The influence of competing perceptual and motor priors in the context of the size–weight illusion

When lifting objects of identical mass but different sizes, people perceive the smaller objects as weighing more than the larger ones (the ‘size–weight’ illusion, SWI). While individual’s grip and load force rates are rapidly scaled to the objects’ actual mass, the magnitude of the force used to lift these SWI-inducing objects is rarely discussed. Here, we show that participants continue to apply a greater loading force to a large SWI-inducing cube than to a small SWI cube, lift after lift. These differences in load force persisted long after initial errors in grip and load force rates had been corrected. Interestingly, participants who showed the largest illusion made the smallest errors in load force. This unexpected relationship suggests that the motor system is consistently biased toward the expectations of heaviness for a particular stimulus in a Bayesian fashion, and that this loading error is subsequently reduced by SWI perceptual errors in the opposite direction.