Abstract
Lifting an object requires precise scaling of fingertip forces based on a prediction of object weight. At object contact, a series of tactile and visual events arise that need to be rapidly processed online to fine-tune the planned motor commands for lifting the object. The brain mechanisms underlying multisensory integration serially at transient sensorimotor events, a general feature of actions requiring hand-object interactions, are not yet understood. In this study we tested the relative weighting between haptic and visual signals when they are integrated online into the motor command. We used a new virtual reality setup to desynchronize visual feedback from haptics, which allowed us to probe the relative contribution of haptics and vision in driving participants’ movements when they grasped virtual objects simulated by two force-feedback robots. We found that visual delay changed the profile of fingertip force generation and led participants to perceive objects as heavier than when lifts were performed without visual delay. We further modeled the effect of vision on motor output by manipulating the extent to which delayed visual events could bias the force profile, which allowed us to determine the specific weighting the brain assigns to haptics and vision. Our results show for the first time how visuo-haptic integration is processed at discrete sensorimotor events for controlling object-lifting dynamics and further highlight the organization of multisensory signals online for controlling action and perception.NEW & NOTEWORTHY Dexterous hand movements require rapid integration of information from different senses, in particular touch and vision, at different key time points as movement unfolds. The relative weighting between vision and haptics for object manipulation is unknown. We used object lifting in virtual reality to desynchronize visual and haptic feedback and find out their relative weightings. Our findings shed light on how rapid multisensory integration is processed over a series of discrete sensorimotor control points.
Highlights
Skilled object lifting involves the planning of a motor command specifying fingertip forces based on a prediction of the object weight
To quantify the relative weighting of visual and haptic information on force scaling, we modeled the force planning in response to a visual delay
We considered three different models in which visual delay could affect the force profile (Fig. 2A): 1) the force generation profile was kept unchanged, but its onset was shifted by visual delay; 2) visual delay slowed down the force generation profile, but its onset was unaltered; and 3) a combination of a “haptic” and “visual” force profile
Summary
Skilled object lifting involves the planning of a motor command specifying fingertip forces based on a prediction of the object weight. In case of a mismatch between the predicted and actual sensory feedback, forces are rapidly adjusted to control the action (Johansson and Westling 1988). During lifting of an object, multiple sources of sensory inputs are available to adjust fingertip forces to the specific object properties. Decreased grip force control is seen in the absence of cutaneous information (e.g., by anesthesia, Johansson and Westling 1984; Monzée et al 2003) or force feedback (Gibo et al 2014). Absence of visual information leads to impaired adaptation of force scaling in repeated object lifting (Buckingham and Goodale 2010). Observing others handling objects can influence subsequent force scaling, as well (Buckingham et al 2014; Uçar and Wenderoth 2012)
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