Illusory motion of the motion aftereffect induces postural sway.

significant role that visual stimulation plays in postural control has been well established. For instance, visual stimuli simulating self-motion through the environment generate potent postural adjustments in observers In all the studies just cited, the pos-tural adjustments occurred as a result of motion information in a visual stimulus that was presented to the observer (i.e., direct visual stimulation). It remains an open question, however, whether this perception-action cycle is the result of direct visual stimulation only, or whether postural adjustments also occur when the motion of the visual stimulus is illusory. Here, we show that the latter is the case. Prolonged viewing of visual motion results in neural adaptation, and subsequent viewing of a stationary stimulus normally results in illusory motion in the opposite direction, a famous phenomenon known as the motion aftereffect (MAE; Anstis, Verstraten, & Mather, 1998). Surprisingly, this sequence of stimulation also causes postural sway in the direction consistent with the perceived illusory motion. Control test patterns that do not generate an MAE after identical adaptation do not induce sway. This suggests that the visuo-vestibular interactions that govern postural control are not influenced by visual stimulation per se, but can be modulated by an illusory motion signal (e.g., the internal neural signal responsible for the MAE). Visual experience is often the direct result of visual stimulation. Hence, it is not surprising that most research on visuo-vestibular interactions has used direct visual stimulation , such as optic-flow stimuli simulating self-motion To be able to disentangle actual (direct) visual stimulation from visual experience, we used the MAE. During our experiment, observers (N = 7) stood in a completely dark room on a force plate (Fig. 1a). The recorded posturo-graphic data were used to analyze the center-of-pressure (COP) displacement in the medial-lateral direction. Observers stood in front of a projection screen (87° × 56°) and visually adapted to a binary random-pixel array (RPA; 50% dark pixels, 50% bright pixels) that was translating leftward or rightward with a speed of approximately 3°/s. The RPA was initially presented for 40 s to build up adaptation; 20-s top-up adaptation epochs were used between trials to keep observers maximally adapted. Each adaptation epoch was followed by a black screen for 2 s and then a 14-s presentation of the test pattern. Observers had to press a button to report when the MAE dissipated, if its duration was shorter than 14 s. Three different test …