Antihysteresis of perceived longitudinal body axis during continuous quasi-static whole-body rotation in the earth-vertical roll plane.

Estimation of subjective whole-body tilt in stationary roll positions after rapid rotations shows hysteresis. We asked whether this phenomenon is also present during continuous quasi-static whole-body rotation and whether gravitational cues are a major contributing factor. Using a motorized turntable, 8 healthy subjects were rotated continuously about the earth-horizontal naso-occipital axis (earth-vertical roll plane) and the earth-vertical naso-occipital axis (earth-horizontal roll plane). In both planes, three full constant velocity rotations (2°/s) were completed in clockwise and counterclockwise directions (acceleration = 0.05°/s(2), velocity plateau reached after 40 s). Subjects adjusted a visual line along the perceived longitudinal body axis (pLBA) every 2 s. pLBA deviation from the longitudinal body axis was plotted as a function of whole-body roll position, and a sine function was fitted. At identical whole-body earth-vertical roll plane positions, pLBA differed depending on whether the position was reached by a rotation from upright or by passing through upside down. After the first 360° rotation, pLBA at upright whole-body position deviated significantly in the direction of rotation relative to pLBA prior to rotation initiation. This deviation remained unchanged after subsequent full rotations. In contrast, earth-horizontal roll plane rotations resulted in similar pLBA before and after each rotation cycle. We conclude that the deviation of pLBA in the direction of rotation during quasi-static earth-vertical roll plane rotations reflects static antihysteresis and might be a consequence of the known static hysteresis of ocular counterroll: a visual line that is perceived that earth-vertical is expected to be antihysteretic, if ocular torsion is hysteretic.

Hysteresis effects of the subjective visual vertical during continuous quasi-static whole-body roll rotation.

Healthy human subjects, when roll tilted in darkness, make systematic errors in estimating subjective visual vertical (SVV). Typically, roll tilt underestimation occurs at angles beyond 60 degrees (A-effect). At smaller tilt angles, overestimation may occur (E-effect). At approximately 135 degrees whole-body roll tilt, Kaptein and Van Gisbergen (2004, 2005) found an abrupt A/E transition, the exact location of which depended on the preceding rotation direction indicating hysteresis. Since this was observed using relatively fast roll velocity, it remains unclear whether the described hysteresis is dynamic or static. To clarify this uncertainty, we continuously rotated nine healthy subjects about the earth-horizontal naso-occipital axis, while they performed SVV adjustments every 2 s. Starting from the upright position, three full quasi-static constant velocity rotations (2 degrees/s) were completed in both directions (CW: clockwise; CCW: counterclockwise). SVV deviation from earth-verticality was plotted as a function of whole-body roll position. A bimodal Gaussian distribution function was fitted to SVV differences between CW and CCW rotations. A-effects (peaks at 88 degrees and 257 degrees chair position) at identical whole-body positions were larger after rotations from upside-down than after rotations from upright (average peak difference: 26 degrees). These results demonstrate static hysteresis for SVV estimation.