The integration of size and weight cues for perception and action: evidence for a weight-size illusion

Humans routinely estimate the size and weight of objects. Yet, when lifting two objects of equal weight but different size, they often perceive the smaller object as being heavier. This size-weight illusion (SWI) is known to have a lesser effect on motor control of object lifting. How the nervous system combines "weight" and "size" cues with prior experience and whether these cues are differentially integrated for perception and sensorimotor action is still not fully understood. Therefore, we assessed not only whether the experience of size biases weight perception, but also if experience of weight biases the size perception of objects. Further, to investigate differences between perceptual and motor systems for cue-experience integration, participants haptically explored the weight of an object with one hand and then shaped the aperture of their other hand to indicate its perceived size. Results-First, next to a SWI, healthy adults (N = 21) perceived lighter objects as being smaller and heavier objects as being larger, demonstrating a weight-size illusion (WSI). Second, participants were more susceptible to either the SWI or WSI. Third, aperture of the non-exploring hand was scaled to perceived weight and not to physical size. Hand openings were consistently smaller than physical size, with SWI-sensitive participants being significantly more affected than WSI-sensitive subjects. We conclude: first, both size and weight perceptions are biased by prior experience. Weight perception is biased by expectations of size, while size perception is influenced by the expectancy of weight. Second, humans have the tendency to use one cue predominantly for both types of perception. Third, combining perceived weight with expected size influenced hand motor control, while online haptic feedback was largely ignored. Finally, we present a processing model underlying the size-weight cue integration for the perceptual and motor system.