Independence of perceptual and sensorimotor predictions in the size-weight illusion.

The smaller of two equally weighted objects is judged to be heavier when lifted. Here we disproved a leading hypothesis that this size-weight illusion is caused by a mismatch between predicted and actual sensory feedback. We showed that when subjects repeatedly lifted equally heavy large and small objects in alternation, they learned to scale their fingertip forces precisely for the true object weights and thus exhibited accurate sensorimotor prediction. The size-weight illusion nevertheless persisted, suggesting that the illusion can be caused by high-level cognitive and perceptual factors and indicating that the sensorimotor system can operate independently of the cognitive/perceptual system.