Motor synergies and mental representations of grasping movements

We investigated hand kinematics of grasping movements towards spherical objects that systematically varied in size. Kinematic data was analyzed using principal component analysis (PCA) in order to extract movement synergies. On top of this we also analysed mental representations of grasping movements using a hierarchical sorting paradigm called Structure Dimensional Analysis in order to draw comparisons between the observed physical synergies and mental representations of those synergies. Results indicate that very early in the movement it is possible to distinguish different end grasps in a low dimensional PC space. We found that 82% of the variance was described by the first two PCs, and this rose to 92% when a third PC was added. Grasps used for the smallest objects were clearly separated from grasps used for medium and larger objects, and a clear separation of grasps used for small objects and larger objects was also found in the results of the analysis of mental representation of grasps. This supports the notion that grasping movements are strongly influenced in a top-down manner by conceptual factors.