Planning grasps for object manipulation: integrating internal preferences and external constraints.

Grasp selection for object manipulation depend on a person's preferences (e.g., comfortable grasp) and the object's shape (i.e., how the object can be grasped). Both have to be matched when planning to grasp an object. According to the simulation hypothesis, humans simulate the action outcome for each of the grasp options to select the best grasp. However, if an object offers many different grasp options, further processing is necessary to reduce the number of possibilities. According to the preference hypothesis, a preferred grasp is first computed and then adjusted to comply with the objects' shape, if necessary. To test the hypotheses, we asked participants to grasp knobs that could be grasped with two, four, or an unconstrained range of grasps. When participants chose among two or four options, planning time increased with the number of possible grasps which is in line with the simulation hypothesis. However, when grasps were unconstrained, planning times were as short as in the two-grasp condition, suggesting another--possibly preference-based--selection process in this case. In contrast to planning time, grasp choices were comparable regardless of the knob's shape. This suggests a common criterion most likely determined grasp selection in all conditions.