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Abstract

We investigated whether computer-generated stereo projected images (virtual objects) are suitable as targets in grasping experiments. For real and virtual objects we measured grasp quality, movement time, preshape aperture, and terminal aperture angle under two different conditions: (a) block presentation of virtual and real objects and (b) mixed presentation (real and virtual objects interspersed). Maximal number of consecutive presentations of virtual objects was limited to seven under the mixed condition. Six subjects took part in condition (a) and ten subjects in condition (b). Objects had a smooth spline-based contour with a limited number of stable grasp points. Six objects were presented in six different orientations and each grasp was repeated five times.

We found that in block presentation mode grasp quality was generally lower for virtual objects (43) than for real objects (78). Analysis of the time course revealed that grasp quality was initially the same for both types of objects, but decreased for virtual objects during the course of the experiment. In mixed presentation mode, no such difference was found: grasp quality was equally high for both object types (75). No significant differences in movement time and other dynamic parameters were found.

One of the major differences between the two conditions is the lack of haptic feedback for virtual objects in block presentation mode. We argue that missing haptic feedback for virtual objects leads to pantomiming and decreased grasp quality. Haptic feedback is necessary for maintaining grasp quality at a stable level.