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Poster A8, Thursday, August 16, 10:15 am – 12:00 pm, Room 2000AB

Quantification and parcellation of posterior inferior frontal cortex connections to auditory association and supplementary motor area targets

Vatche Baboyan1, Gregory Hickok1, Nitin Tandon2,3;1Department of Cognitive Sciences, University of California, Irvine, 2Vivan L Smith Department of Neurosurgery, University of Texas Medical School at Houston, 3Memorial Hermann Hospital, Texas Medical Center, Houston

Diffusion imaging studies over the past decade suggest that a variety of intra- and inter-lobar association fibers important for language terminate near the posterior inferior frontal cortex (pIFC). These studies rarely examine the asymmetry of the connections to posterior temporal and superior frontal cortex pathways and their inter-relationship to language dominance. Here, we sought to quantify the structural connections of the pIFC, bilaterally, to well-known targets in auditory and motor association cortex within a set of neurosurgical patients having undergone the Intracarotid Amytal (Wada) Test for language lateralization. We tested whether left-right asymmetries exist among pIFC connections to the supplementary motor Area (preSMA/SMA), supramarginal gyrus, and posterior temporal cortex (STG/MTG). To infer a relationship between structural asymmetry and language laterality, only unilateral left-hemisphere language dominant subjects were enrolled in this study. Anatomical regions of interest were generated for the pIFC (comprising BA45, BA44, and ventral BA6), the supplementary (SMA proper) and pre-supplementary motor cortex (preSMA), the supramarginal gyrus (SMG), and posterior temporal cortex (STG/MTG). Probabilistic fiber tracking was implemented from each pIFC subregion to each of the target regions and inter-hemispheric differences in seed-to-target connectivity was compared. In addition, we performed a pIFC parcellation at the individual subject level by clustering seed-to-target connectivity computed from the pIFC as a whole. The resulting cluster areas were also statistically compared to determine whether the pIFC shows inter-hemispheric differences in it’s preferred cortico-cortical connections. We found that BA45 showed a leftward asymmetry to the SMA and a rightward asymmetry to the SMG and pSTG. BA44 showed a significant leftward asymmetry of connections to the posterior MTG and rightward asymmetry of connections to the SMG. vBA6 showed a leftward asymmetry to both the pSTG and pMTG. These findings demonstrate that the anterior portion of Broca’s area (specifically, BA45) has a leftward asymmetry of connections to the supplementary motor area while posterior portions (BA44/vBA6) have leftward asymmetries of connections to posterior temporal cortex. Consistent with prior research, the present work implicates the significance of a left-dominant structural network of both pIFC to SMA and pIFC to MTG/STG connections for language function while a right-dominant network of pIFC to SMG network likely contributes to lateralized visuospatial function. This work also indicates that, similar to studies of the thalamus, a so-called “hard parcellation” of the pIFC on the basis of its structural connections may present a clinically useful imaging biomarker to lateralize speech and language function.

Topic Area: Speech Motor Control and Sensorimotor Integration