Vertebrates are remarkable for their ability to select and execute goal-directed actions: motor skills critical for thriving in complex, competitive environments. A key aspect of a motor skill is the ability to execute its component movements over a range of speeds, amplitudes and frequencies (vigor). Recent work has indicated that a subcortical circuit, the basal ganglia, is a critical determinant of movement vigor in rodents and primates. We propose that the basal ganglia evolved from a circuit that in lower vertebrates and some mammals is sufficient to directly command simple or stereotyped movements to one that indirectly controls the vigor of goal-directed movements. The implications of a dual role of the basal ganglia in the control of vigor and response to reward are also discussed.
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