The contribution of motion, the visual frame, and visual polarity to sensations of body tilt.
Three types of visual information contribute to the sense of self orientation with respect to gravity: visual polarity of objects with a distinct top and bottom, the principal vertical and horizontal lines of the visual environment, and visual motion. Three visual displays were designed to investigate the contribution of each visual feature to illusory self tilt: a large sphere lined with dots, a cubic room lined with dots, and a furnished room with floor and ceiling. In experiment 1 the dotted room and the furnished room were tilted to various angles about the roll axis of the erect subject who set a visual line and an unseen rod to the apparent vertical. In the dotted room, settings were made either with respect to the nearest surface to the horizontal or with respect to the nearest diagonal of the room. In the furnished room, settings were made with respect to the nearest horizontal wall but not with respect to diagonals. In experiment 2 each of the three displays was rotated at constant velocity and subjects' responses were classified into four categories: illusory self tilt at a constant angle, alternating self tilt with the body becoming erect each time a surface became horizontal, continuous head-over-heels self rotation, and a feeling that the body was supine. Almost all responses were of constant tilt in the sphere. Constant and alternating tilt were the most common responses in the dotted room. In the furnished room 60% of subjects experienced full head-over-heels self rotation.