The sulcus diagonalis (ds) and the anterior ascending ramus of the lateral fissure (aalf) are two defining sulci of the posterior ventrolateral frontal cortex, which is also known as the anterior language region in the language dominant hemisphere. The aalf extends dorsally from the lateral fissure, separating the pars opercularis from the pars triangularis of the inferior frontal gyrus. The ds, which is a relatively vertical sulcus, is found within the pars opercularis. Given the proximity and similar orientation of these two sulci, it can be difficult to identify them properly. The present study provides a means of differentiating these two sulci accurately using magnetic resonance imaging (MRI). Voxels within the ds and the aalf were labeled in 40 in vivo MRI volumes (1.5 T) that had been linearly registered to the Montreal Neurological Institute stereotaxic space to examine the morphological patterns of these two sulci and classify these patterns based on relations with neighboring sulci. The morphological variability and spatial extent of each sulcus was then quantified in the form of volumetric and surface spatial probability maps. The ds, a rather superficial sulcus, could be identified in 51.25% of hemispheres. The aalf, on the other hand, could be identified in 96.25% of hemispheres and was observed to extend medially, deep below the surface of the hemisphere, to reach the circular sulcus of the insula. Understanding the details of the sulcal morphology of this region, which, in the language dominant left hemisphere, constitutes Broca's area, is crucial to functional and structural neuroimaging studies investigating language.

The horizontal ascending ramus of the lateral fissure (half) is a characteristic sulcus of the ventrolateral frontal cortex that forms the morphological boundary between the pars triangularis and the pars orbitalis of the inferior frontal gyrus. The present study examined the morphology of this sulcus to provide a means of identifying it accurately with magnetic resonance imaging (MRI). Voxels within the half were labeled in 50 in vivo MRI volumes (1.5 T) that had been linearly registered to the Montreal Neurological Institute stereotaxic space and the morphology of the half was categorized based on relations with neighboring sulci. The spatial variability and extent of the half were then quantified across subjects using volumetric (MINC Toolkit) and surface (FreeSurfer) spatial probability maps. The half could be identified in 95% of hemispheres, and the main morphological patterns were classified into three categories: Types I, II, and III. There were no statistically significant interhemispheric differences in the frequency of the half or its morphological patterns. Understanding the details of the sulcal morphology of this ventrolateral region is critical for an accurate interpretation of the location of activation peaks generated in functional neuroimaging studies investigating language, working memory, and other cognitive processes.

Objectives In the human brain, the inferior frontal gyrus (IFG) is comprised of three morphological regions, which include the pars opercularis, pars triangularis, and pars orbitalis. These brains regions are implicated in a number of cognitive and linguistic functions, and the pars opercularis and pars triangularis of the language dominant hemisphere are collectively referred to as Broca's area. Evolution of the morphology of the IFG has been a topic of study in comparative neuroscience. Materials and Methods Using magnetic resonance images (MRI) from 294 chimpanzees with known pedigrees, we quantified the folding patterns, surface area, and depth of three sulci and their subdivisions that define the IFG. Results Chimpanzees show considerable individual variation in IFG sulci. A higher proportion of males and chimpanzees with better orofacial motor control had a bifurcation in the dorsal limb of the fronto‐orbital sulcus. For surface area and depth, chimpanzees with better orofacial motor control had greater leftward asymmetries in surface area and increased gyrification for the fronto‐orbital sulcus and greater rightward biases for the inferior precentral sulcus. Quantitative genetic analyses revealed that the average surface area and depth for three of the five sulci subdivisions were significantly heritable. By contrast, we found no evidence for heritability in asymmetries for any of the sulci. Discussion These findings provide context to understanding evolutionary selection for increasing motor, cognitive and linguistic functions among primates, and contribute an essential comparative framework for interpreting IFG evolution in the human lineage.