ABSTRACT
The simple spike firing of cerebellar Purkinje cells encodes information on the kinematics of limb movements. However, these conclusions have been primarily based on averaging the discharge of Purkinje cells across trials and time and there is little information on whether Purkinje cell simple spike firing encodes specific motor errors during limb movements. Therefore, this study investigated single-trial correlations between the instantaneous simple spike firing of Purkinje cells with various kinematics and error signals. Purkinje cells (n = 126) were recorded in the intermediate and lateral zones centered on the primary fissure while three monkeys intercepted and tracked a target moving in a circle. Cross-correlation analysis was performed between the instantaneous simple spike firing rate and speed, the directional component of the velocity vector, and error signals during single movement trials. Significant correlations at physiologically relevant lags of +/-250 ms were observed with tracking speed for 37% of Purkinje cells, with the velocity component in 39%, with direction error in 6% and speed error in 25%. Simple spike firing of the majority of Purkinje cells with significant correlation showed a negative lag with respect to speed and a positive lag with respect to error signals. We hypothesize that the cerebellum is involved in movement planning and control by continuously monitoring movement errors and making intermittent corrections that are represented as fluctuations in the speed profile.
Subject(s)
Action Potentials/physiology , Cerebellar Cortex/physiology , Movement/physiology , Orientation/physiology , Psychomotor Performance/physiology , Purkinje Cells/physiology , Animals , Arm/innervation , Arm/physiology , Biomechanical Phenomena , Electrophysiology , Feedback/physiology , Female , Learning/physiology , Macaca mulatta , Motion Perception/physiology , Motor Skills/physiology , Neuropsychological Tests , Photic Stimulation , Reaction Time/physiology , Space Perception/physiology , Time FactorsABSTRACT
The cerebellum has been hypothesized to provide internal models for limb movement control. If the cerebellum is the site of an inverse dynamics model, then cerebellar neural activity should signal limb dynamics and be coupled to arm muscle activity. To address this, we recorded from 166 task-related Purkinje cells in two monkeys performing circular manual tracking under varying viscous and elastic loads. Hand forces and arm muscle activity increased with the load, and their spatial tuning differed markedly between the viscous and elastic fields. In contrast, the simple spike firing of 91.0% of the Purkinje cells was not significantly modulated by the force nor was their spatial tuning affected. For the 15 cells with a significant force effect, changes were small and isolated. These results do not support the hypothesis that Purkinje cells represent the output of an inverse dynamics model of the arm. Instead these neurons provide a kinematic representation of arm movements.
Subject(s)
Cerebellum/physiology , Purkinje Cells/physiology , Algorithms , Animals , Arm/physiology , Biomechanical Phenomena , Cerebellum/cytology , Electromyography , Electrophysiology , Female , Functional Laterality/physiology , Hand/physiology , Macaca mulatta , Models, Neurological , Movement/physiology , Muscle, Skeletal/innervation , Muscle, Skeletal/physiology , Psychomotor Performance/physiologyABSTRACT
Complex limb movements exhibit segmentation into submovements characterized as bell-shaped speed pulses. Submovements have been implicated in both feedback and feedforward control, reflecting an intermittent error-correction process. This study examines submovements occurring during a circular manual tracking task in humans, focusing on the amplitude-duration scaling of submovements and the properties of this scaling across changes in movement speed and external force load. The task consisted of intercepting and tracking a circularly moving target using a two-jointed, robotic arm that allowed external force fields to be imposed during tracking. Different speed levels and different levels of three types of force field were examined. Submovements were defined as fluctuations in the speed profile. The properties of the amplitude-duration ratio of the speed pulses were examined in relation to target speed and external force fields. The results show that the amplitude and duration of the submovements scale linearly in human manual tracking. The slope of the scaling was independently influenced by both target speed and external force fields. A common element in the increase in the scaling slope was increased tracking errors. Control experiments using passive movements and power spectral analysis showed that the submovements were not artifacts of the mechanical/acquisition system or the imposed force field. These results are consistent with the concept of stereotypy in which movements are constructed of scaled versions of a single prototype. Furthermore, the results support the hypothesis that submovements are integral to an error detection and correction control process.
