A quantitative analysis of pallidal discharge during targeted reaching movement in the monkey

Neurons in the globus pallidus have been studied during reaching movements of the arm made at varying speeds. The reaching task is similar to one used in earlier experiments, in which disruption of normal pallidal output caused changes in movement time. The pallidal cells studied were those that showed task-related changes in activity and a modification of discharge when the arm was manipulated outside of the task. Neuronal discharge was assessed to evaluate two possible models, one in which the timing of task-related discharge varied as a function of movement time and the other in which the amplitude (mean firing rate) of the change in discharge varied as movement time varied. The relation between neuronal discharge and movement time was examined quantitatively on a trial-by-trial basis using a statistical algorithm that identified each phase of the change in neuronal discharge on each trial. A non-parametric statistic was used to determine the correlation between movement time and the duration or latency of changes in neural firing or the mean discharge during each phase of the cell's response. For fifty-five percent of the 40 neurons studied, the timing of the cell's response (duration or latency) varied as a function of movement time. For only 10 cells (25%) was there a significant correlation between movement time and the mean firing rate during one or more phases of the cell's response. Both timing and frequency modulation with movement time were limited to cells responsive to manipulation at the wrist or the shoulder.