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On the Shapes of the Precentral Gyrus of Humans and Chimpanzees
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Edité par CCSD -
International audience. The cerebral neocortex, unique to mammals, has the particularity of being folded in many species. This folding is not random but related to the internal structural and functional organization[1,2]. Some patterns can be shared between species, such as the central (CS) and precentral (preCS) sulci in great apes that constitute the boundaries of the precentral gyrus, place of motor cortex (BA 4, and a part of BA6)[3].We have developed a method to capture the shape variability of cortical sulci within and between species. Here we examine the interindividual variability of the precentral gyrus (preCG) in humans and chimpanzees.77 chimps (Pan troglodytes) were scanned in vivo at 3T according to US Department of Health and Human Services guidelines. Using the "Morphologist 2012" pipeline [4], CSs and preCSs were extracted from T2-weighted images, as well as T1-weighted images of sixty human HCP subjects. All sulci were affinely normalized in Talairach space and then with a species-specific homothetic transformation. Right sulci were flipped to allow comparison with left ones. A pairwise shape similarity matrix was computed after rigid alignment [5]. Isomap-based manifold learning was used to capture two axes describing the greatest parts of variance in the space spanned by the sulci. For better visualization, the 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 isomap axes. We performed three experiments for intra- and inter-species comparisons.Our moving averages capture preCG well in both species. The CS highlights the motor knob of the hand, which is larger, deeper, and more ventral in chimps than in humans. The preCS is mainly composed of two parts, far apart in chimps, corresponding to the inferior preCS (IP) and the superior preCS (SP) [7]. Between the IP and SP is a transverse gyrus that merges with the posterior part of the middle frontal gyrus. The width of the preCG is relatively larger in chimps, which is consistent with [6].In humans, the CS and preCS are not exactly aligned, with the preCS being more ventral. This may be due to the higher operculation in humans.The shape variability features expressed on each of the isomap dimensions for the chimp and human experiments are different. No axis exhibits left-right asymmetry. Chimp dimension1 shows a shortening of the ventral portion of the preCG while keeping the transverse gyrus of the preCG aligned with the knob of the hand; as well as a shift of the orientation of SP to the knob. Dimension2 represents the two parts of preCG moving towards the knob.In humans, dimension1 corresponds to a narrowing of the gyrus dorsally while the hand area develops and a somatosensory knob aligned with the transverse gyrus appears. Dimension2 represents the transverse gyrus reaching the surface simultaneously with the development of a somatosensory knob in the same middle part of the preCG.The dimension1 that describes the variability of chimps and humans mixed together, shows that they separate along the axis: humans have a dorsal knob and the transverse gyrus connects at the somatosensory knob, while in chimps the gyrus arrives at a more ventral knob.The preCG narrows in this same area.The dimension2 does not separate the species (p=0.10). The hand knob remains in dorsal position while the SP shrinks, no major change ventrally.Their knob being more dorsal, humans seem to have a longer oro-facial area allowing more complex structures. The transverse gyrus is mainly aligned with the hand knob in chimps while aligned with a more ventral sensory knob in humans. This location may correspond to the midprCG, a region defined by neurosurgical investigations [8] and functionally located between dorsal hand and ventral orofacial cortical representations. Further work may benefit from the addition of fibers tracts and cytoarchitectonic to better understand what these folding variations may reflect.
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