Motor sequences and the basal ganglia: kinematics, not habits

Despite a lack of definitive evidence, it is frequently proposed that the basal ganglia (BG) motor circuit plays a critical role in the storage and execution of movement sequences (or motor habits). To test this hypothesis directly, we inactivated the sensorimotor territory of the globus pallidus internus (sGPi, the main BG motor output) in two monkeys trained to perform overlearned and random sequences of four out-and-back reaching movements directed to visual targets. Infusion of muscimol (a GABA(A) agonist) into sGPi caused dysmetria and slowing of individual movements, but these impairments were virtually identical for overlearned and random sequences. The fluid predictive execution of learned sequences and the animals' tendency to reproduce the sequence pattern in random trials was preserved following pallidal blockade. These results suggest that the BG motor circuit contributes to motor execution, but not to motor sequencing or the storage of overlearned serial skills.