Fumiyuki Goto, Tomoko Tsutumi, Hironari Kobayashi, Akira Saito, Jin Kanzaki, Takanobu Kunihiro and Kaoru Ogawa
Objective: The aim of this study was to investigate changes of the subjective visual vertical (SVV) in healthy, male subjects during prolonged lateral body tilt both in light and dark conditions.
Methods: Static SVVs were obtained in a sitting and in lying down positions. Following the measurement in the sitting position, the subject was instructed to lay down and remain that way for the next set of recordings. SVV was measured immediately and repeated another 8 times at 15-minute intervals. Finally, the subject returned to the sitting position, and SVV was measured immediately and 15 minutes later, both in light and dark conditions.
Results: Tilt of the SVV shifted in the same direction as the head tilt soon after the subject assumed the lateral body tilts position from the sitting position. This SVV deviation gradually increased, eventually reaching a plateau. Although subjects reached the plateau at various rates, the mean latency to reach the plateau were approximately 30 minutes. This phenomenon was reproduced with intra-subject repeated testing. SVV deviation increased more rapidly in the light than in the dark.
Conclusion: Comparison of SVVs under dark and light conditions informs us that the phenomenon, which is the subjective recognition of contralateral tilt of the true vertical, could function in the maintenance of one’s head in the vertical position. We propose this visual effect is one of important inputs to stabilize visual vertical. Therefore visual input would be essential for maintaining one’s upright position on earth.