The effect of preceding tilt on the perceived vertical. Hysteresis in perception of the vertical.

The subjective vertical (SV) was measured at various positions of roll-tilt (R). Positions R90-right, R 45-right, R0, R45-left, R90-left were reached either in clockwise sequence (i.e. starting with R 90-left) or in counterclockwise sequence (starting with R90-right); position R135-right was attained from R 110-right or R 160-right. The SV was affected by the preceding tilt (hysteresis). Clockwise position sequence produced, for example, a counterclockwise SV deviation (from the medium value at the obtained position) as though indicating a more advanced tilt position (Fig. 5). It is concluded that the aftereffects and hysteresis differences in perception of position and of SV depend on adaptational processes in the somatoreception system which interacts with the labyrinth posture-receptors.