Identification of sub-groups with signs of hyperplasia, hypoplasia and normal-sized cerebellum [95] reflects the heterogeneity of the autistic population. Piven et al. [87] reported that cerebellar volume correlates with an increased total brain volume. In the majority of autistic subjects, reduced size of the cerebellar hemisphere is observed [42, 82], but this trend is not detectable in cohorts of high-functioning autistic individuals [56]. Regional hypoplasia affects the vermis in autistic individuals relatively often [20, 22, 52] and may be associated with the deficits in attention-orienting [49, 104], stereotypic behavior and reduced exploration observed in autism [86]. In the examined autistic cohort, selective and severe hypoplasia of lobes 1–4 associated with hypoconvolution of a large portion of the dentate nucleus appears to correspond to clinically detected defects of movement coordination. These findings suggest that differences in the type, topography and severity of cerebellar developmental defects may contribute to different clinical manifestations.
