Anticipatory adjustments in the unloading task: is an efference copy necessary for learning?

In the unloading task, a weight is held in the palm of one hand. When an external agent removes the weight, an upward perturbation of the loaded hand is observed. However, when a person removes the weight by lifting it with their own hand, the perturbation is attenuated due to an anticipatory adjustment of the flexor muscles in the load-bearing arm. An experiment was conducted to examine conditions under which this anticipatory response could be learned. Using a virtual reality system with force-feedback robotic arms, normal subjects performed the unloading task under one of four learning conditions: (a) the participant initiated the unloading by pressing a button, (b) the unloading was cued by a brief visual stimulus, (c) the unloading was performed by a virtual "hand" that moved smoothly towards the object, and (d) the unloading followed three rhythmic force-pulses applied to the finger of the participant. After extended practice (192 trials) we found a significant reduction of the upward perturbation only in the button pressing condition. Control conditions indicated that the acquired response was due to an anticipatory feedforward response rather than due to a change in tonic state such as an increase in arm stiffness. These results indicate that a voluntary action is necessary to acquire an anticipatory adjustment in the unloading task.