Move on up: Fingertip forces and felt heaviness are modulated by the goal of the lift

Attention, Perception, & Psychophysics https://doi.org/10.3758/s13414-019-01703-w TIME FOR ACTION: REACHING FOR A BETTER UNDERSTANDING OF THE DYNAMICS OF COGNITION Move on up: Fingertip forces and felt heaviness are modulated by the goal of the lift Gavin Buckingham 1 & Heather Donald 2 # The Author(s) 2019 Abstract When we interact with objects, we usually do so for a purpose. It is well known that the specific goal of an action can have a substantial effect on initial reach kinematics. No research, however, has examined the effect that the goal of a lift can have on the fingertip forces and perception of object weight when picking up an object to move it. Here, we report a study in which participants were asked to move objects laterally to a higher platform, to a lower platform, or to a platform of the same height. The objects were rated, on average, as feeling heavier after they were moved to a higher platform than after they were moved to a lower platform or to a platform of the same height. Furthermore, participants gripped and lifted with more force, and used higher rates of force, when moving objects to a higher platform compared with moving it to a platform of the same height. These findings suggest that the goal of movement in the context of object interaction may affect how heavy an object feels and the way in which it is lifted. Keywords Perception and Action . Haptics We interact with objects hundreds of times every day. When lifting objects, even ones we have not touched before, we typically apply forces in a predictive manner, with fingertip forces and pre-lift-off force rates reflecting the object’s expected weight (Flanagan & Johansson, 2011). Although we typically grasp objects to use them in some way, many of our interactions with objects are perceptual in nature, with the goal of evaluating nonvisual properties such as weight. Interestingly, many factors can influence how heavy an object feels when it is lifted. For example, it is well known that an object will feel heavier when it is cold than when it is at room temperature (Ross & Murray, 1978). Furthermore, the surface friction of an object can influence how heavy it feels, such that slippery objects feel heavier than nonslippery objects do (Flanagan, Wing, Allison, & Spenceley, 1995)—an effect caused by the increased grip force required to maintain an * Gavin Buckingham 1 Sport and Health Sciences, University of Exeter, Exeter EX2 4LU, UK 2 Department of Psychology, Heriot-Watt University, Edinburgh, UK appropriate friction coefficient when lifting slippery objects. In fact, how an object is gripped and lifted can influence how heavy it feels. For example, it is easily demonstrated that lifting an object rapidly will make it feel less heavy than it would feel when it is lifted slowly. Similarly, lifting an object with a wider grip aperture, or using more digits to grip the object surface, can make it feel heavier than it actually is (Flanagan & Bandomir, 2000). In addition to these low-level factors, a range of higher level influences on the perception of object weight results in dramatic weight illusions. The most famous example of how humans misperceive object weight can be experienced with the size–weight illusion (SWI), in which small objects feel substantially heavier than identically weighted large objects (Charpentier, 1891). This illusory weight difference has been shown to be unrelated to sensorimotor factors (Flanagan & Beltzner, 2000; Grandy & Westwood, 2006; Mon-Williams & Murray, 2000) and is instead thought to reflect the role of cognitive expectations on our perception of heaviness. A lifetime of experiencing the positive correlation between size and mass cause lifters to expect the large object to outweigh the small object, and therefore to experience it as lighter than expected and vice versa (for review, see Buckingham, 2014). Indeed, even a single object can be made to feel it is Atten Percept Psychophys of a substantially different weight if an individual merely expects to be lifting something heavier or lighter than the object they eventually interact with (Buckingham & Goodale, 2010; Buckingham, Ranger, & Goodale, 2011). This effect is ubiquitous, having been demonstrated in a wide range of populations—from children as young as 2 years (Robinson, 1964), to patients with unilateral brain injury (Buckingham, Bieńkiewicz, Rohrbach, & Hermsdörfer, 2015a), to blind human echolocators (Buckingham, Milne, Byrne, & Goodale, 2015b)—and can only be influenced by thousands of trials of perceptual learning (Flanagan, Bittner, & Johansson, 2008). Similar, albeit much smaller, effects can be experienced with the material–weight illusion, in which objects that appear to be made from a light material feel slightly heavier than objects that appear to be made from a more dense material (Buckingham, Cant, & Goodale, 2009; Buckingham et al., 2011; Ellis & Lederman, 1999). These weight illusions are considered to be a unique instance of an individual’s perception reflecting a combination of sensory input with the inverse of perceptual prior expectations (Brayanov & Smith, 2010). The nature of the prior expectation that drives these weight illusions is, however, far from clear. One reason for this lack of a mechanistic understanding might stem from the tasks typically employed in perceptual weight-judgement tasks. In the majority of weight perception studies, the participant is simply told to lift and report (or compare with a standard) the weight of an object, before replacing it on the table surface. In our daily lives, however, actions typically have a goal with an end state that is distinct from the originating movement. Indeed, very few studies have examined weight perception in the context of a more natural, goal-directed movement. Interestingly, a growing body of work in the context of reach-to-grasp movements suggests that the end-state goal of an action can affect how the movement itself is planned. Typically, these studies assess movement kinematics during early phases of a grasping movement, showing that the likely end posture of the grasp will affect the start posture of the grasp, with individuals typically prioritising end-state comfort over initial-state comfort (for review, see Rosenbaum, Chapman, Weigelt, Weiss, & van der Wel, 2012). It remains unclear, however, the extent to which premovement parameters, such fingertip-force parameterization, and subsequent experiences of heaviness (Flanagan et al., 1995) can be affected by movement goals. Understanding the relationship between the prior expectations that can drive heaviness perceptions and the goal-directed effects that can influence grasp planning might shed light on the interplay between hedonic perception and motor planning. In order to better understand the nature of the expectations that appear to influence heaviness perception, we examined how weight perception is affected by varying the goal of lift. To (...truncated)