Subregions Of The Human Superior Frontal Gyrus And Their Connections

Objective:The superior frontal gyrus (SFG) is located at the superior part of the prefrontal cortex and is involved in a variety of functions, suggesting the existence of functional subregions. However, parcellation schemes of the human SFG and the connection patterns of each subregion remain unclear. To identify the subregions of SFG and their connectivity, we firstly parcellated the human SFG into three different subregions based on diffusion tensor tractography.Subjects and Methods:Two different data sets were obtained in this study. Data set1included diffusion tensor imaging (DTI), structural MR imaging, and resting-state functional MRI (fMRI) data, whereas data set2only included DTI with different scan parameters and structural MR imaging data. All MR images were acquired using a Signa HDx3.0Tesla MR scanner (General Electric, Milwaukee, WI, USA) with an eight-channel phased-array head coil. DTI data were acquired by a single-shot echo planar imaging sequence.The resting-state fMRI datas for all subjects were acquired using a Signa HDx3.0Tesla MR scanner (General Electric, Milwaukee, WI, USA), during fMRI scans, all subjects were instructed to keep their eyes closed, to stay as motionless as possible, to think of nothing in particular, and not to fall asleep.The DTI and the Tl-weighted images were both preprocessed using tools including FMRIB’s Diffusion Toolbox (FSL4.0; http://www.fmrib.ox.ac.uk/fsl) and statistical parametric mapping (SPM8) package (http://www.fil.ion.ucl.ac.uk/spm). After correction for eddy current and head motion, the skull-stripped T1-weighted images were firstly co-registered to the b=0images in native DTI space, and then transformed to the MNI space. Finally, the inverted transformation parameters were used to transform the seed and target masks from MNI space to the native DTI space with nearest-neighbor interpolation.Probabilistic tractography was then performed using the FSL software package. Finally, the results for the probabilistic tractography were stored at a connectivity matrix. Based on the native connectivity matrix, a cross-correlation matrix was calculated that quantified the similarity between the connectivity profiles of the seed voxels. The cross-correlation matrix was then fed into a spectral clustering algorithm for image segmentation.After that, we identified the different anatomical and functional connectivity patterns of the three subregions, then we reparcellated the anteriormedial and the dorsolateral subregions using the same methods and analyzed the anatomical and functional connectivity of the subclusters. Results:Using probabilistic tractography and spectral clustering algorithm, the SFG was parcellated into three separable subregions with different anatomical connection patterns. They were the anteromedial SFG, the dorsolateral SFG, and the posterior SFG.This parcellation scheme was validated by similar analysis of the bilateral SFGs in another independent data set. Using probabilistic DTT, we identified the anatomical connection pattern of each SFG subregion. The SFGam was mainly connected with the anterior cingulate cortex and the mid-cingulate cortex; the SFGdl was connected with the middle and inferior frontal gyri; and the SFGp was connected with the thalamus, precentral gyrus, and inferior frontal gyrus.Overall, these three SFG subregions showed different rsFC patterns. Because the functional significance of the negative rsFC is a matter of debate, we only focused on the positive rsFCs of each SFG subregion. Both the SFGam and the SFGdl were correlated with PCC, precuneus, ACC, medial prefrontal cortex, dorsolateral prefrontal cortex, angular gyrus, and anterior temporal lobe. However, the SFGam was also strongly correlated with the MCC, whereas the SFGdl was also strongly correlated with the middle frontal gyrus. The SFGp was correlated with the precentral gyrus, postcentral gyrus, SMA, MCC, and parts of the parietal cortices.Conclusion:1. We firstly parcellated the human SFG into the anteromedial (SFGam), dorsolateral (SFGdl), and posterior (SFGp) subregions based on diffusion tensor tractography.2. The SFGam was anatomically connected with the anterior and mid-cingulate cortices, which are critical nodes of the cognitive control network and the default mode network (DMN). The SFGdl was connected with the middle and inferior frontal gyri, which involve in the cognitive execution network. The SFGp was connected with the precentral gyrus, caudate, thalamus, and frontal operculum, which are nodes of the motor control network.3. Resting-state functional connectivity analysis further revealed that the SFGam was mainly correlated with the cognitive control network and the DMN; the SFGdl was correlated with the cognitive execution network and the DMN; and the SFGp was correlated with the sensorimotor-related brain regions.4. The SFGam and SFGdl were further parcellated into three and two subclusters that are well corresponding to Brodmann areas.