capsule projections have been reported previously for non-human primates and rats, however not for fibers originating in the IFG. Thus, these findings may in part represent a species-specific structural connectivity pattern. The presence of rostrocaudally distributed putaminal fiber projections from the IFG suggests a unique and possibly integrative role for the putamen in cognitive processing. Is the human putamen structurally equipped to orchestrate executive coordination of language/motor integration? If so, this study’s findings might in part illustrate why humans are able to perceive and program the extraordinarily diverse set of speech sounds and combinations that characterize human verbal communication. Moving forward, broader conceptualization of the putamen as a dual language/motor structure and further vetting of human putaminal connectivity might lead to novel insights regarding the substrate, development, and execution of language acquisition and speech articulation.

Topic Area: Speech Motor Control and Sensorimotor Integration