VISUALLY PERCEIVED VERTICAL AND VISUALLY PERCEIVED HORIZONTAL ARE NOTORTHOGONAL

The authors examined the difference in errors made by eight subjects i n setting a bar of light in an otherwise darkened room to either visua lly perceived vertical (VPV) or visually perceived horizontal (VPH) du ring maintained roll-tilted positions around the naso-occipital axis. Two viewing distances were examined, 25 and 60 cm. Subjects were teste d at roll-tilt angles of 10 degrees intervals from upright to body hor izontal (both left ear down (LED) and right ear down (RED)) in a rando mized fashion. Settings were made only after a 1 min delay at each til t angle to allow for decay of the semicircular canal signal. Chair rot ation speed was 2 degrees/s with subjects being re-tested using 1/2 de grees/s (at 25 cm) to determine the effect of rotation speed. Average errors for vertical versus horizontal were significantly different fro m each other (P < 0.01) at both the 25 and 60 cm viewing distances. Th e errors follow a complex function, with VPH showing smaller errors th an VPV for large roll-tilts, while the opposite was true for medium-si zed roll-tilts. This was true at both chair velocities. That is, VPV a nd VPH are not orthogonal to one another under the conditions examined . There are large differences between individuals but each individual showed a repeatable pattern. The average extent of non-orthogonality w as found to be as high as 7 degrees at some large roll-tilt angles. Th ese findings raise questions about the appropriateness of comparing th e results of studies using the different tasks VPV and VPH. Factors th at might contribute to this effect are discussed, including somatosens ory input and ocular counterrolling (OCR). (C) 1998 Elsevier Science L td. All rights reserved.