Abstract
Estimating forces acting between our hand and objects is essential for dexterous motor control. An earlier study suggested that vision contributes to the estimation by demonstrating changes in grip force pattern caused by delayed visual feedback. However, two possible vision-based force estimation processes, one based on hand position and another based on object motion, were both able to explain the effect. Here, to test each process, we examined how visual feedback of hand and object each contribute to grip force control during moving an object (mass) connected to the grip by a damped-spring. Although force applied to the hand could be estimated from its displacement, we did not find any improvements by the hand feedback. In contrast, we found that visual feedback of object motion significantly improved the synchrony between grip and load forces. Furthermore, when both feedback sources were provided, the improvement was observed only when participants were instructed to direct their attention to the object. Our results suggest that visual feedback of object motion contributes to estimation of dynamic forces involved in our actions by means of inverse dynamics computation, i.e., the estimation of force from motion, and that visual attention directed towards the object facilitates this effect.
Highlights
In the second experiment, we examined how attending to either the hand cursor or the object cursor influenced the coordination of the grip force
The position of the object and its inertial reaction force was calculated at 1 kHz using the 4th-order Runge-Kutta method, and the force was applied to the cube
Measured load force delayed from the simulated load force by 44.3 ± 10.3 and 22.0 ± 10.2 ms in the small and large delay conditions, possibly due to the property of the haptic device. The average of these delays was approximately equal to the delay of the visual feedback, and the temporal discrepancy between the load force and the visual feedback was estimated to be below 5 degrees of phase difference in both load force conditions
Summary
We examined how attending to either the hand cursor or the object cursor influenced the coordination of the grip force. We examined three visual feedback conditions: a condition in which both the hand and object positions are shown and the participants were instructed to control the hand to move with the specified amplitude and frequency (attend-to-hand condition), a similar condition in which the participants were instructed to control the object to move with the specified amplitude and frequency (attend-to-object condition), and the condition without the two visual feedback sources (control condition). In the attend-to-object condition, the color of the edge markers changed according to the position of the object rather than the cube. Together with the load force condition, there were 6 conditions in total. Each block contained 4 trials for each condition and there were 8 blocks (32 trials for each condition in total).
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