Grasping partly occluded objects: effects of global stimulus information on action

We investigated here how partly occluded stimuli affect early and late prehension kinematics. Stimuli were pictures of partly occluded cylindrical objects with indentations or protrusions at regular intervals along the contour. They diverged into two plausible completions: a local completion, comprising a linear continuation of the contour, and a global completion, revealing a continuation of the contour indentations. Convergent stimuli with one plausible completion acted as controls. Twenty-eight participants repeatedly performed a grasping movement in darkness, as if they grasped the earlier-seen partly occluded cylinder. The peak acceleration of the hand opening (occurring at about 20% of the movement time) and the maximum hand aperture (at about 60% of the movement time) were taken to reflect the representation of the width of the stimuli used for action. The results show that the grasping movements were affected not only by local but also by global stimulus properties of which the representation remained stable over time.