Fig. 1.

Experimental setup. A: lateral view showing the arrangement of the experimental apparatus. Subjects sat facing a mirror onto which the start positions and targets were projected using a 52-in HDTV. Subjects rested their arms in an air-sled system placed on a glass tabletop. The air sled support was attached to a robotic manipulandum at its distal end. B: top view of the setup showing attachment of the robot arm to the air sled, attachment of the Flock of Birds (FOB) sensors on the subject's arm and a typical screen display. The robot arm was connected to the air-sled using an L-shaped bracket. Two FOB sensors were used to record subjects' movements per arm. One sensor was attached to the air sled while the other was attached to the subject's upper arm. The screen display consisted of filled circles representing the start positions and targets for each arm. In addition, a cursor representing the position of the left and right index fingertip was displayed on the screen. A trial was typically initiated by bringing the cursor into the start circle followed by the presentation of an audiovisual “go” signal. Note that subjects could not see their arms during the experiment because of the mirror placed above. The mirror has been “cut out” in this figure for display purposes.