ATYPICAL WHITE MATTER MICROSTRUCTURE IN AUTISM AND ASPERGER'S SYNDROME

Background: Growing evidence supports the idea that integration among functionally specialized brain regions is atypical in autistics, variously described as reduced long-range interactions, enhanced intraregional interactions or increased functional independence among cortical regions. However, it is not known whether these hypothesized differences in effective connectivity are associated with, or possibly result from, atypical anatomical connectivity. In addition, while reduced fractional anisotropy (FA) has been demonstrated in the corpus callosum and temporal white matter, it is not known if these effects are confined solely to autistics, or are also characteristic of Asperger’s syndrome.

Objectives: We used diffusion tensor imaging (DTI) to estimate regional white matter microstructural properties in autistic and Asperger participants.

Methods: Our sample included 21 autistic, 18 Asperger and 21 typical participants, matched on IQ, age (range 14-35), sex and manual preference. Using a 3T MRI system, we collected seventy-five 2mm axial slices, covering the entire cerebrum and cerebellum. For each slice, ten reference (b=0) and sixty diffusion-weighted (b=700) images with unique non-collinear diffusion encoding were obtained. The diffusion tensor and derivative parametric values including fractional anisotropy were estimated for each 2mm voxel. After transformation to a common anatomical space, FA image values were used as measures in a one-way ANOVA.

Results: Linear contrasts revealed a large cluster of voxels encompassing the corpus callosum and adjacent white matter, in which the FA values were linearly decreasing among the comparison, Asperger and autistic groups. Reductions in cerebellar white matter FA were also seen in the autistic group.

Conclusions: We observed supratentorial alterations in white matter microstructure in both the Asperger and autistic groups, consistent with the assertion that differences in anatomical connectivity may be responsible for altered patterns of intra- and inter-hemispheric effective connectivity in autism and Asperger's syndrome.