Illusions of force perception: the role of sensori-motor predictions, visual information, and motor errors.

Internal predictions influence the perception of force. When we support an object with one hand and lift it up with the other, we expect the force to disappear from the first, postural hand. In a virtual reality system, we violated this prediction by maintaining the force on the postural hand, whereas the object was still seen and felt to be lifted by the lifting hand. In this situation, participants perceived an illusionary increase in force on the postural hand, which was, in reality, constant. We test three possible mechanisms of how force perception may be influenced in this context. First, we showed that part of the illusion can be linked to a sensorimotor prediction--the predicted sensory consequences based on an efference copy of the lifting action. The illusion is reduced when the object is lifted by an external force. We also showed that the illusion changes on a trial-by-trial basis, paralleling the fast adaptation of the postural response. Second, motor errors that arise from a miscalibrated forward model do not contribute to the illusion; the illusion was unchanged even when we prevented motor errors by supporting the postural hand. Finally, visual information signaling the removal of the object is sufficient to elicit part of the illusion. These results argue that both sensorimotor predictions and visual object information, but not motor errors, influence force perception.