Abstract
During manipulation, object slipping is prevented by modulating the grip force (GF) in synchrony with motion-related inertial forces, i.e., load force (LF). However, due to conduction delays of the sensory system, GF must be modulated in advance based on predictions of LF changes. It has been proposed that such predictive force control relies on internal representations, i.e., internal models, of the relation between the dynamic of the environment and movement kinematics. Somatosensory and visual feedback plays a primary role in building these internal representations. For instance, it has been shown that manipulation-dependent somatosensory signals contribute to building internal representations of gravity in normal and altered gravitational contexts. Furthermore, delaying the timing of visual feedback of object displacement has been shown to affect GF. Here, we explored whether and the extent to which spatial features of visual feedback movement, such as motion direction, may contribute to GF control. If this were the case, a spatial mismatch between actual (somatosensory) and visual feedback of object motion would elicit changes in GF modulation. We tested this hypothesis by asking participants to generate vertical object movements while visual feedback of object position was congruent (0° rotation) or incongruent (180° or 90°) with the actual object displacement. The role of vision on GF control was quantified by the temporal shift of GF modulation as a function of visual feedback orientation and actual object motion direction. GF control was affected by visual feedback when this was incongruent in the vertical (180°), but not horizontal dimension. Importantly, 180° visual feedback rotation delayed and anticipated GF modulation during upward and downward actual movements, respectively. Our findings suggest that during manipulation, spatial features of visual feedback motion are used to predict upcoming LF changes. Furthermore, the present study provides evidence that an internal model of gravity contributes to GF control by influencing sensory reweighting processes during object manipulation.
Highlights
A substantial body of evidence indicates that the human ability to perform dexterous manipulation depends on two control modes, known as reactive and predictive
We tested the hypothesis that spatial features of visual feedback kinematics, such as motion direction, would elicit prediction of the timing of load force (LF) fluctuations based on an internal model of gravity, i.e., visual gravity
The results from the control experiment cross-validated our hypothesis of the contribution of visual gravity in grip force (GF) modulation, since incongruent visual feedback oriented along the horizontal axis did not influence GF timing
Summary
A substantial body of evidence indicates that the human ability to perform dexterous manipulation depends on two control modes, known as reactive and predictive (for review see, Flanagan and Johansson, 2002). Reactive force control is implemented by modulating GF in response to unexpected LF changes detected by tactile afferents (Cutkosky in Uygur et al, 2012; Prescott et al, 2018) This strategy is useful when manipulating an object in novel dynamic contexts, e.g., interacting with unfamiliar objects or responding to external perturbations. Models, have been proposed as mechanisms for predictive force control, as they are assumed to capture invariant features of the arm (e.g., dynamic), the object (e.g., size, weight), and the environment (e.g., gravity; Flanagan and Johansson, 2002; Augurelle et al, 2003; White et al, 2005; White, 2015)
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