Effects of altered transport paths and intermediate movement goals on human grasp kinematics.

It has been observed that grip opening is delayed when participants are asked to execute complex grasping movements, such as passing over an obstacle or a via-position (Haggard and Wing 1998; Alberts et al. 2002). This finding was proposed to indicate a shift toward sequential performance, meaning that complex movements are carried out in independent motor steps. In our experiments we investigated which aspects of a grasping task determine whether a movement is executed holistically or sequentially. Therefore, participants had to perform different types of curved movements in order to reach and grasp a target object. When only the complexity of the transport paths was varied, no indication of sequential movement execution was found. However, when participants additionally had to either stop at, or pass over a certain via-position the pre-shaping pattern changed considerably indicating a movement segmentation effect. This effect became stronger with increasing difficulty of the sub-task, suggesting that attentional factors are involved.