The impact of real and illusory target perturbations on manual aiming.

This experiment was designed to determine if real and illusory shifts in target position at movement initiation affect the same online corrective processes. Adult participants completed rapid goal-directed movements toward the vertex of a target "T" located at the midline, 25 cm distal to a small home position. At movement initiation, the target either stayed the same, shifted its real position, its illusory position or both. The real perturbation involved a 2.5 mm shift either toward or away from the body. For the illusory perturbation, the horizontal portion of the "T" changed to inward or outward Müller-Lyer wings. Both the real and the illusory perturbation affected movement outcome. The two manipulations began to have their impact at peak velocity. Because both perturbations affected mid to late trajectory control and because their effects were not independent, we concluded that real and illusory target shifts impact late visual motor control associated with a comparison between the position of the limb and the perceived position of the target.