| In order to understand better how the brain adaptively controls voluntary movements, this work examined human and monkey subjects performing a knob turning task, similar to tuning a radio dial. Subjects sometimes made these target-directed hand movements as a single, smooth motion, but often produced a series of discrete movement segments. The central mechanisms responsible for the production of discrete movement segments were investigated via three approaches: analysis of human kinematics, modeling the neuromusculoskeletal system, and single-unit recordings of Purkinje cells (PCs) in the cerebellum. A direct, objective algorithm was developed to detect when a movement was composed of multiple segments, and movements were separated into three segment types: primary movements, overlapping submovements, and delayed submovements. The kinematic properties of each of these segments were remarkably similar, suggesting they were generated by some common central process. Movements only became irregular and had shapes that differed from a bell-shaped velocity when they were made as a series of two or more overlapping segments. When torque perturbations were applied, the size and frequency of corrective submovements increased, but subsequently decreased as subjects adapted over time. As a result, the kinematics of the overall movement returned near normal.;A simplified model of the motor control system was used to show how changing pulse-like commands affected kinematics, and how multiple pulse-like commands could produce discrete movement segments. Many PCs fired simple spike (SS) discharge related to both the primary movement and the corrective submovements, and most likely encoded for muscle activation levels. SS activity often changed when random perturbations were applied, showing that these cells were involved in the continuous monitoring and adjustment of movement commands. PCs fired complex spikes (CSs) after natural and electrical stimulation, and CS firing at movement onset may have helped subjects make faster movements. Around 35--50% of PCs fired CSs after the monkeys made corrective submovements. In summary, PCs monitor feedback and efference copy, and make discrete and continuous adjustments to movement commands. PCs may learn to generate single, accurate primary movements through changing synaptic efficacy of their inputs under the guidance of CS firing following discrete corrective submovements. |
