Positional errors introduced by transient perturbations applied to a multi-joint limb.

We explored a recently discovered phenomenon that smooth transient perturbations applied to the hand can lead to violations of equifinality. Healthy subjects occupied an initial hand position against a bias force and tried not to interfere with hand displacements produced by changes in the force. The force changes were smooth and transient (ending up with the same bias force value), with or without a time interval (dwell time) between the force change application and removal. They could lead to an increase or a decrease in the bias force. The subjects performed the task with eyes open and closed. After the force change was over, the hand stopped consistently short of the initial position only when the initial force change increased the bias force. No consistent positional errors were seen for the opposite force change direction. These results were consistent across trials with and without dwell time performed with and without vision. We conclude that the positional errors were not due to muscle properties but reflected a drift in the hand referent coordinate within the central nervous system triggered by the perturbation and driven by the difference between the actual and referent hand coordinates during the dwell time.