Subthalamic nucleus discharge patterns during movement in the normal monkey and Parkinsonian patient

The pathophysiology of Parkinson disease (PD) is characterized by derangements in the discharge rates, bursting patterns, and oscillatory activity of basal ganglia (BG) neurons. In this study, subthalamic nucleus (STN) neuronal activity patterns in humans with PD were compared with that in the normal monkey during performance of similar volitional movements. Single-unit STN recordings were collected while PD patients and animals moved a joystick in the direction of targets presented on a monitor. When discharge rates in all PD human and normal monkey neurons were compared, no significant differences were observed. However, when neurons were classified by peri-movement response type (i.e., excited, inhibited, or unresponsive to movement) statistical differences were demonstrated — most significantly among PD excited neurons. Analysis of burst activity demonstrated inter- and intra-burst activities were greater in the PD human compared to the monkey irrespective of neuronal response type. Moreover, simultaneously recorded neurons in the human demonstrated consistent oscillatory synchronization at restricted frequency bands, whereas synchronized oscillatory neurons in the monkey were not restricted to distinct frequencies. During movement, discharge and burst rates were positively correlated, independent of subject or neuronal response type; however, rates and oscillatory activity were more strongly correlated in the PD human than the normal monkey. Interestingly, across all domains of analysis, STN neurons in PD demonstrated reduced response variability when compared to STN neurons in the normal monkey brain. Thus, the net effect of PD may be a reduction in the physiological degrees of freedom of BG neurons with diminished information carrying capacity.