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

Goodale MA (2013) Size Matters: A Single Representation Underlies Our Perceptions of Heaviness in the Size-Weight Illusion. PLoS ONE 8(1): e54709. doi:10.1371/journal.pone.0054709 Size Matters: A Single Representation Underlies Our Perceptions of Heaviness in the Size-Weight Illusion Gavin Buckingham 0 Melvyn A. Goodale 0 Joy J. Geng, University of California, Davis, United States of America 0 The Brain and Mind Institute, Department of Psychology, The University of Western Ontario , London, Ontario , Canada 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. - 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. It is believed that we consciously perceive the weight of an object relative to an expectation (or representation) of how heavy it is likely to be, based on its size, material, and/or other contextual properties. 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]. As alluded to above, it is thought that the SWI is caused by lifters incorrect expectations of heaviness [4]. Individuals expect large objects to outweigh similar looking small objects, because they encounter this relationship between size and weight over and over again in the natural world. When lifting objects that induce the SWI, these environmentally-induced expectations are violated (i.e., the larger object does not outweigh the smaller one), leading to the percept that that the small object outweighs the large object. However, the mechanism by which confounded expectations are translated into this perceptual effect remains elusive. One promising line of inquiry suggested that the SWI is caused by lifting errors specifically from a mismatch between the expected and the actual haptic feedback of the lift [5]. This sensorimotor mismatch explanation has, however, proved unworkable, given that lifters rapidly correct their initial erroneous sensorimotor predictions [2,6] even though they continue to experience an unchanging perceptual illusion. The independence of lifting kinetics and heaviness perception has not forced researchers to abandon the notion that the illusion is caused by our expectations. Instead, the concept has been refined to incorporate distinctly adapting representations for the perceptual and sensorimotor systems [7]. In this framework, the sensorimotor systems predictions are based on a rapidly adapting set of representations, whereas the perceptual system makes use of slowly adapting representations. The rates at which these separate representations adapt are proposed to be a function of necessity. On the one hand, the rapid sensorimotor adaptation facilitates interacting with objects in the world, which may change in mass from lift to lift (i.e., a bottle of water from which you are drinking). On the other hand, the extraordinarily slow adaptation of conscious perception ensures that encountering an unusuallyweighted item does not define the new norm for that particular class of object. It is precisely because these perceptual expectations are so resistant to change that the magnitude of the SWI does not diminish with repeated experiences. There is a growing body of evidence for the roles that cognitive factors play in our conscious perception of heaviness, with numerous reports of weight illusions where top-down factors must play a role. Ellis and Lederman [8] demonstrated that inappropriately-weighted practice golf balls induce a weight illusion in golfers, but not in individuals without golf experience (i.e., who would have no expectations associated with a practice golf ball). Another recent top-down weight illusion comes from Dijker [9], who noted that dolls which would be expected to feel lighter (in this case, a female doll) tended to feel heavier than dolls which were expected to be heavier (a muscular male doll). The most wellstudied variants of the SWI are, however, weight illusions caused by manipulating the apparent material properties of the lifted stimuli [1014]. These demonstrations of a so-called materialweight illusion (MWI) are, at face value, very similar to the SWI objects which seem to be made from a light-looking material feel heavier than identically-weighted objects which seem to be made from a heavy-looking material. Furthermore, individuals make lifting errors which reflect their expectations of heaviness, initially lifting the heavy-looking material with a higher force rate than the lighter-looking one. And, like with the SWI, these mistakes which are rapidly corrected with practice [10]. In short, an individuals cognitive expectations of heaviness can have a wide range of effects on their conscious perception of heaviness. Although this representation-based view of weight illusions is certainly consistent with much recent work on the topic [1518], it can at this time (...truncated)