Morphology of the sulci of the ventrolateral frontal cortex in the human brain and relations to cytoarchitecture

The ventrolateral frontal cortex of the human brain consists of the inferior frontal gyrus, which can be subdivided into the pars opercularis, the pars triangularis, and the pars orbitalis. The pars opercularis is delineated from the pars triangularis by the anterior ascending ramus of the lateral fissure (aalf), while the pars triangularis is delineated from the pars orbitalis by the horizontal ascending ramus of the lateral fissure (half). The sulcus diagonalis (ds) is a sulcus that can often be found within the pars opercularis. In the language dominant hemisphere, the inferior frontal gyrus forms the anterior language zone and is necessary for language production. More recent research has demonstrated a role for the pars triangularis and pars orbitalis of both hemispheres in certain aspects of working memory. Despite the research that has been conducted on the functional involvement of the different subdivisions of the inferior frontal gyrus, there remains a discrepancy in the literature regarding the frequency of the sulci that define this region and confusion regarding the morphological patterns that these sulci form with their neighbours. Furthermore, it remains unclear how the specific sulci relate to underlying cytoarchitectonic borders. The first study of this thesis used in vivo MRI to examine the morphology of the aalf and the ds. These sulci were explored in three dimensions, in 40 MRI volumes, to determine their frequency and main morphological patterns with neighbouring sulci. We then quantified the spatial variability and extent of these two sulci using volumetric and surface spatial probability maps to establish the average location of each sulcus in the Montreal Neurological Institute (MNI) stereotaxic space. The results demonstrate that the aalf is a much more common sulcus than the ds and that these two sulci can be differentiated based on their hemispheric depth, the aalf being a much deeper sulcus that extends to the level of the insula. Furthermore, these sulci form three major patterns with their neighbours. The morphological descriptions and MNI space coordinates of the aalf and the ds provided here can be used to improve the identification of the pars opercularis in functional imaging studies of language, as well as during neurosurgery in the frontal lobe in the language dominant hemisphere.The second study consists of a similar morphological analysis of the half. This sulcus was explored in three dimensions, in 50 MRI volumes, to determine its frequency across hemispheres and its main morphological patterns with neighbouring sulci. We also quantified the spatial variability and extent of this sulcus. The results indicate that, as with the aalf, the half is a deep sulcus that extends to the level of the anterior insula. Furthermore, this sulcus forms three major morphological patterns with neighbouring sulci. The results presented here can be used, in combination with those of the first study, to improve the anatomical interpretation of functional neuroimaging studies of the inferior frontal gyrus, as well as the localization of this region during neurosurgery. In the third study, we examined the relationship between the half and the cytoarchitectonic border of areas 45 and 47/12 in BigBrain, the first high resolution histological volume of the human brain. In both hemispheres of BigBrain, we established that area 45 occupies the ventral pars triangularis and the dorsal bank of the half, while area 47/12 lies on the ventral bank of the half and the dorsal pars orbitalis. Importantly, rather than observing a sharp border between these areas, we observed a gradual change in cytoarchitectonic features. The cytoarchitectonic labels of BigBrain provided here will be valuable for the anatomical interpretation of activation foci generated in this part of the inferior frontal gyrus during functional neuroimaging studies of language and memory-related processes

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