Claude Ghez, M.D
“Separate planning and control of hand trajectory and final position in reaching”
We examine the questions of whether in reaching for an object, the brain plans the trajectory of the hand or the final posture of the limb and whether, in adapting to external disturbances, the brain estimates errors in relation to the intended trajectory or the intended final position of the hand?
Three sets of experiments addressed these questions by examining the transfer of adaptative adjustments to visuomotor rotations and force fields between tasks requiring either accurate control of hand trajectory or control of final position. Our findings and model simulations that replicate them point to the following conclusions.
1. Implementation of reaching movements makes use of two separate controllers, one regulating the hand trajectory and the other the hand's final position or limb posture. The realized trajectory reflects the superimposed actions of these two controllers.
2. Adaptive adjustments in planned trajectory and final position are governed by different kinematic errors and represented in different coordinate systems.
3. Intended rectilinear paths are planned with reference to the initial position of the hand prior to movement (i.e. in vectorial or hand-centered coordinates) and adjusted by errors computed in this coordinate system. Intended final positions are planned with reference to the final position of the hand and adjusted by errors in eye- or gaze- centered coordinates.
4. When encountering dynamic perturbations, the recovery of endpoint accuracy and precision and trajectory linearity depends upon the nature of the errors experienced by the subjects.