CONTROL OF TRAJECTORY MODIFICATIONS IN TARGET-DIRECTED REACHING

Human reaching movements to fixed and displaced visual targets were re corded and compared with simulated movements generated by using a two- joint arm model based on the equilibrium-point (EP) hypothesis (lambda model) of motor control (Feldman, 1986). The aim was to investigate t he form of central control signals underlying these movements. Accordi ng to this hypothesis, movements result from changes in control variab les that shift the equilibrium position (EP) of the arm. At any time, muscle activations and forces will depend on the difference between th e arm's EP and its actual position and on the limb's velocity. In this article, we suggest that the direction of EP shift in reaching is spe cified at the hand level, whereas the rate of EP shift may be specifie d at the hand or joint level. A common mechanism underlying reaching t o fixed and displaced targets is proposed whereby the EP of the hand s hifts in a straight line toward the present target. After the target i s displaced, the direction of the hand EP shift is modified toward the second target. The results suggest that the rate of shift of the hand EP may be modified for movements in different parts of the work space . The model, with control signals that vary in a simple fashion over t ime, is able to generate the kinematic patterns observed empirically.