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.

Experimental Brain Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 F o r P e e r R e v i e w AbstractWeight illusions -where one object feels heavier than an identically-weighted counterpart -have been the focus of many recent scientific investigations. The most famous of these illusions is the 'size-weight illusion', where a small object feels heavier than an identically-weighted, but otherwise similar-looking, larger object. There are, however, a variety of similar illusions which can be induced by varying other stimulus properties, such as surface material, temperature, colour, and even shape. Despite well over 100 years of research, there is little consensus about the mechanisms underpinning these illusions. In this review, I will first provide an overview of the weight illusions which have been described. I will then outline the dominant theories which have emerged over the past decade for why we consistently misperceive the weights of objects which vary in size, with a particular focus on the role of lifters' expectations of heaviness. Finally, I will discuss the magnitude of the various weight illusions, and suggest how this largely-overlooked facet of the topic might resolve some of the debates surrounding the cause of these misperceptions of heaviness.Keywords: size-weight illusion, material-weight illusion, object lifting, grip force, weight perception, expectations 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 F o r P e e r R e v i e w Some of the earliest illusions to be formally described are illusions of heaviness, where objects subjectively feel lighter or heavier than they actually are. These are most compellingly demonstrated in situations where identically-weighted objects are made to feel as if they weigh different amounts from one another. This article is intended to provide a brief overview of the weight illusions which have been described since the late 1800s, and provide an up-to-date commentary on the possible causes of the size-weight illusion. This article is not intended to be a review of the history of weight illusion research, and the interested reader can read comprehensive historical treatments from other authors ). Page 2 of 35 Experimental Brain Research -A brief taxonomy of weight illusionsThe obvious place to start this review is with the famous 'size-weight illusion' (SWI), which is by far the most well-studied of all the illusions of heaviness. The SWI occurs when small and large objects are adjusted to have identical masses. When these identically-weighted objects are lifted, the smaller object invariably feels heavier than the larger object (Charpentier 1891). The illusion is cognitively impenetrable, mean...

BackgroundOur expectations of an object's heaviness not only drive our fingertip forces, but also our perception of heaviness. This effect is highlighted by the classic size-weight illusion (SWI), where different-sized objects of identical mass feel different weights. Here, we examined whether these expectations are sufficient to induce the SWI in a single wooden cube when lifted without visual feedback, by varying the size of the object seen prior to the lift.Methodology/Principal FindingsParticipants, who believed that they were lifting the same object that they had just seen, reported that the weight of the single, standard-sized cube that they lifted on every trial varied as a function of the size of object they had just seen. Seeing the small object before the lift made the cube feel heavier than it did after seeing the large object. These expectations also affected the fingertip forces that were used to lift the object when vision was not permitted. The expectation-driven errors made in early trials were not corrected with repeated lifting, and participants failed to adapt their grip and load forces from the expected weight to the object's actual mass in the same way that they could when lifting with vision.Conclusions/SignificanceVision appears to be crucial for the detection, and subsequent correction, of the ostensibly non-visual grip and load force errors that are a common feature of this type of object interaction. Expectations of heaviness are not only powerful enough to alter the perception of a single object's weight, but also continually drive the forces we use to lift the object when vision is unavailable.

Background While studies have shown that feminine faces are perceived as both trustworthy and attractive 1 ,

Observing the actions of others has been shown to modulate cortico-spinal excitability and affect behaviour. However, the sensorimotor consequences of observing errors are not well understood. Here, participants watched actors lift identically-weighted large and small cubes which typically elicit expectation-based fingertip force errors. One group of participants observed the standard overestimation and underestimation-style errors that characterise early lifts with these cubes (Error Video -EV). Another group watched the same actors performing the well-adapted error-free lifts that characterise later, well-practiced lifts with these cubes (No Error Video -NEV). We then examined actual object lifting performance in the subjects who watched the EV and NEV. Despite having similar cognitive expectations and perceptions of heaviness, the group that watched novice lifters making errors themselves made fewer overestimation-style errors than those who watched the expert lifts. To determine how the observation of errors alters cortico-spinal excitability, we measured motor evoked potentials in separate group of participants while they passively observed these error and no-error videos. Here, we noted a novel size-based modulation of cortico-spinal excitability when observing the expert lifts, which was eradicated when watching errors. Together, these findings suggest that individuals' sensorimotor systems are sensitive to the subtle visual differences between observing novice and expert performance.