Investigating function and connectivity of morphometric findings - Exemplified on cerebellar atrophy in spinocerebellar ataxia 17 (SCA17)

Spinocerebellar ataxia type 17 (SCA17) is a rare autosomal dominant neurodegenerative disorder characterized by progressive cerebellar ataxia but also a broad spectrum of other neuropsychiatric signs. As anatomical and structural studies have shown severe cerebellar atrophy in SCA17 and a differentiation of the human cerebellum into an anterior sensorimotor and posterior cognitive/emotional partition has been implicated, we aimed at investigating functional connectivity patterns of two cerebellar clusters of atrophy revealed by a morphometric analysis in SCA17 patients. In particular, voxel-based morphometry (VBM) revealed a large cluster of atrophy in SCA17 in the bilateral anterior cerebellum (lobule V) and another one in the left posterior cerebellum (lobules IX, VIIb, VIIIA, VIIIB). These two cerebellar clusters were used as seeds for functional connectivity analyses using task-based meta-analytic connectivity modeling (MACM) and task-free resting state connectivity analysis. Results demonstrated first consistent functional connectivity throughout the cerebellum itself; the anterior cerebellar seed showed stronger connectivity to lobules V, VI and to some extent I-IV, and the posterior cerebellar seed to the posterior lobules VI-IX. Importantly, the cerebellar anterior seed also showed consistently stronger functional connectivity than the posterior one with pre- and motor areas as well as the primary somatosensory cortex. In turn, task-based task-independent functional connectivity analyses revealed that the cerebellar posterior seed was linked with fronto-temporo-parietal areas as well as partly the insula and the thalamus, i.e., brain regions implicated in cognitive and affective processes. Functional characterization of experiments activating either cerebellar seed further corroborated this notion, revealing mainly motor-related functions for the anterior cluster and predominantly cognitive functions were associated for the posterior one. The differential functional connectivity of the cerebellar anterior and posterior cluster highlights the manifold connections and dichotomy of the human cerebellum, providing additional valuable information about probably disrupted cerebellar-cerebral connections and reflecting the brunt of motor but also the broad spectrum of neuropsychiatric deficits in SCA17.