Association between White Matter Microstructure and Autistic Traits across the Diagnostic Divide
There is growing evidence that suggests autistic traits lie on a continuum in the general population. Many studies have examined the relationship between white matter (WM) microstructural properties and autistic traits with few studies reporting associations [Lidaka et al. 2012; Gibbard et al. 2013] whilst other studies showed lack of associations [Cedric et al. 2015]. The discrepancies in the results of previous studies led us to carry out this study to find whether the autistic traits in the non-clinical population are associated with WM microstructural abnormalities. Moreover, previous Diffusion Tensor Imaging (DTI) studies in Autism spectrum disorders (ASD) compared to healthy controls have consistently shown reduced fractional anisotropy (FA) and increased mean diffusivity (MD) values in the regions including the superior longitudinal fasciculus (SLF), uncinate fasciculus (UF), inferior longitudinal fasciculus (ILF), and inferior fronto-occipital fasciculus (IFOF) [Catani et al., Brain 2016; Boets et al., 2018; Gibbard et al., 2013]. These brain regions play a significant role in acquiring socio-communication skills, emotional recognition and visual processing skills. In this study, we sought to replicate the above findings and to correlate the diffusion indices with Autism spectrum quotient (AQ) scores in a combined sample of neurotypicals and ASD. It was hypothesized to determine the possible relationship between the WM microstructural integrity and autistic traits.
To determine the white matter microstructural correlates of the autistic traits in the combined sample of neurotypicals and ASD.
Fifty three adults (28 neurotypicals, 25 ASD: 31 males, 22 females, age 18-50 years) were recruited for this study. This study was approved by University Research Ethics Committee. Siemens 3T MRI scanner was used to acquire the whole brain DTI images with a 32-channel head coil and high resolution MPRAGE at the Centre for Integrative Neuroscience and Neurodynamics, University of Reading. All the participants also completed the AQ questionnaire. Tract based spatial statistics was employed, on the output of which standard space masks were applied to extract all the mean FA and MD values from the bilateral SLF, UF, ILF and IFOF which were then correlated with the AQ scores.
Pearson correlation (one-tailed) was used to find the associations between FA, MD values and AQ scores in the combined sample of neurotypicals and ASD while controlling for age, gender and IQ. Negative correlation was found between FA and AQ in right inferior longitudinal fasciculus (r = -0.275, P = 0.027) which was the only statistically significant region. No significant positive correlation between MD and AQ was found.
In this study, the results replicate the significant findings between the white matter microstructural properties and autistic traits in the right inferior longitudinal fasciculus. This finding is consistent with previous reports of case-control differences in this tract (Gibbard et al., 2013), and provides evidence for the relationship between the white matter microstructural properties and autistic traits across the diagnostic divide. The absence of association in other regions may be due to the inadequate sample size and/or unknown patterns of interaction between age and autistic traits.