Habitual and goal-directed factors in (everyday) object handling.

A habitual and a goal-directed system contribute to action selection in the human CNS. We examined to which extent both systems interact when selecting grasps for handling everyday objects. In Experiment 1, an upright or inverted cup had to be rotated or moved. To-be-rotated upright cups were more frequently grasped with a thumb-up grasp, which is habitually used to hold an upright cup, than inverted cups, which are not associated with a specific grasp. Additionally, grasp selection depended on the overarching goal of the movement sequence (rotation vs. transport) according to the end-state comfort principle. This shows that the habitual system and the goal-directed system both contribute to grasp selection. Experiment 2 revealed that this object-orientation-dependent grasp selection was present for movements of the dominant- and non-dominant hand. In Experiment 3, different everyday objects had to be moved or rotated. Only if different orientations of an object were associated with different habitual grasps, the grasp selection depended on the object orientation. Additionally, grasp selection was affected by the horizontal direction of the forthcoming movement. In sum, the experiments provide evidence that the interaction between the habitual and the goal-directed system determines grasp selection for the interaction with every-day objects.