Corpus Callosum Structure and Interhemispheric Information Transfer in Autism

Background:  Although behavioral evidence supports the notion that interhemispheric information transfer is atypical in autism, the neural mechanisms responsible for these phenomena are unclear. One of the most common neurostructural findings in autism involves the corpus callosum (CC), the main white matter fiber bundle connecting the hemispheres. In autism both CC size and fiber integrity are atypical, suggesting that information transfer efficiency may be adversely affected. However, direct evidence supporting a link between atypical structure and interhemispheric information transfer is lacking.

Objectives:  To investigate the relation between task performance requiring interhemispheric information transfer and the length, size, and integrity of the callosal fibers in autism.

Methods:  22 autistics and 23 non-autistics were studied, with the groups matched on Wechsler PIQ (77-127), Raven’s Progressive Matrices, age (14-38) and laterality. Visuomotor tasks requiring interhemispheric information transfer included: (1) the Purdue Pegboard Test, involving hand-eye coordination and bimanual coordination, and (2) the Poffenberger Task, measuring interhemispheric transfer time, based on the principle that reaction time to a stimulus presented in the visual periphery is faster when the involved visual and motor cortical areas are in the same hemisphere than in opposite hemispheres (where interhemispheric information transfer is required).  White matter microstructure was studied with diffusion tensor imaging (b=0 and 700s/mm², 128 directions) and callosal macrostructure with high-resolution T1-weighted images collected using a 3T MRI system. The size of 25 CC subregions was determined from the T1-weighted images, and their microstructural integrity was estimated from the diffusion images.  The surface area of the cortex to which each CC subregion projected, and the corresponding fiber length, was computed using probabilistic tractography. The size of each CC subregion (relative to the surface area of the left and right gray matter regions it connects) and its diffusion properties were then related to the behavioural measures of interhemispheric transfer using linear models.

Results:  Relative CC size was significantly smaller in autistics compared to controls in frontal subregions connecting the motor cortical areas. For fiber length, no group differences were observed. As expected, in controls, bimanual task performance (Purdue) was positively correlated with the size of the CC in the subregions connecting sensorimotor cortices. However, for autistics a negative relationship between motor performance and CC size was observed in regions connecting occipital cortical areas. Although interhemispheric transfer time (Poffenberger Task) was related to CC size in sensorimotor regions for controls, it was more strongly related to the length of the CC fibers connecting occipital areas in autism.

Conclusions:  Different relationships between regional CC macrostructural properties and visuomotor behavior in autism suggest that associated CC size reductions reflect atypical interhemispheric connectivity. Stronger visuomotor behavioral correlations with CC regions connecting visual areas in autism supports the existence of an atypical pattern of interhemispheric information transfer, possibly reflecting a more prominent role for visual mechanisms in sensorimotor behavior.