Efficient grasping requires attentional resources

We investigated the effects of visuo-spatial attention on the kinematics of grasping movements by employing a dual-task paradigm. Participants had to grasp cylindrical objects of different sizes (motor task) while simultaneously identifying a target digit presented at a different spatial location within a rapid serial visual presentation (perceptual task). The grasping kinematics in this dual-task situation were compared with the those measured in a single-task condition. Likewise, the identification performance was also measured in a single-task condition. Additionally, we kept the visual input constant across conditions by asking participants to fixate. Without instructions about the priority of tasks (Experiment 1) participants showed a considerable drop of identification performance in the dual-task condition. Regarding grasping kinematics, the concurrent perceptual task resulted in a less accurate adaptation of the grip to object size in the early phase of the movement, while movement times and maximum grip aperture were unaffected. When participants were instructed to focus on the perceptual task (Experiment 2), the identification performance stayed at about the same level in the dual-task and the single-task conditions. The perceptual improvement was however associated with a further decrease in the accuracy of the early grip adjustment. We conclude that visual attention is needed for the effective control of the grasp kinematics, especially for a precise adjustment of the hand to object size when approaching the object.