Fig. 1.

Experimental setup. A, Subjects reached to visual targets while holding the handle of a manipulandum. The location of their hand was displayed directly above their hand via a video projector on a horizontal screen that was mounted at <1 cm above the handle. B, Performance was measured in three small work spaces, each a semicircle of radius 10 cm. When the hand was in a given work space, it was initially positioned at the center target. For odd-numbered movements, targets were chosen randomly from the marked locations on the circumference of the work space. Targets for even-numbered targets were always at the center of the work space. The typical joint angle vector at the left work space (Left) was q_l = (104°,71°), that at the center work space (Center) was q_c = (63°,90°), and that at the right work space (Right) wasq_r = (13°,65°). A typical arm link length for the upper arm was l₁ = 33 cm, and that for the forearm wasl₂ = 34 cm.