2.1 Participants
All participants were recruited by bulletin advertisements in the Shanxi province of China. The exclusion criteria for this study included subjects having neurological or psychiatric diseases, cognitive disabilities, contraindications to imaging studies, history of substance abuse (i.e., illicit drugs or alcohol), or any other major medical illnesses. After applying the exclusion criteria, the remaining 109 subjects were included in this study, consisting of 24 males and 85 females with an average age of 24.8 ± 2.4 years (range: 21–33 years). The subjects provided written informed consent before the study and were reimbursed for their time. Approval for this study was obtained from the Shanxi Medical University Ethics Committee.
2.2 Personality assessment
The FPPI consists of 103 “yes” (score = 1) or “no” (score = 0) items. These questions assess personality in terms of five different traits: Tai Yang, Shao Yang, Yin Yang, Shao Yin, and Tai Yin. In addition, “lie,” which includes 8 items, is another personality dimension used to measure the truthfulness of subject responses. If the score on this dimension was > 4, the questionnaire was regarded as invalid and excluded from the study. The FPPI has shown good reliability, construct validity, and convergent validity with other personality scales [14].
2.3 Data acquisition
All fMRI images were collected on a 3.0T Siemens Trio scanner (Siemens Healthcare GmbH, Erlangen, Germany) at Shanxi Provincial People’s Hospital. The standard eight-channel phase-array head coil was employed, and image artifacts, such a head motion and scanner noise, were reduced with foam padding and headphones. The images were transversely collected using the echo-planar sequence with the following protocol: 32 slices, TR of 2,500 msec, TE of 40 msec, FA of 90°, matrix of 64 × 64, voxel size of 3 mm × 3 mm × 3 mm, FOV of 240 mm × 240 mm, and 212 volume.
2.4 Image pre-processing
The fMRI data were preprocessed with the SPM12 toolset (http://www.fil.ion.ucl.ac.uk/spm). Due to signal instability and subject acclimation, the initial 10 volumes were dismissed. For the 202 images that remained, corrections were implemented to account for delays in slices and head movement. The translational parameters were greater than allowable (2.5 mm) in five datasets. After excluding these five datasets, the remaining images were normalized with respect to the traditional SPM8 echo-planar image template and resampled to 3 mm cubic voxels. Next, spatial smoothing was applied (4 mm full width at half maximum [FWHM] Gaussian kernel), and the linear trends were removed from the final images. Lastly, we regressed the cerebrospinal fluid (CSF), white matter, and six head movement parameters.
2.5 Computation of functional connectivity networks
Anatomical parcellation was accomplished with automated anatomical labeling (AAL) by separating the images into 45 regions of interest (ROI) for each hemisphere of the brain, which resulted in 90 total ROI. Edges of the network were defined by generating 90 × 90 correlation matrixes and calculating the region-wise Pearson’s correlation coefficients. The threshold was set as the sparsity (S), or the ratio of actual existing edges divided by the maximum possible edges in the network. The construction of the brain networks is shown in Fig. 1.
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2.6 Graph theoretical analysis: Network metrics
The topological features of the brain functional networks were assessed by the nodal and global network measures. First, the global metrics, consisting of the normalized clustering coefficient (γ) and characteristic path length (λ), were computed. In comparison to the random networks with low clustering coefficients and shorter path lengths, small-world networks display higher clustering coefficients and similar path lengths (i.e., γ = Cp/Crand > 1, λ = Lp/Lrand ≈ 1) [15]. Small-worldness is the quantitative measurement that includes the combination of these two conditions (i.e., σ = γ/λ > 1) [16]. Meanwhile, the nodal measures encompassed Freeman’s betweenness centrality (BCi) and degree (Ki). More details on the mathematical definitions and interpretations of the network metrics may be found in the Supplementary Material.
2.7 Associations between the network metrics and personality traits
All of the nodal measures reached the threshold in the range of 0.05 ≤ T ≤ 0.40 (interval = 0.01). The area under the curve (AUC) was calculated for each network metric in order to obtain a summarized scalar for topological characterization that was independent of the single-threshold selection [17]. A partial correlation was determined between each of the five trait scores and the AUC of each network metric, with age, gender, and education (years) as covariates. The false discovery rate (FDR) was applied to the data for correction [18] using the code developed by Groppe in 2010 with Matlab (MathWorks, Natick, MA, USA).
2.8 Statistical analysis
P-values < 0.05 were considered statistically significant.