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 orie ntation with respect to gravity: visual polarity of objects with a dis tinct top and bottom, the principal vertical and horizontal lines of t he visual environment, and visual motion. Three visual displays were d esigned to investigate the contribution of each visual feature to illu sory self tilt: a large sphere lined with dots, a cubic room lined wit h dots, and a furnished room with floor and ceiling. In experiment 1 t he dotted room and the furnished room were tilted to various angles ab out the roll axis of the erect subject who set a visual line and an un seen rod to the apparent vertical. In the dotted room, settings were m ade either with respect to the nearest surface to the horizontal or wi th respect to the nearest diagonal of the room. In the furnished room, settings were made with respect to the nearest horizontal wall but no t with respect to diagonals. In experiment 2 each of the three display s was rotated at constant velocity and subjects' responses were classi fied into four categories: illusory self tilt at a constant angle, alt ernating self tilt with the body becoming erect each time a surface be came horizontal, continuous head-over-heels self rotation, and a feeli ng that the body was supine. Almost all responses were of constant til t in the sphere. Constant and alternating tilt were the most common re sponses in the dotted room. In the furnished room 60% of subjects expe rienced full head-over-heels self rotation.