Poster B65, Thursday, August 16, 3:05 – 4:50 pm, Room 2000AB

An in-depth examination of the morphology of two sulci of the frontal language zone and comparisons to cytoarchitectonic Brodmann areas 44 and 45

Trisanna Sprung-Much1,2, Michael Petrides1,2;1McGill University, 2Montreal Neurological Institute

The sulcus diagonalis and the anterior ascending ramus of the lateral fissure are located within the posterior ventrolateral frontal cortex, otherwise known as the frontal language zone in the language dominant hemisphere. On the surface of the cortex, the anterior ascending ramus divides the inferior frontal gyrus into the pars opercularis, posteriorly, and the pars triangularis, anteriorly. The sulcus diagonalis is a relatively-vertically oriented sulcus located within the pars opercularis. We labeled voxels within the sulcus diagonalis and the anterior ascending ramus in 40 in-vivo MRI (1.5T) volumes (i.e. 80 hemispheres) that had been linearly registered to the Montreal Neurological Institute (MNI) stereotaxic space, in order to determine the morphological patterns formed by these two sulci. Our results demonstrate that the sulcus diagonalis is a fairly superficial sulcus that is less frequent than the anterior ascending ramus. The latter is a much more reliable sulcus that consistently extends medially to reach the insula. Furthermore, we were able to identify four major morphological patterns formed by these two sulci, which we classify here as Types I-IV. In Type I, the sulcus diagonalis was absent, and the anterior ascending ramus lay directly anterior to the inferior precentral sulcus. Type II consisted of those hemispheres in which both the anterior ascending ramus and the sulcus diagonalis could be easily identified on the surface of the hemisphere. In Type III, the sulcus diagonalis was present but joined superficially with either the anterior ascending ramus (Type IIIa), the inferior precentral sulcus (Type IIIb), or the inferior frontal sulcus (Type IIIc). In Type IV, a third anterior branch of the lateral fissure could be identified, that is, in addition to the anterior ascending ramus of the lateral fissure and its counterpart, the horizontal ramus of the lateral fissure. This third branch tended to form a 45° angle to the ascending and horizontal rami and always extended medially to reach the insula. Finally, we compared the average location of the anterior ascending ramus, generated across all 40 subjects, with the location of cytoarchitectonic Brodmann areas 44 and 45 based on data freely available in the open-access pipeline FreeSurfer. The results illustrate what traditional cytoarchitectonic maps suggest, namely that, posterior to the anterior ascending ramus lies Brodmann area 44 on the pars opercularis and, anterior to it, lies Brodmann area 45, on the pars triangularis. Establishing a thorough understanding of the sulcal morphology of this region will enhance the anatomical accuracy of functional imaging studies investigating language.

Topic Area: Methods

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