In dyadic conditions (see below), each subject used a robotic manipulandum, a twin visuomotor and haptic interface system (Tvins)37 (link), to control a simulated virtual beam (Fig. 1 a). Each robot’s arm consists of parallel links which are driven by electric motors placed under the display board on which the task was visually rendered by a projector. The handles of the manipulanda are aero-magnetically floated on the support table to minimize friction, such that they move freely on the flat surface of the support table. In the present study, the handle was programmed to move only in forward–backward dimension (y; Fig. 1 b). The forces exerted by a participant on the robot handle were measured by a six-axis force/torque sensor. Data were collected at a sampling rate of 2 kHz.
Participants sat on a reclining adjustable chair and wore a seat belt. We set the chair’s height so as to align the shoulder-arm-hand with the handle’s height on the horizontal plane. We positioned each participant by having his/her shoulder 45 cm away from the origin of the hand position. Each participant's forearm rested on a cuff that was mechanically supported on the same horizontal plane. Therefore, participants were not required to hold their arm against gravity. For safety reasons, movement of the Tvins could be stopped if one of the following criteria was met: when the emergency hand switch was pressed by the participant, when the force/torque sensor measured force greater than 20 N, or when two participants’ hands were 20 mm away from each other on the y- axis. The latter criterion was used to ensure that both subjects would initiate the movement at similar onset latencies from the time at which they saw the start area color change (movement start cue), thus avoiding one subject only starting the virtual beam transport task. Depending on the stiffness condition, the virtual beam and hand position may be apart. In such a case, as a safety measure, the 20-mm criterion was used for two reasons: to avoid causing a large driving force to the beam (see Eq.3 ) and prevent the robot from exerting large feedback force on the subjects (see Eq. 4 ). Each participant grasped the robot handle under the display board with his/her dominant (right) hand. Additionally, we placed a partition to prevent participants from seeing each other. Participants wore earplugs and soundproof earmuffs to mute the robot sound.
Participants sat on a reclining adjustable chair and wore a seat belt. We set the chair’s height so as to align the shoulder-arm-hand with the handle’s height on the horizontal plane. We positioned each participant by having his/her shoulder 45 cm away from the origin of the hand position. Each participant's forearm rested on a cuff that was mechanically supported on the same horizontal plane. Therefore, participants were not required to hold their arm against gravity. For safety reasons, movement of the Tvins could be stopped if one of the following criteria was met: when the emergency hand switch was pressed by the participant, when the force/torque sensor measured force greater than 20 N, or when two participants’ hands were 20 mm away from each other on the y- axis. The latter criterion was used to ensure that both subjects would initiate the movement at similar onset latencies from the time at which they saw the start area color change (movement start cue), thus avoiding one subject only starting the virtual beam transport task. Depending on the stiffness condition, the virtual beam and hand position may be apart. In such a case, as a safety measure, the 20-mm criterion was used for two reasons: to avoid causing a large driving force to the beam (see Eq.
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