Structural connectivity of left cortical speech regions defined by direct cortical stimulation during awake language mapping

• ¹ Goethe University, Department of Neurology, Germany
• ² Palacky University, Department of Neurology, Czechia
• ³ Goethe University, Department of Neurosurgery, Germany

The location of eloquent cortex is highly variable. Left hemispheric surgery thus requires functional language mapping via intraoperative direct cortical electric stimulation during neuropsychological testing, which provides a detailed functional map of regions that are critically involved in the tested task. Although the patient's brains are abnormal due to mass lesions, these results provide nevertheless insights into general Neurophysiological principles. We previously could not find a clear relationship between individual regions involved in articulation, speech planning and picture naming and individual gyrification. To test whether the lack of clear attributable functional roles of individual gyri was related to different structural connectivity, we obtained pre-operative T1 and DWI datasets (3T, 60 dir) in so far 4 patients and performed probabilistic fiber tracking (FDT in FSL) using 5mm spheres around the functionally positive sites as seeds, yielding posterior probability distribution maps thresholded at the 90th percentile of all non-zero values. Independent of individual function, the triangular inferior frontal gyrus (IFG) projected to more rostral parts of the mesial prefrontal cortex (mPFC) compared to the opercular IFG that projected to the more caudal pre-SMA and lateral articulatory motor cortex (M1). Yet, independently from individual location on a given gyrus, anomia sites projected to the mPFC and locally to the opercular IFG. Speech arrest sites had local projections to frontal operculum, M1 and postcentral gyrus but also long-range connections to the dorsal premotor cortex, pre-SMA and via the arcuate fascicle to the posterior middle temporal gyrus. Even adjacent sites on the same gyrus that differed in function showed dissociable structural connectivity. Our results suggest that structure-function relationships are more evident between function and structural connectivity than between function and gyrification. This implies a developmental dissociation between gyrification and functional specialization.