Size Matters: A Single Representation Underlies Our Perceptions of Heaviness in the Size-Weight Illusion

Gavin Buckingham,Melvyn A Goodale,Joy J Geng

doi:10.1371/journal.pone.0054709

Abstract

In the size-weight illusion (SWI), a small object feels heavier than an equally-weighted larger object. It is thought that this illusion is a consequence of the way that we internally represent objects’ properties – lifters expect one object to outweigh the other, and the subsequent illusion reflects a contrast with their expectations. Similar internal representations are also thought to guide the application of fingertip forces when we grip and lift objects. To determine the nature of the representations underpinning how we lift objects and perceive their weights, we examined weight judgments in addition to the dynamics and magnitudes of the fingertip forces when individuals lifted small and large exemplars of metal and polystyrene cubes, all of which had been adjusted to have exactly the same mass. Prior to starting the experiment, subjects expected the density of the metal cubes to be higher than that of the polystyrene cubes. Their illusions, however, did not reflect their conscious expectations of heaviness; instead subjects experienced a SWI of the same magnitude regardless of the cubes’ material. Nevertheless, they did report that the polystyrene cubes felt heavier than the metal ones (i.e. they experienced a material-weight illusion). Subjects persisted in lifting the large metal cube with more force than the small metal cube, but lifted the large polystyrene cube with roughly the same amount of force that they used to lift the small polystyrene cube. These findings suggest that our perceptual and sensorimotor representations are not only functionally independent from one another, but that the perceptual system represents a more single, simple size-weight relationship which appears to drive the SWI itself.

Highlights

Despite our impressive repertoire of perceptual abilities, humans tend to make rather imprecise judgments about the veridical weight of an object, instead making relative judgments about how heavy an item is
To determine how size and material properties influenced lifters’ perceptions of heaviness we examined the perceptual ratings of heaviness in a repeated-measures ANOVA (Figure 2, bottom). This test revealed significant main effects of size (F(1,27) = 1371.7, p,.001, partial eta2 = .98) and material (F(1,27) = 19.6, p,.001, partial eta2 = .42). These main effects indicate the presence of a large size-weight illusion (SWI) and a smaller materialweight illusion’ (MWI) respectively
In this study we investigated how size and material properties interact with one another when lifting objects and judging their weights

Summary

Introduction

Despite our impressive repertoire of perceptual abilities, humans tend to make rather imprecise judgments about the veridical weight of an object, instead making relative judgments about how heavy an item is. A prime example of the subjective nature of this process comes from illusions of heaviness such as the size-weight illusion (SWI), where a small object feels heavier than a larger, but otherwise similar looking object of the same weight [1] This powerful illusion does not lessen after prolonged experience with the stimuli, and even persists when the individual is told that the stimuli have the same mass [2,3]. One promising line of inquiry suggested that the SWI is caused by lifting errors – from a mismatch between the expected and the actual haptic feedback of the lift [5] This sensorimotor mismatch explanation has, proved unworkable, given that lifters rapidly correct their initial erroneous sensorimotor predictions [2,6] even though they continue to experience an unchanging perceptual illusion

Methods

Results

Conclusion