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Neuroticism is well known as a personality trait which enables us to predict individual vulnerability to suffer from affective disorders (MDD, BPD). Over the last years, MRI-based neuroimaging could provide evidence for lots of different parts of the human brain to correlate with this personality trait. White matter correlates are mostly localized in frontotemporal tracts. The orbitofrontal cortex and prefrontal cortex dominate as gray matter correlates of neuroticism. As many studies could not find any results, the question whether there are consistent correlates of neuroticism at all remains unclear. Further, there is no consensus on concrete localisations of possible correlates of neuroticism yet. The aim of this study is the MRI-based, multimodal analysis of human brain correlates of neuroticism in a large sample of healthy adults.
In our sample, the analysis of gray matter has been carried out using surface- / volume-based morphometry to measure cortical volume, cortical thickness and gyrification (Gesamtkohorte, n = 670). Additionally, in a subgroup we performed diffusion tensor imaging (DTI) to analyse white matter (Teilkohorte, n = 570), in particular FA (fractional anisotropy) and RD (radial diffusivity). White matter analysis was performed with TBSS, gray matter analysis was carried out using SPM12 / CAT12. Neuroticism was measured with the help of the NEO-FFI personality questionnaire.
In the subgroup (Teilkohorte), after controlling for age, sex and site, we found positive correlations between the averaged FA within a single tract and neuroticism in the right cingulum, the right and left inferior frontooccipital fasciculus, further in the right and left uncinate fasciculus. Measuring the averaged RD within a single tract, negative correlations with neuroticism were detected in the left cingulum, the right and the left inferior frontooccipital fasciculus and the left uncinate fasciculus (p ≤ 0,05). Regarding the voxel-wise DTI analysis, we additionally found positive correlations between neuroticsm and FA in the anterior thalamic radiation and the inferior longitudinal fasciculus in both hemispheres. In our maingroup (Gesamtkohorte) results did not survive correction for familiy-wise error (FWE). Nevertheless, measures of cortical thickness in the right hemisphere showed uncorrected positive correlations with neuroticism in the medial orbitofrontal cortex, the superior temporal, superior frontal and middle frontal rostral cortex. In a follow-up analysis, the mean vertex-values of cortical thickness in the superior temporal and medial orbitofrontal cortex revealed a significant correlation with the averaged FA in the right uncinate fasciculus.
The main result of this study reads as follows: people who tend to suffer from affective disorders as major depressive disorder or bipolar disorder, thus scoring high in neuroticism, show increased integrity of white matter microstructure (elevated FA, reduced RD). As this hypothesis is contrary to the results of former studies and local changes in RD in healthy adults scoring high on neuroticism are rarely investigated yet, it needs to be replicated. Future research needs to be aware of specific characteristics of the analyzed sample, such as aspects like the distribution of age or neuroticism-scores which may influence the direction of the association between neuroticism and imaging parameters like the FA. The linkage between personalitiy, brain structure and function becomes very clear as regards the results in the orbitofrontal cortex. From a functional perspective, the orbitofrontal cortex contains in addition to the structural correlates also aspects like emotion regulation, which partly determine the amount of neuroticism. At least, there remain some difficulties when comparing our results with those in clinical samples with an oppositional direction of the assoiation between neuroticism and brain structure. A possible explanation for this phenomenon could be a nonlinear development of the correlation between neuroticism and brain structure in the course of a mental disease.