Held’s answer to the first question is that subjects must receive visual feedback from active movement, i.e., reafferent visual stimulation, in order for significant and stable adaptation to occur (Held &amp; Hein 1958; Held 1961; Held &amp; Bossom 1961). Evidence for this conclusion came from experiments in which participants wore laterally displacing prisms during both active and passive movement conditions. In the active movement condition, the subject moved her visible hand back and forth along a fixed arc in synchrony with a metronome. In the passive movement condition, the subject’s hand was passively moved at the same rate by the experimenters. Although the overall pattern of visual stimulation was identical in both conditions, adaptation was reported only when subjects engaged in self-movement. Reafferent stimulation, Held and Bossom concluded on the basis of this and other studies,