Parallel pathways for movement initiation of monkeys

Participation of the pathway from the cerebellum through "motor" thalamus to motor cortex in movement initiation was tested in three. Cebus monkeys. The animals performed a simple reaction time (RT) task involving prompt elbow flexion or extension in response to an audiovisual "GO!" signal. RTs were not changed by reversible cooling of the arm area of the contralateral ventral lateral (VL) thalamus, that includes rostral VPL. In contrast, cooling the cerebellar dentate nucleus ipsilateral to the operant arm prolonged RTs by about 80 ms, as tested in two of the same monkeys. The results confirm cerebellar participation in prompt generation of well-learned intended arm movements; but they also indicate that although the route through VL-thalamus may be important, it is not essential for initiation of the same movements.