Comparison of the shape of the Central Sulcus in Hominids Ophelie Foubet1, Zhong Yi Sun1, William Hopkins2, Jean-François Mangin1 Institutions: 1Université Paris-Saclay, CEA, CNRS, Neurospin, Gif-sur-Yvette, France 2University of Texas MD Anderson Cancer Center, Bastrop, TX Introduction: Gyrification of the neocortex appears during the third trimester of gestation in humans. While the folding pattern is stable and simple in small mammalian brains, it becomes rich and complex in larger brains, reaching a peak of complexity and inter-individual variability in humans [1,2]. However, among this variability some major features are stable within species and even between species. The central sulcus (CS) is one of them, dividing the motor cortex from the somatosensory cortex in primates [3]. We examined here the morphological variability of CS in hominids, with a focus on the motor hand region, a significant issue of human evolution. Methods: T2-weighted images scanned at 4.7T or 7T of 30 chimps (Pan troglodytes), 30 gorillas (Gorilla gorilla) and 18 orangutans (Pongo pygmaeus) were acquired under the guidelines of the US Department of Health and Humans Services. CSs were extracted from these MRIs, as well as from T1-weighted images of thirty human HCP subjects, using the "Morphologist 2012" pipeline[4]. All sulci were affinely normalized in Talairach space and a species-specific homothetic transformation was applied before right sulci were flipped to be compared to the left ones. A pairwise comparison of their shape was performed after a rigid alignment [5]. Isomap-based manifold learning was used to capture a one-dimensional approximation of the space spanned by the sulci. To exhibit the variability trait described on the isomap, all sulci were rigidly aligned to the sulcus with the minimum distance to the set and local shape averages were performed at regular intervals along the one-dimensional manifold. We performed different experiments for intra and inter-species comparisons. Results: Beyond their differences in overall CS shape, the 4 species appear to share a common variability trait described on each of their isomap axis. This trait is similar to previous results on large human databases [5,6] and corresponds to a shape variation ranging from a single motor hand knob configuration to a double knob configuration. The axis displaying all sulci reveals a separation of species. While chimp and gorilla populations share the same variation in CS shape (p=0.79), humans and orangutans appear to have their own variability trait and share little with the other species. Interestingly, humans find themselves in the middle, between the shape variability of orangutans and the common one of chimps and gorillas. The axis seems to describe again a variation in shape from a single knob to a double knob, with the addition however of a rotation of the sulcus on the orangutan side. We note the appearance of a shift between the two parts of the SC, dorsal and ventral to the knob, at the level of humans and which develops towards the orangutans. This shift seems to allow for the development of a more ventral somatosensory knob rather than a real second motor knob. Conclusions: The inter-individual variability within all hominid species appears to be characterized primarily by a shape variation ranging from a single to a double knob configuration. When the 4 species are mixed, the single-to-double-knob trait is no longer enough to describe all the variability. The distributions of the different species do not mix, suggesting that differences in overall sulci shape between species prevail. Compared to chimps and gorillas, humans and orangutans appear to have a larger buried gyrus in the central region of the CS, suggesting greater sensory-motor connectivity and integration between their motor and somato-sensory cortices [3]. This common feature could explain the similarity in shape of their hand-knob, as it has to fold over the large buried gyrus. Furthermore, it could explain the appearance of a knob in the somato-sensory cortex of humans and orangutans on the axis common to all 4 species. Their better sensory-motor integration could reflect an increase in hand and digits use for bipedal humans and an increase in hand and foot grasping for tree-dwelling orangutans. Acknowledgement: HBP-SGA2 (785907), FRM (DIC20161236445), ANR(IFOPASUBA), Chaire Blaise Pascal of Region Ile de France to W. Hopkins [1] Fischl B (2008), ‘Cortical folding patterns and predicting cytoarchitecture.’, Cereb Cortex 18:1973-1980. [2] Welker W (1988), ‘Why does cerebral cortex fissure and fold?’, Cereb Cortex 8B:3-135. [3] Hopkins W.D. (2014), ‘Evolution of the Central Sulcus Morphology in Primates’, Brain, behavior and evolution 84.1 (2014): 19-30. [4] Fischer C (2012), ‘Morphologist 2012: the new morphological pipeline of BrainVISA.’, In. HBM. Beijing. [5] Sun Z. Y. (2012), ‘The Effect of Handedness on the Shape of the Central Sulcus’, Neuroimage, 60(1):332-9 [6] Sun Z. Y. (2016), ‘Linking morphological and functional variability in hand movement and silent reading.’, Brain Structure and Function 221(7):3361-71