The purpose was to examine the effect of instructions about body position on vertical orientation. In order to emphasize the effects, the surface of support was tilted 15 degrees, and subjects were tilted 5 degrees to the counter side of the oblique plane, and asked to adjust a luminous line to a vertical position. Moreover, subjects were instructed that their body was either upright (False Instructed Condition) or not upright (Control Condition). The results indicated that in the Control Condition, the apparent vertical was shifted to the counter side of their body (E-effect), and that in the False Instructed Condition, the apparent vertical was shifted in the opposite direction. These results were discussed in terms of the over-compensation and under-compensation of the cognitive body position. The resuls suggested that the cognitive body position was an important factor in orientation.
