ABSTRACT
The subjective vertical (SV, the angle ß between a subjective vertical line and the medial plane of the head) exhibits a distinct time-dependency, when measured over several minutes. To show this, data from different experiments are presented. Subjects were tested in different head-to-trunk-pitch and head-to-trunk-roll postures. They were tilted to the side from either a standing or a prone position, into positions of different angles of clockwise or counterclockwise rotation (i.e., lateral tilt). Other variables were the magnitude and time of preceding tilt; interfering stimuli, such as a background field of parallel lines placed to the left or right of the luminous reference line; and, in experiments carried out under water, the influence of the somatoreceptor system. In addition, the auditory subjective vertical was also measured, by estimating when the direction of a sound was vertical.
Under all experimental conditions the amount of change of the subjective vertical in time depends on the degree of lateral tilt. With a tilt of 30° and 45°, there is a significant increase of ß in time, whereas a significant decrease of ໟ occurs at tilts of 90°, 120°, 135° and 150° (Figs. 14.1, 14.2). The effect of the background field and the hysteresis effect (i.e., the ໟ-difference between clockwise and counterclockwise attainment of position) are superimposed on these time dependent changes. Findings indicate that the time which is spent in the pre-tilt position magnifies the hysteresis effect of the subjective vertical (Fig. 14.3). The comparison between the visual and the auditory subjective vertical reveals similarities. The time course of the auditory subjective vertical shows a significant increase at a body tilt of 30°, as does the corresponding curve of the visual subjective vertical (Fig. 14.4).
The results are discussed on the basis of the hypothesis that the influence of the statolith organs decreases with increasing body tilt. This amplifies the effect of 242interfering stimuli (e.g., somatoreceptor input and optical reference). Adaptation of the somatoreceptors is assumed to account for the temporal change of the subjective vertical. Furthermore, it is supposed that the aftereffects and hysteresis differences in the perception of position and of the subjective vertical also depend on the adapting processes in the somatoreceptor system which interact with the labyrinthine posture-receptors. But the possibility that a central mechanism processing statolithic information is also involved cannot be ruled out.
