Visual Cues of Object Properties Differentially Affect Anticipatory Planning of Digit Forces and Placement

Abstract

Studies on anticipatory planning of object manipulation showed initial task failure (i.e., object roll) when visual object shape cues are incongruent with other visual cues, such as weight distribution/density (e.g., symmetrically shaped object with an asymmetrical density). This suggests that shape cues override density cues. However, these studies typically only measured forces, with digit placement constrained. Recent evidence suggests that when digit placement is unconstrained, subjects modulate digit forces and placement. Thus, unconstrained digit placement might be modulated on initial trials (since it is an explicit process), but not forces (since it is an implicit process). We tested whether shape and density cues would differentially influence anticipatory planning of digit placement and forces during initial trials of a two-digit object manipulation task. Furthermore, we tested whether shape cues would override density cues when cues are incongruent. Subjects grasped and lifted an object with the aim of preventing roll. In Experiment 1, the object was symmetrically shaped, but with asymmetrical density (incongruent cues). In Experiment 2, the object was asymmetrical in shape and density (congruent cues). In Experiment 3, the object was asymmetrically shaped, but with symmetrical density (incongruent cues). Results showed differential modulation of digit placement and forces (modulation of load force but not placement), but only when shape and density cues were congruent. When shape and density cues were incongruent, we found collinear digit placement and symmetrical force sharing. This suggests that congruent and incongruent shape and density cues differentially influence anticipatory planning of digit forces and placement. Furthermore, shape cues do not always override density cues. A continuum of visual cues, such as those alluding to shape and density, need to be integrated.

Visual cues, expectations, and sensorimotor memories in the prediction and perception of object dynamics during manipulation

“…Both the handle-induced torques as well as previous external torques were higher correlated with T ΔCOP*GF than with T ΔLF*w/2 (Table S2) in separate models in which the torque components were the dependent variables. This finding stands in contrast with a recent study by Lee-Miller et al 2016 in which subjects failed to partition their finger positioning according to visual cues. We presume that the differences in the handle geometry and magnitude of the external torques account for this discrepancy as the handle was only half as wide as in the study by Lee-Miller et al 2016, whereas the maximal external torques were over three time as large.…”

“…This finding stands in contrast with a recent study by Lee-Miller et al 2016 in which subjects failed to partition their finger positioning according to visual cues. We presume that the differences in the handle geometry and magnitude of the external torques account for this discrepancy as the handle was only half as wide as in the study by Lee-Miller et al 2016, whereas the maximal external torques were over three time as large. This could have necessitated anticipatory finger positioning to comfortably generate the required torques.…”

“…However, when judging well-known everyday objects of similar natural weights, subjects judged bigger objects to be heavier than smaller ones, leading to the assumption that explicit expectations are added and not contrasted with sensory feedback (Buckingham and MacDonald 2016). Regarding the perception of the mass distribution or torques, subjects were shown to be capable of correctly estimating the center of mass (CoM) of an assembled object based on the visible object configuration after hefting the object components (Craje et al 2013;Lee-Miller et al 2016), while viewing a mirrored image of an unbalanced hand-held object compromised the correct judgement of the direction of the CoM (Xu et al 2012).…”

“…When lifting objects with an asymmetric center of mass, it is necessary to exert compensatory torques at the moment of lift-off to prevent the object from tilting. It has been shown that subjects can successfully utilize the shape of objects to anticipate its torque (Fu and Santello 2012;Lee-Miller et al 2016;Salimi et al 2003). However, as previous studies only investigated two opposing object shapes, Communicated by Melvyn A. Goodale.…”

Smooth at one end and rough at the other: influence of object texture on grasping behaviour

“…Bingham and Muchisky (1993) showed that humans can accurately judge the location of the COM using object symmetry. Lee-Miller et al (2016) have shown that an object's COM is judged from its visual appearance and that anticipatory planning of digit forces and placement is done on the basis of this visual estimate of the COM. Lederman and Wing (2003) showed that subjects actually grasp in such a way that the grasp axis (axis between the thumb and index finger) passes near or through the COM (also see Kleinholdermann et al 2007;Goodale et al 1994;Wing and Lederman 1998;Endo et al 2011).…”