Date of Award

Summer 2017

Document type


Degree Name

MD (Medical Doctor)

First Supervisor

Prof. Kieran Murphy

Second Supervisor

Dr Eileen Daly

Third Supervisor

Dr Michel Thiebaut de Schotten


Autism Spectrum Disorders, Diffusion Tensor Imaging, Twin Study


Autism Spectrum Disorder (ASD) is a lifelong condition characterised by deficits in language use, reciprocal social interaction, and restricted and repetitive interests. Diffusion Tensor Imaging (DTI) visualises white matter tracts in vivo, giving information about volume and white matter integrity as measured by Fractional Anisotropy and Mean Diffusivity. Changes seen on DTI have been associated with ASD. Both ASD and DTI measures have been shown to be heritable traits.

36 twin pairs (27 twin pairs where at least one had ASD and 9 control pairs) participated in the study. ASD diagnosis was confirmed using ADOS-G and ADI-R. Each subject had a DTI scan virtually dissected using deterministic tractography techniques.

An ASD group of 35 individuals were compared to 16 control individuals using independent samples t-tests. White matter volume was greater in the ASD group for the long and posterior segments of the left arcuate fasciculus, the posterior segment of the right arcuate, the right uncinate and the whole brain. White matter volume was lower in the left cingulum, and the inferior fronto occipital fasciculus (IFOF). Fractional anisotropy was lower in the left cingulum and mean diffusivity higher in the posterior segment of the right arcuate. Changes in volume of the posterior segment of the left arcuate and IFOF remained significant after correcting for multiple observations.

Data from these traits were fit to a constrained bivariate liability threshold twin model. Correlations between liability to ASD and the total volume of white matter in the brain, and the volume of the posterior segment of the left arcuate were seen. These traits were estimated to have heritable factors and were estimated to share heritable factors with liability to ASD. This supports prior research identifying specific gene variants associated with both ASD and volumetric changes in brain structure.

Creative Commons License

Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 4.0 License.

File Size

16.5 MB


A thesis submitted for the degree of Doctor of Medicine from the Royal College of Surgeons in Ireland in 2017.

Available for download on Saturday, November 30, 2019