Figure 1.
Experimental setup and protocol. a, Target movement: the performer had to continuously move both wrists back and forth, being required to maintain 90° out of phase (a quarter of the cycle) between them. b, Sketch of the experimental setup. c, During some trials, AVF was provided on a computer screen in front of the performer, by displaying the position of both wrists as the orthogonal coordinates of a single cursor. Perfect 90°-out-of-phase cyclical movement corresponds to a circular trajectory on the screen. d, Experimental protocol. The daily experimental session was divided into three parts, delimited by the vertical lines. Part I consisted of a single block (white) of four trials during which subjects had to perform in-phase cyclical movements, without AVF. In parts II and III, the target movement was the 90° out of phase. Part II consisted of four “learning” (red–magenta) or “phase-shift” (gray) blocks, each containing eight trials during which subject received AVF during four randomly selected trials (50%). In the “phase-shift” blocks, an artificial phase shift was inserted between the actual movement and the visual feedback, when present. Randomly inserted between these four blocks, part II also contained a “shakers” block (blue–orange) with four trials during which the subject received AVF and four trials without, and had shakers fixed onto the dorsal and palmar section (flexors and extensors) of both wrists. Finally, part III was done on days 1, 3, and 5 and consisted of three “visual noise” blocks, each containing seven trials during which AVF was provided but corrupted by noise, whose level was randomly selected for each trial.