Reaching beyond reach

The analysis of errors in two-joint reaching movements has provided clues a bout sensorimotor processing algorithms. The present study extends this foc us to situations where the head, trunk, and legs join with the arm to help reach targets placed slightly beyond arm's length. Subjects reached accurat ely to touch "real targets" or reached to the remembered locations of "virt ual targets" (i.e., targets removed at the start of the reach). Subjects ma de large errors in the virtual-target condition and these errors were analy zed with the aim of revealing the implications for whole-body coordination. Subjects were found to rotate the head less in the virtual-target conditio n (when compared with accurate movements to real targets). This resulted in a more limited range of head postures, and the final head angles at the en d of the movements were geometrically related to the incorrect hand locatio ns, perhaps accounting for some portion of the errors. This suggests that h ead-eye-hand coordination plays an important role in the organization of th ese movements and leads to the hypothesis that a representation of current gaze direction may serve as a reference signal for arm motor control.