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  • Seventy-one right-handed healthy university students (34 males) underwent magnetic resonance imaging (MRI) scans at Jinling Hospital, Nanjing, China. The mean age of the participants was 22.35 years (standard deviation = 1.5, age range: 19–26 years). Informed written consents were obtained from all participants before any study procedure was initiated. Exclusion criteria included any history of psychiatric or neurological illness, brain injury, and alcohol or drug abuse. An informal interview prior to the MRI scanning was conducted to confirm that subjects did not use any psychotropic drugs. Left-handed individuals, who were assessed using Chinese revised version of the Edinburgh Handedness Inventory, were excluded [19]. The present study was approved by the local Medical Ethics Committee at Jinling Hospital, Nanjing University. To assess personality traits, all participants completed the Eysenck Personality Questionnaire-Revised, Short Scale for Chinese (EPQ-RSC), a self-report questionnaire [20]–[23]. We chose the EPQ-RSC since it has been extensively used for clinical and research purposes in China. Furthermore, the validity and stability of this scale have been authenticated in Chinese subjects. The EPQ-RSC includes four dimensions: Extraversion (E), Neuroticism (N), Psychoticism (P), and Lie (L). Subjects responded to each item in the questionnaire with “yes” or “no” (coded as 1 or 0) depending on the applicability of the statement. The raw scores were translated into T-scores using the following formula [22]:where mean is the mean value of the personality scores over all participants; SD is the standard deviation of the personality scores. Qian et al. [22] suggested that EPQ-RSC had satisfactory reliability and validity of extraversion, neuroticism and lie, whereas the reliability and validity of psychoticism were relatively lower. We focused our analyses on extraversion and neuroticism, two important and significant personality dimensions whose resultant T-scores were used for measuring correlations with GMV values in the present work. High-resolution 3D structural MRI scans of all subjects were performed on a 3T MRI scanner (Siemens-Trio, Erlangen, German) using a T1-weighted spoiled grass gradient recalled sequence at the Jinling Hospital, Nanjing, China. Tight but comfortable foam padding was used to minimize head motion, and earplugs were used to reduce scanner noise. The following parameters were used: repetition time (TR) = 2300 ms, echo time (TE) = 2.98 ms, flip angle = 9°, slice thickness = 1 mm, FOV = 24 cm×24 cm, matrix size = 512×512, and voxel size = 0.5 mm×0.5 mm×1 mm. Images were initially visually inspected for artifacts or structural abnormalities unrelated to healthy subjects. Subjects with general MRI contraindications were excluded in the following analyses. VBM analyses were performed using SPM8 (http://www.fil.ion.ucl.ac.uk/spm) as previously described [24]. The detailed procedures were as follows. First, the origin of each participant’s structural images was set to the anterior commissure manually. Second, all images were divided into gray matter, white matter, and cerebrospinal fluid, and then imported into a strictly aligned space [25]. Third, the segmented images were iteratively registered by the Diffeomorphic Anatomical Registration Through Exponentiated Lie Algebra toolbox [26]. This process created a template for a group of individuals. The resulting images were spatially normalized into the MNI space using an affine spatial normalization. An extra processing step was performed to multiply each spatially normalized image by its relative volume before and after normalization to maintain the total amount of each tissue. Finally, images were smoothed with an isotropic Gaussian kernel of 8 mm full width at half maximum. Voxel-based multiple regression analyses (based on general linear model) were performed by SPM8, with voxel-wise GMV value as dependent variable and N and E scores of personality traits as covariates of interest. In addition, sex, age, and total intracranial volume were used as external regressors to control their effects on both brain structure [27], [28] and personality [29]. We set the significance value at p<0.05 using the AlphaSim correction (combined height threshold of p<0.005 and a minimum cluster size of 172 voxels). This correction was conducted using the AlphaSim program embedded into the REST Software (http://www.restfmri.net/forum/REST_V1.8), which applied Monte Carlo simulation to calculate the probability of false positive detection by considering both the individual voxel probability threshold and cluster size [30].
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