Optotrak 3020 system

Movements were recorded using the Optotrak 3020 system (Northern Digital, Waterloo, Ontario). Using skin-friendly tape, six rigid PVC plates, each with three infrared light-emitting diodes, were attached to the participant's sternum, to the acromion, on the left side of the right upper arm below the insertion of the deltoid, proximal to the ulnar and radial styloids, to the dorsal surface of the hand (van Andel, Wolterbeek, Doorenbosch, Veeger, & Harlaar, 2008) (link), and to the index finger (Van Der Steen & Bongers, 2011) (link). Following the procedure described by van Andel et al. (2008) (link), for each individual participant, the 19 anatomical positions were recorded together with the rigid body position data using a standard pointer device. A small aluminum plate was taped under the index finger to prevent flexion-extension in the interphalangeal joints while allowing for flexion-extension and adduction-abduction in the metacarpophalangeal joint (Van Der Steen & Bongers, 2011) (link).

To develop the simulator, we used a 19-inch CRT monitor (39.5 Â 29.6 cm, 800 Â 600 pixels, Mitsubishi Diamondtron M2 RDF223G) that was refreshed at 120 Hz, stereo shutter goggles (Crystal Eyes 3, StereoGraphics), and a personal computer (DELL, Inc., Precision T7400, 3.2-GHz CPU, 3.25-GB RAM, graphics card: NVIDIA Quadro FX 5800). Subjects observed stimuli presented by the CRT through the goggles (Fig. 4). To minimize real crosstalk between the images presented to their eyes, we used only red phosphor on the monitor, which was comparatively faster. The minimum and maximum luminances measured through the stereo goggles were less than 0.001 cd/m 2 and 1.99 cd/m 2 , respectively.
The viewing distance was 160 cm, at which theoretically, the pixel structure cannot be distinguished by a person with a visual acuity of 1.0. This way of determining the viewing distance was used for the standard viewing distance of HDTV screens [75] , [76] .
We used an infrared (IR) position sensor (Optotrak 3020 System, Northern Digital) to track the subject's head position during the experiment. An IR emitter was attached to the goggles. The sensor collected the head-position data every 10 ms, resulting in no awareness of the latency of the image presentation based on the head motion.

The position of eight active infrared markers was recorded via an Optotrak 3020 system (Northern Digital INC). Three markers were placed on the subject's right hand: on the wrist at the styloid process of the radius, on the nail of the thumb and on the nail of the index (sampling rate at 300Hz; spatial resolution: 0.1 mm). Two markers were positioned on the bottles and the remaining three on the table to define the 3D workspace. Data pre-processing consisted in applying a second-order Butterworth dual pass filter (cutoff frequency: 10Hz). Kinematic parameters were assessed for each trial individually via Optodisp software (Optodisp -copyright INSERM-CNRS-UCBL, (Thévenet, Paulignan, & Prablanc, 2001) . For each movement we measured the whole movement time as the time between the beginning of the hand movement and the moment when the bottle was put in its landing position. We measured the latency and amplitude of the highest wrist acceleration, velocity and deceleration peaks for each movement phase. In addition, for the Reach phase we measured the amplitude and latency of the maximum grip aperture and for the Displace phase, the maximal height at which the bottle was lifted. Parameters as recorded from a single trial are illustrated in figure 1.

We used a setup that allowed us to create 3-D virtual stimuli (see Figure 1). In this setup mirrors reflected the images from two CRT monitors (1096 3 686 pixels, 47.3 3 30.0 cm) that were to the sides of the subjects' head to the two eyes. Subjects looked straight ahead at these mirrors and had the illusion that the 3-D virtual objects were in front of them. New images were created for each eye with the frequency of the refresh rate of the monitors (160 Hz). We recorded the position of the head and of the index finger of the preferred hand at 250 Hz using infrared emitting diodes (IREDs) and an Optotrak 3020 System (Northern Digital, Waterloo, ON, Canada). One IRED was attached to the nail of the index finger and three to a mouthpiece with a dental imprint. Subjects were allowed to freely move their head during the experiments (although the setup did not encourage large head movements since subjects had to look into the mirrors). Tracking the head's position allowed us to adapt the images to movements of the head with a very short delay (about 20 ms). The positions of subjects' eyes relative to the mouthpiece were determined in advance following the same calibration procedure as in Sousa et al. (2010) .