Morphological patterns and spatial probability maps of two defining sulci of the posterior ventrolateral frontal cortex of the human brain: the sulcus diagonalis and the anterior ascending ramus of the lateral fissure

Abstract

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.

Appendix

See Figs. 13, 14, 15, 16, 17, and 18.

Fig. 13

Volumetric probability map of the anterior ascending ramus of the lateral fissure (aalf) from the 13 left hemispheres of the Type I morphological group, i.e., those hemispheres in which the aalf could be found directly anterior to the inferior precentral sulcus. The x coordinates of the sagittal sections are indicated in the upper right corner of each section; the z and y coordinates are shown on the appropriate axes. The probability map has been overlaid onto the MNI152 2009c asymmetric template used for registration. The anterior ascending ramus extends from an x coordinate of − 59, laterally, to an x coordinate of − 37, medially. The color bar indicates the extent of overlap of the labeled voxels, with a maximum overlap of 65% occurring at voxel position x − 46, y + 20, z + 6

Fig. 14

Volumetric probability map of the anterior ascending ramus of the lateral fissure (aalf) from the 9 right hemispheres of the Type I morphological group, i.e., those hemispheres in which the aalf could be found directly anterior to the inferior precentral sulcus. The x coordinates of the sagittal sections are indicated in the upper right corner of each section; the z and y coordinates are shown on the appropriate axes. The probability map has been overlaid onto the MNI152 2009c asymmetric template used for registration. The anterior ascending ramus extends from an x coordinate of + 59, laterally, to an x coordinate of + 37, medially. The color bar indicates the extent of overlap of the labeled voxels, with a maximum overlap of 70% occurring at voxel position x + 54, y + 23, and z + 5

Fig. 15

Surface probability maps of the anterior ascending ramus of the lateral fissure (aalf) from a the 13 left hemispheres and b the 9 right hemispheres of the Type I morphological group, i.e., those hemispheres in which the aalf could be found directly anterior to the inferior precentral sulcus. All probability maps have been overlaid onto the surface template, fsaverage, used for registration. The color bar indicates the extent of overlap of the labeled vertices. The x, y, and z coordinates below each surface indicate the position, in MNI305 stereotaxic space, of the vertex with the maximum overlap

Fig. 16

Volumetric probability map of the sulcus diagonalis (ds) from a total of 25 left hemispheres that include the 7 left hemispheres classified as sulcal extension cases. The x coordinates of the sagittal sections are indicated in the upper right corner of each section; the z and y coordinates are shown on the appropriate axes. The probability map has been overlaid onto the MNI152 2009c asymmetric template used for registration. The sulcus starts at an x coordinate of − 60, laterally, and finishes, medially, at an x coordinate of − 44. The color bar indicates the level of overlap of the labeled voxels. Two distinct peaks are generated, with a maximum overlap of 28% occurring at voxel position x − 49, y + 17, z + 5

Fig. 17

Volumetric probability map of the sulcus diagonalis (ds) from a total of 27 right hemispheres that include the 4 right hemispheres classified as sulcal extension cases. The x coordinates of the sagittal sections are indicated in the upper right corner of each section; the z and y coordinates are shown on the appropriate axes. The probability map has been overlaid onto the MNI152 2009c asymmetric template used for registration. The sulcus starts at an x coordinate of x + 60, laterally, and finishes, medially, at an x coordinate of + 46. The color bar indicates the level of overlap of the labeled voxels. Two distinct peaks are generated, with a maximum overlap of 33% occurring at voxel position x + 54, y + 21, z + 16

Fig. 18

Surface probability maps of the sulcus diagonalis (ds) from a total of a 25 left hemispheres and b 27 right hemispheres that include the sulcal extension cases. All probability maps have been overlaid onto the surface template, fsaverage, used for registration. The color bar indicates the extent of overlap of the labeled vertices. The x, y, and z coordinates below each surface indicate the position, in MNI305 stereotaxic space, of the vertex with the maximum overlap