Perceived body position and the visual horizontal.

This contribution examines the relation between the subjective visual vertical, the subjective visual horizontal, and the perceived body position of human subjects. Firmly fixed on a tiltable chair with head and torso restrained, 11 healthy subjects were rolled sideways and indicated their subjective horizontal body position. In these positions the subjects were also asked to adjust a luminous line alternately to the vertical and to the horizontal. The adjustments of the subjective horizontal body position cluster around a mean of 96.3 degrees with a remarkably broad range (SD: 19.7 degrees). In the subjective horizontal body position, the luminous line does not appear horizontal when in line with one's own spinal axis. It is set further down by 27.4 degrees on average and, therefore, perpendicular to the subjective visual vertical. This finding supports the idea that the judgement of the own body position and the judgement of the orientation of a seen object respective to gravity are based on different references. Contradictory to other investigations [23,24], is the empirical fact that the individual subjects were not able to adjust the horizontal body position with the reported accuracy (range of mean adjustments 77.5 to 117.6 degrees).