Fragile x syndrome shares most of the behavioral phenotypic similarities with autism. How are these similarities reflected in brain morphology? Ten children with autism and seven with fragile x underwent morphological (T1) 1.5T magnetic resonance imaging. We found no significant difference in total brain volumes, regional volumes, gyrification index, sulcul depth and cerebral cortical thickness. However, children with autism showed significant decrease in the medial prefrontal bilaterally and the left anterior cingulate cortices. Regression analysis revealed positive correlation between the medial prefrontal cortical thickness and social IQ. We suggest that the difference between the two groups in the medial prefrontal and anterior cingulate cortices thickness may entail an altered social cognitive style. Functional magnetic resonance imaging studies directly differentiating between social indifference (autism) and social avoidance (fragile x) are needed in order to further characterize the spectrum of social abnormalities between these two groups.
Key words: autism, fragile x syndrome, magnetic resonance imaging, cerebral cortical thickness, medial prefrontal, anterior cingulate.