The influence of obstacles on grasp planning.

When reaching to grasp for an object in the absence of obstacles, the choice of contact points is highly consistent within and between healthy humans, suggesting a preplanning of grasping movements (Gilster et al. in Exp Brain Res 217:137-151, 2012). In real life, objects may obstruct the favored contact points at a target object, requiring adjustments to avoid collision. In the present study, we investigated how an obstacle that directly obstructs the favored contact points for two-digit grasping changes the planning and execution of reach-to-grasp movements. Furthermore, we elucidated to what extent an obstacle placed at various angular positions around the target object (thereby not directly obstructing the favored contact points) still influences trajectories, contact points, and time-related parameters. When obstacles directly obstructed favored contact points participants either chose a completely new contact point or grasped the object only slightly away from the favored contact point. Obstacles located near the favored contact points but not directly obstructing them still resulted in a repulsive effect, meaning that contact points were shifted away from the obstacle to ensure sufficient distance to the obstacle. We found that the position of an obstacle even influences the direction in which the fingers set off. This leads to a deviation of the trajectory very early in the time course, yielding longer movement times if the main contact points are obstructed. Taken together, the early significant influence of obstacles on the grasping movement supports the assumption that grasping movements are preplanned.