Visually perceived vertical and visually perceived horizontal are not orthogonal

AbstractThe authors examined the difference in errors made by eight subjects in setting a bar of light in an otherwise darkened room to either visually perceived vertical (VPV) or visually perceived horizontal (VPH) during maintained roll-tilted positions around the naso-occipital axis. Two viewing distances were examined, 25 and 60 cm. Subjects were tested at roll-tilt angles of 10° intervals from upright to body horizontal (both left ear down (LED) and right ear down (RED)) in a randomized fashion. Settings were made only after a 1 min delay at each tilt angle to allow for decay of the semicircular canal signal. Chair rotation speed was 2°/s with subjects being re-tested using 1/2°/s (at 25 cm) to determine the effect of rotation speed. Average errors for vertical versus horizontal were significantly different from each other (P