Motor function of the monkey globus pallidus. 2. Cognitive aspects of movement and phasic neuronal activity.

In order to study the role of the basal ganglia (BG) in cognitive aspects of movement, we recorded extracellularly from pallidal neurons in conscious monkeys while they performed a sequential wrist movement task consisting of a series of holds and ballistic jumps. The movement sequence had to be performed within specified time restraints and was predictable. We recorded the activity of 297 neurons whose discharges were related to the movement task. The movement-related response was found to be influenced by the contextual setting and by the degree of difficulty of the task in a subgroup of 82 neurons with a clear response to the first ballistic movement. Predictable and easy movements were usually represented by more prominent movement-related responses in 46% of these neurons; 35% of neurons from a different subset of 105 neurons also demonstrated a second phasic response just before the end of the final hold period of the task. This response was also found to be influenced by the predictability of the final hold period, both in its time duration and also by the direction of the following ballistic movement in double jump tasks. These findings were in keeping with a cognitive role for the BG in movement performance. In particular we suggest that the phasic neuronal activity was an internal cue generated by the BG for predictable movements of a subconscious nature which signals the end of a component of movement in a movement sequence. This cue is appropriately timed to terminate sustained neuronal activity in the SMA and to allow the next movement in the sequence to be executed.