Cerebellar networks with basal ganglia: feasibility for tracking cerebello-pallidal and subthalamo-cerebellar projections in the human brain.

Neuroanatomical studies using transneuronal virus tracers in macaque monkeys recently demonstrated that substantial interactions exist between basal ganglia and the cerebellum. To what extent these interactions are present in the human brain remains unclear; however, these connections are thought to provide an important framework for understanding cerebellar contributions to the manifestation of basal ganglia disorders, especially with respect to tremor genesis in movement disorders such as Parkinson's disease. Here, we tested the feasibility of assessing these connections in vivo and non-invasively in the human brain with diffusion magnetic resonance imaging and tractography. After developing a standardized protocol for manual segmentation of basal ganglia and cerebellar structures, masks for diffusion tractography were defined based on structural magnetic resonance images. We tested intra- and inter-observer stability and carried out tractography for dentato-pallidal and subthalamo-cerebellar projections. After robustly achieving connection probabilities per tract, the connectivity values and connectional fingerprints were calculated in a group of healthy volunteers. Probabilistic diffusion tractography was applicable to probe the inter-connection of the cerebellum and basal ganglia. Our data confirmed that dentato-thalamo-striato-pallidal and subthalamo-cerebellar connections also exist in the human brain at a level similar to those that were recently suggested by transneuronal tracing studies in non-human primates. Standardized segmentation protocols made these findings reproducible with high stability. We have demonstrated that diffusion tractography in humans in vivo is capable of revealing the structural bases of cerebellar networks with the basal ganglia. These findings support the role of the cerebellum as a satellite system of established cortico-basal ganglia networks in humans.