Lifting without Seeing: The Role of Vision in Perceiving and Acting upon the Size Weight Illusion

Abstract

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.

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

“…Thus, the cortico-spinal consequences of observing an error (even an error made by oneself) could automatically drive the correction of that error for future lifts, contributing to processes underpinning fingertip force adaptation and perhaps motor learning in general. If so, observation of one's own actions may a critical cue for error-based learning, possibly accounting for our recent findings that lifting without visual feedback impairs fingertip force adaptation (Buckingham and Goodale, 2010a;Buckingham et al, 2011). Thus, one role of the sensorimotor system is to gather evidence about the likely requirements of an upcoming motor plan from a variety of sources to drive successful behaviour.…”

“…When they are deprived of vision individuals show deficits in their ability to correct their fingertip force errors, continually lifting objects with forces that reflect how heavy the objects look, rather than how heavy the objects actually are (Buckingham and Goodale, 2010a;Buckingham, Ranger, and Goodale, 2011). These findings indicate that individuals receive valuable information describing the direction and magnitude of a lifting error from visual kinematic cues.…”

Weightlifting exercise and the size–weight illusion

“…In contrast, when asked how heavy each dumbbell felt after completing the experiment, participants reported that the small dumbbell felt heavier than the large dumbbell (t(17) = 2.33, p<.05, Figure 2B). Thus, participants expected the dumbbells to weigh different amounts and experienced a SWI as a consequence (e.g., Buckingham & Goodale, 2010a).…”

“…We have recently provided further indications of how expectations can affect weight perception, by demonstrating that the SWI can even be induced in a single medium-sized cube. In this study, participants were given a short visual preview of a larger or smaller cube, which they expected to eventually lift without visual feedback (Buckingham & Goodale, 2010a). Prior to actually lifting the cube without vision, however, the viewed large or small cube was discreetly replaced with the same lifted cube, unbeknownst to the lifter.…”

Effects of expectations on loudness and loudness difference

“…In the material-weight illusion, the one of two equally heavy objects that is made from what seems to be a heavier material seems lighter. The dominant theories of the illusions hold that expectations play a major role in the illusions (see, e.g., Buckingham & Goodale, 2010;Buckingham, Ranger, & Goodale, 2011;Ellis & Lederman, 1998). Objects' weights are, to some degree, compared with their expected weights.…”