The visibility of contact points influences grasping movements.

When humans grasp an object, the thumb habitually contacts the object on the visible part, whereas the index finger makes contact with the object on the occluded part. Considering that the contact points play a critical role in object-oriented actions, we studied if and how the visibility of the points of contact for the thumb and index finger affects grasping movements. We adapted reach-to-point movements (visual feedback displacement: 150 mm in depth) performed with either the thumb or the index finger to measure how a newly learned visuomotor mapping transfers to grasping movements. We found a general transfer of adaptation from reach-to-point to reach-to-grasp movements. However, the transfer of adaptation depended on the visibility of contact points. In the first experiment, in which only the thumb's contact point was visible during grasping, the transfer of adaptation was larger after thumb than after index finger perturbation. In the second experiment, in which both contact points were equally visible, the transfer of adaptation was of similar magnitude. Furthermore, thumb trajectories were less variable than index trajectories in both experiments weakening the idea that the less variable digit is the digit that guides grasping movements. Our findings suggest that the difficulty in determining the contact points imposes specific constraints that influence how the hand is guided toward the to-be-grasped object.