Investigating the Role of Haptic and Visual Perception in the Material-Weight Illusion

Weight illusions provide a compelling demonstration that prior experience affects perception. Here we investigated how the expectation-inducing modality affected the Material-Weight Illusion (MWI), where dense-looking objects feel lighter than less dense-looking objects. Participants lifted equally-weighted polystyrene, cork, and granite cubes whilst viewing computer-generated images of the cubes in virtual reality (VR). The representation of the object in VR was manipulated to create four illusion-inducing sensory conditions: visual differences only, haptic differences only, congruent visual-haptic differences, and incongruent visual-haptic material differences. Although an MWI was induced in all conditions, whereby the polystyrene object was reported to feel heavier than the granite object, the strength of the MWI differed across conditions, with haptic material cues having a stronger influence on perceived heaviness than visual material cues. These results are consistent with optimal integration theories of multi-modal perception, highlighting that perception reflects individual cues’ reliability and relevance in specific contexts.

Perceiving and acting upon weight illusions in the absence of somatosensory information

When lifting novel objects, individuals' fingertip forces are influenced by a variety of cues such as volume and apparent material. This means that heavy-looking objects tend to be lifted with more force than lighter-looking objects, even when they weigh the same amount as one another. Expectations about object weight based on visual appearance also influence how heavy an object feels when it is lifted. For instance, in the “size-weight illusion,” small objects feel heavier than equally weighted large objects. Similarly, in the “material-weight illusion,” objects that seem to be made from light-looking materials feel heavier than objects of the same weight that appear to be made from heavy-looking materials. In this study, we investigated these perceptual and sensorimotor effects in IW, an individual with peripheral deafferentation (i.e., a loss of tactile and proprioception feedback). We examined his perceptions of heaviness and fingertip force application over repeated lifts of objects that varied in size or material properties. Despite being able to report real weight differences, IW did not appear to experience the size- or material-weight illusions. Furthermore, he showed no evidence of sensorimotor prediction based on size and material cues. The results are discussed in the context of forward models and their possible influence on weight perception and fingertip force control.

The Golf-Ball Illusion: Evidence for Top-down Processing in Weight Perception

Theories of weight illusions have traditionally emphasised either the primary contribution of low-level sensory cues or the role of expectation based on knowledge and past experience. Current models of weight illusions lean quite strongly towards sensory-based interpretations. The current experiment raises a problem for such approaches by generating a weight illusion that is difficult to explain other than by the participants' knowledge. Golfers (who expect a weight difference between ball types) reliably judged practice golf balls to weigh more than real golf balls of the same weight. In contrast, non-golfers (who expect no weight difference between ball types) judged practice and real balls of equal weight to weigh the same. Furthermore, within the group of golfers, those who expected the weights of the two ball types to be the most discrepant prior to lifting tended to report the strongest illusions subsequent to lifting. Because there is no low-level sensory cue between ball types that on its own would signal a weight difference, the current finding suggests that there is a top-down component to weight perception that is based on experience with specific objects.