Equipment and data recording.

Following the description presented by Fabbri-Destro et al. (2009), we considered as an index of preserved second-order motor planning the reduced duration of the first part of the action (i.e., the reaching act) for the easier condition compared with the more difficult one (i.e., placing the cube in the large or small container, respectively). Kinematics of movements were recorded using an optoelectronic system for motion capture (OEP System; BTS Bioengineering, Milan, Italy), and spatiotemporal parameters were computed. This system (spatial resolution 0.3 mm) was composed of eight charge-coupled device (CCD) cameras with axial light-emitting diodes that emit infrared light.

The beam of infrared light emitted by the camera flash was reflected by each marker and captured by the cameras working at a sampling rate of 60 Hz. Each marker consisted of adhesive plastic spheres or hemispheres (diameter, 10 mm) covered with reflective paper. The three-dimensional coordinates of each marker were determined by stereophotogrammetry using at least two CCD cameras. A set of five markers was placed on specific body landmarks: one on the shoulder (acromion, hereafter ACR), one on the elbow (lateral epicondyle, hereafter ELB), two on the wrist (ulnar and radial styloid processes, hereafter _UWRI and _RWRI), and one on the hand (fifth metacarpal head, hereafter HAND). Moreover, there were three markers on the cube: one on two adjacent sides and one on the top (see, Fig. 2.1).