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Layer-resolved FMRI activation and connectivity of the left inferior frontal cortex during reading

Poster B118 in Poster Session B, Tuesday, October 24, 3:30 - 5:15 pm CEST, Espace Vieux-Port

Daniel Sharoh1,2, David G. Norris1,3,4, Peter Hagoort1,2; 1Donders Institute for Brain, Cognition and Behaviour, Radboud University, Nijmegen, Netherlands, 2Max Planck Institute for Psycholinguistics, Nijmegen, Netherlands, 3Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Essen, Germany, 4Faculty of Science and Technology, Magnetic Detection and Imaging, University Twente, Enschede, Netherlands

Layer-resolved FMRI is becoming established as a feasible measure of bottom-up and top-down signal contributions to BOLD activation within brain regions (Lawrence et al., 2018). This method supports the direct measurement of these signals by exploiting high spatial resolution FMRI and neuroanatomical information (Koopmans et al.,2010). While previous research has focused primarily on sensory cortices, new research is emerging which focuses on questions in cognitive neuroscience (Finn et al.,2019; Sharoh et al.,2019), as this method transitions from proof-of-concept research to a new role as a methodological tool in application focused brain imaging. Although connectivity of cortical layers is a goal of the field, developments in this area have been limited. This abstract describes results which demonstrate simultaneous bottom-up and top-down connectivity from a portion of BA 44 to regions hierarchically inferior and superior to it. These results were obtained in the context of a word-reading experiment. A network of left hemisphere regions which includes posterior STG (TPJ), BA 44 and (pre-)motor cortex are believed to support acoustic-phonetic processing and articulation. While the connections between these regions have been extensively studied and are known to be supported by the SLF III (Giampiccolo&Duffau,2022), much is unknown concerning connections between BA 44 and (pre-)motor cortex, as well as the network dynamics these regions exhibit under different conditions. METHODS: Submillimeter FMRI data were acquired on a 7T scanner during a reading experiment. We exploited the tendency of the articulatory system to exhibit a BOLD response in some contexts during silent reading (Hagoort et al., 1999). Items were presented individually and displayed for 800ms. We manipulated lexicality (real/pseudo) and length (long/short) of the stimulus items to interrogate relative differences in the contribution of lower and higher order regions to the BOLD signal in BA 44. The grey matter volume in this region was partitioned into three equivolume bins (Waehnert et al.,2014), and the signal from each of the bins was extracted using a spatial GLM (van Mourik et al.,2019). The signal in each of these depth-bins was analyzed to determine the relative bottom-up and top-down contribution to BA 44 for each of the task conditions. Furthermore, a layer-dependent GPPI (Sharoh et al.,2019)--a type of task-based connectivity analysis--was performed to map bottom-up and top-down signal through BA 44 as it traveled through the left hemisphere network. RESULTS AND DISCUSSION: The layer-dependent activation results within BA 44 demonstrated that reading real-words compared to pseudo-words resulted in reduced bottom-up input to to this region. Bottom-up input was additionally observed to decrease for long compared to short real-words. We furthermore determined that bottom-up signal propagated from posterior STG through BA 44 and to orofacial motor cortex, whereas top-down signal propagated from motor cortex to BA 44, and likely back to posterior STG. This finding does not preclude the existence of other connections throughout this network. This result demonstrates that layer-dependent connectivity analysis can be used to directly map BOLD signal changes related to top-down and bottom-up signal sources through an interacting network during word reading.

Topic Areas: Methods, Reading

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