Cerebellar pathway diffusion MRI measures are linked to core autism symptoms in early adolescents aged 9 to 11 years.

In this preregistered study, we used diffusion MRI (dMRI) to characterize the tissue properties of cerebellar pathways in 9-11-year-old early adolescents from the Adolescent Brain Cognitive Development (ABCD) Study. We examined fractional anisotropy (FA), mean diffusivity (MD), and number of streamlines (NoS) across five cerebellar pathways-the inferior, middle, and superior cerebellar peduncles, the input and Purkinje fibers, and the parallel fibers-in adolescents with (n = 135) and without (n = 7276) a parent-reported ASD diagnosis. We further tested whether cerebellar dMRI measures differentially related to core ASD symptom domains of social communication and interaction (SCI) and restricted and repetitive behaviors (RRB). Group comparisons revealed significant differences in cerebellar NoS (Pillai's trace p = .033), driven by greater NoS of the superior cerebellar peduncle in early adolescents with ASD; FA and MD showed no significant group effects. Cerebellar pathway dMRI measures exhibited significant interactions with diagnosis in relation to ASD symptom severity. The superior cerebellar peduncle demonstrated the strongest diagnosis-dependent interaction effects, with the NoS showing markedly stronger associations with both social communication (Δβ = 0.14, p = .0019) and restricted and repetitive behaviors (Δβ = 0.21, p < .0001) in children with ASD. These findings highlight the superior cerebellar peduncle as a key pathway of structural variability and highlight dMRI measures of the cerebellar pathways as meaningful correlates of ASD symptoms in early adolescence.