A visually-driven postural after-effect

Viewing a large-scale moving scene typically causes a free standing observer to lean in the direction of the seen motion. It was also noted that when the visual motion ceases, the observer drifts back toward the upright position, but instead of this body movement stopping at the vertical, it tends to continue so that the observer remains leaning in the opposite direction for several seconds before finally returning to the upright. The two experiments reported here were designed to investigate the determinants of this postural after-effect in relation to a pitch vection stimulus. Our findings clearly showed that the after-effect was dependent upon (1) the establishment of a prior visually-induced body lean, and (2) sight of the static display on the cessation of motion. The notion of an internalised representation of body position was invoked to account for these results.