Autism as a disconnection syndrome: A qualitative and quantitative review of diffusion tensor imaging studies

• Widespread disruption of white matter integrity in individuals with ASD.
• Both long-range and short-range connections are disrupted.
• Developmental shift from greater to lower fractional anisotropy.
• Corpus callosum and ventral tracts are most prominent areas of aberrant connectivity.

This review aims at evaluating the leading hypothesis of lower long-range and greater short-range cortical connectivity in individuals with autism spectrum disorder (ASD) by the available literature on diffusion tensor imaging (DTI) studies. DTI, coupled with tractography, assesses the structural connections between cortical regions and quantifies their white matter integrity. First, we provide an extensive qualitative overview of DTI findings in ASD. Next, to reveal convergence between studies, results are quantitatively analyzed using Activation Likelihood Estimation (ALE) and fibre tracking is performed to visualize the white matter tracts running through the obtained ALE clusters. Finally, findings from DTI research are related to specific symptoms characteristic of ASD. Overall, the qualitative analysis yields a widespread disruption of white matter integrity in the brain of individuals with ASD as compared to typically developing controls. This is the case for both the long-range and the local short-range connections, partially contradicting the leading hypothesis. However, several studies investigating very young children with ASD report greater structural connectivity, suggesting a developmental switch in white matter integrity in the ASD brain. Based on the combined qualitative and quantitative analysis, the corpus callosum and the ventral tracts emerge as particularly affected connections in individuals with ASD.