Signaling patterns of globus pallidus internal segment neurons during forearm rotation

We recorded extracellular single unit discharges of globus pallidus internal segment (GPi) neurons in monkeys performing a visually driven forearm rotation movement task in order to quantify how discharge patterns changed in relation to kinematic parameters. Subjects grasped a handle that rotated about its axis while facing a video screen displaying visual targets. Continuous visual feedback of handle rotation position was provided. Monkeys generated forearm rotation movements of +/-35 degrees and +/-70 degrees amplitude in order to align the cursor and targets. Trial records were aligned to forearm rotation onset in order to compare the discharge patterns that were associated with movements of different amplitudes, velocities, and directions. In addition, we quantified the depth of modulation of neuronal discharge associated with movements generated in two different task phases. Comparisons of discharge patterns were made between the visually guided, rewarded phase ("cued movements") and the self-paced, unrewarded phase that returned the monkey to the task start position ("return movements") by quantifying the goodness of fit between neuronal discharge during cued and return movements. Our analyses revealed no systematic relationship between the depth of modulation of GPi neurons and forearm rotation amplitude, direction, or velocity. Furthermore, comparisons between the two behavioral contexts revealed a systematic attenuation of modulation that could not be attributed to differences in movement velocity. Collectively, these findings suggest that the GPi neurons that we studied were not significantly involved in mediating movement kinematics, but may have instead been instrumental in the processing of information about the behavioral context during which movements were generated.