Frontal posture of the neck and trunk in sitting influences subjective visual vertical

[Purpose] To investigate whether postures of the head and trunk in the frontal plane affect the subjective visual vertical (SVV). [Subjects] Nineteen healthy adults were enrolled. [Methods] We measured SVV under five sitting conditions: upright, inclining the head, inclining the trunk, inclining the trunk with the head upright, inclining both the head and trunk. Subjects were instructed to stop a moving line on a PC monitor when they thought that the line was vertical. The values of SVV, and the differences in degree between the true vertical and subjective vertical, were compared among the five conditions. [Results] The absolute values of SVV in the upright and inclining the trunk with the head upright conditions were significantly smaller than those in the three other conditions. The values of SVV in the inclining the trunk condition was significantly smaller than those in the inclining the head or in inclining the head and trunk conditions. [Conclusions] The effects of posture on SVV measurement must be considered.