Living in A Material World: How Visual Cues to Material Properties Affect the Way That We Lift Objects and Perceive Their Weight

Abstract

The visual properties of an object provide many cues as to the tensile strength, compliance, and density of the material from which it is made. However, it is not well understood how these implicit associations affect our perceptions of these properties and how they determine the initial forces that are applied when an object is picked up. Here we examine the effects of these cues on such forces by using the classic "material-weight illusion" (MWI). Grip and load forces were measured in three experiments as participants lifted cubes made from metal, wood, and expanded polystyrene. These cubes were adjusted to have a different mass than would be expected for a particular material. For the initial lifts, the forces were scaled to the expected weight of each object, such that the metal block was gripped and lifted with more force than the polystyrene one. After a few lifts, however, participants scaled their forces to the actual weight of the blocks, implicitly disregarding the misleading visual cues to each block's composition (experiments 1 and 2). Despite this rapid rescaling, participants experienced a robust MWI throughout the duration of the experiments. In fact, the grip and load forces never matched the perception of weight until the differences in the visual surface properties between the blocks were removed (experiment 3). These findings are discussed in relation to recent debates about the underlying causes of weight-based illusions and the effect of top-down visual cues on perception and action.

The material-weight illusion is a Bayes-optimal percept under competing density priors

“…Our model's predicted MWI magnitude is in qualitative agreement with the MWI magnitudes reported by Buckingham et al (2009) , albeit with some differences in predicted order of MWI magnitude as a function of material pair that may be unaccounted for by the simplicity of our model ( Figure 1a, Table 4). Nevertheless, that our competing density priors model can produce MWI magnitudes qualitatively in line with MWI magnitudes observed in the literature supports the theory that the MWI, like the SWI, may occur as a result of optimal inference over competing density priors.…”

“…In their Experiment 2, Buckingham et al (2009) modified their original stimuli such that the wood block still weighed 700g, but the polystyrene block now weighed 680g and the aluminum block now weighed 720g. They then repeated their MWI experiment.…”

Observing object lifting errors modulates cortico-spinal excitability and improves object lifting performance

“…This means that when lifting something for the first time, a lifter's fingertip forces reflect their initial predictions about an object's weight, rather than the actual mass of the object (Gordon et al, 1991). The feed-forward nature of human lifting behaviour often results in grip and load force errors, which can be especially dramatic when objects have an unusual weight for their appearance (e.g., Buckingham, Cant, and Goodale, 2009;Johansson and Westling, 1988). These errors do not generally persist and individuals are rapidly able to overcome their expectations of heaviness, tuning their fingertip forces to the actual, rather than expected, weight of the object(s) being lifted (Flanagan and Beltzner, 2000;Grandy and Westwood, 2006;Mon-Williams and Murray, 2000).…”

The Influence of Material Cues on Early Grasping Force

“…In addition, the force trace was di↵erentiated and the maximum force rate (maximum of the derivative of the force) was determined. Maximum force, force after 10 ms, and maximum force rate were averaged over trials for each texture, condition, and subject separately, excluding the first two trials for each texture in order to eliminate any confound introduced by the material-weight illusion [1]. Since no visual information is available to anticipate the object's friction before touching it in the blindfolded condition, whereas there is in the sighted condition, we expected di↵erent e↵ects of material in the two conditions.…”

“…We do this by measuring the force used by human subjects when picking up objects of equal mass but di↵erent textures. An e↵ect of material on grip force, independent of surface friction, has been found in an experiment in which subjects picked up objects of equal mass made of di↵erent materials, by a handle [1]. For the initial lifts, the grip forces were scaled to the expected mass of the objects based on a visual estimate of the material's density (material-weight illusion).…”