Figure 1.

The cerebellum operating in a feedforward control system. (A) The mossy fibers are thought to provide information referring to the desired plant motor output from motor cortex and the current sensory information referring to the actual state of the body parts (i.e., joint positions/velocities of the upper-limbs of the body-plant). According to the Marr–Albus model (Marr, 1969; Albus, 1971) the climbing fibers are assumed to carry error-related information when moving, thus providing a teaching signal to the cerebellum. By using this error-based-teaching signal the cerebellum is able to learn the corrective actions in a trial-and-error process. When the cerebellar model is not able to deliver add-on torque terms to compensate deviations in the system (for instance during the early learning stages) the general rule consists of adding a feedback to stabilize the open-loop system. (B) Different control pathways during the learning process. The relevant information flow is represented by dashed lines in each learning stage. A fast response gain control is delivered by IO-DCN connection, thus supplying stability in early learning-process stages (dashed blue lines). In later learning-process stages the two control pathways (dashed red lines); the internal MF-GrC-PC-DCN and the more external MF-DCN command the control action. Whilst IO-DCN action decays throughout the learning process its control action is assumed and improved by these two long-term adaptive pathways.