Sensorimotor mismatch but not rotational inertia contributes to the size-weight illusion and weight perception for handheld objects

Weight perception, despite extensive historical study, remains an unresolved process. A well-documented phenomenon within this field is the size-weight illusion (SWI), where smaller objects feel heavier than larger, equally weighted counterparts. One hypothesis attributes this illusion to rotational inertia, the resistance of an object to changes in its rotational motion. This study re-evaluates the role of rotational inertia in the SWI and general weight perception using handheld objects, rather than typical rod-based tools used in past research. Two experiments were conducted. The first manipulated the rotational inertia of ten spheres, identical in appearance and weight, to see if increased inertia led to a perception of greater weight. The second compared three sets of SWI stimuli with varied mass distributions to determine if differences in rotational inertia influenced the illusion's magnitude. Both experiments measured participants' weight perception and lifting forces. Results from Experiment 1 showed no impact of rotational inertia on weight perception. Experiment 2 confirmed the presence of the SWI but found no meaningful relationship between rotational inertia and the illusion's magnitude. Interestingly, lifting force correlated with perceived weight, suggesting a role for sensorimotor mismatches rather than rotational inertia in the SWI. Overall, these findings challenge the notion that rotational inertia is a critical factor in weight perception and the SWI, indicating its influence may be confined to specific contexts involving rod-based objects. The study underscores the importance of lifting dynamics and applied forces in understanding weight perception, suggesting a need to reconsider rotational inertia's broader applicability in perceptual theories.