Transformation of the kinematic characteristics of a precise movement after a change in a spatial task.

The central mechanism of motor programming was studied using a model of precise horizontal flexion of the arm at the elbow joint. Training was performed in the dark to ensure that formation of the motor program was based predominantly on the use of proprioceptive afferentation. The target was not demonstrated before training: subjects determined the angle of arm flexion during training, the moment at which the target position was reached being identified by a brief LED flash. Subjects had to perform the movement as quickly and accurately as possible. The amplitude, speed, and accuracy of the movement were measured in real time. The ten subjects were divided into two groups for initial training to precise movements, using two different protocols: flexion of the elbow to 70 degrees and to 55 degrees . At the second stage of the experiment, each subject's initial target position was suddenly changed (from 70 degrees to 55 degrees and vice versa). Training was continued until a stable accuracy in the new conditions was achieved (with errors of no more than 5% of the specified amplitude). The nature of the transformation in the kinematics of the precise movement in response to the change in the single task parameter illuminated the fundamental principle of organization of the supraspinal motor command for movements of this type. For both specified flexion amplitudes, the ratio between the acceleration and deceleration phases of the movement were identical during the period of skill fixation. On average, 70% of the total amplitude of flexion was accounted for by the acceleration phase and 30% by the deceleration phase. Adaptation of the precise movement to changes in the specified horizontal elbow flexion angle (i.e., re-achievement of the required movement accuracy in the changed conditions) during rearrangement was completed by inversion of these values. According to the results of previous studies, the most informative measure for analysis of the dynamics of the controlling central command was the acceleration of the movement. In terms of current concepts of the mechanism of motor control, the acceleration plateau can be regarded as mirroring long-term depression--the voltage plateau in Purkinje cells and motoneurons. Data processing involved calculation of the integral acceleration in both phases of the movement in relation to the angle of flexion (phase plots). These data underlie our understanding of the mechanism of transformation of movement kinematics responsible for the formation of a new central command.