Grasping movements toward seen and handheld objects

Abstract

Grasping movements are typically performed toward visually sensed objects. However, planning and execution of grasping movements can be supported also by haptic information when we grasp objects held in the other hand. In the present study we investigated this sensorimotor integration process by comparing grasping movements towards objects sensed through visual, haptic or visuo-haptic signals. When movements were based on haptic information only, hand preshaping was initiated earlier, the digits closed on the object more slowly, and the final phase was more cautious compared to movements based on only visual information. Importantly, the simultaneous availability of vision and haptics led to faster movements and to an overall decrease of the grip aperture. Our findings also show that each modality contributes to a different extent in different phases of the movement, with haptics being more crucial in the initial phases and vision being more important for the final on-line control. Thus, vision and haptics can be flexibly combined to optimize the execution of grasping movement.

Figure 1

Experimental setup: top view of the subject’s start (a) and end (b) positions.

Figure 2

Representation of the task under Haptic, Visual and Visuo-Haptic condition. The grasping action was always performed with the right hand. In H and VH participants were already holding the object with their left hand before the start of the grasping action.

Figure 3

Average performance in terms of (a) movement duration, (b) maximum grip aperture, (c) maximum wrist velocity, (d) maximum wrist deceleration, in the haptic (H), visual (V), and visuo-haptic (VH) conditions. Error bars represent the standard error of the mean.

Figure 4

Top view of the average thumb and index digit trajectories in the three conditions. The movement unfolded from right to left. The lines on the top represent the trajectories of the index digit, the lines on the bottom represent the trajectories of the thumb. H, V and VH modalities are represented respectively in light green, dark green and blue. Triangles represent the position along the trajectory where participants reached the MVW, squares the MWD and circles the MGA. The vertical position of the symbols coincides with the midpoint between the thumb and index digit positions. The pMWV, pMWD and pMGA occurred at 0.45 ± 0.04, 0.81 ± 0.03 and 0.90 ± 0.02 in H, at 0.47 ± 0.04, 0.84 ± 0.03, and 0.94 ± 0.01 in V, and, in 0.48 ± 0.04, 0.86 ± 0.02, and 0.94 ± 0.01 in VH. The pMGA of the V condition is not visible, because it is occluded by the VH symbol.

Figure 5

Average (a) grip aperture and (b) wrist velocity in the H, V and VH conditions along the space-normalized trajectory. Shaded horizontal bars below each panel represent the evolution of the p-value along the trajectory for each comparison.