The ability to almost certainly perceive "vertical" is extremely important in life, such as maintaining correct posture.However, there are cases where this ability does not work properly and optical illusions occur.Therefore, the research team conducted an experiment to investigate the sense of vertical orientation in Hong Kong's Victoria Peak cable car "Peak Tram", which is known for the illusion of tilting vertical buildings.

　In the experiment, participants oriented the rod at an angle they felt was vertical and reported a 'subjective tactile vertical' (SHV).When the body was tilted in the laboratory and the SHV was reported in that state, the error was less than 1 degree.However, in experiments inside a moving cable car, unlike the results of conventional laboratory experiments, the SHV was greatly distorted, resulting in large errors.

　Additional experiments were performed to investigate the cause of this error.It was tested under various conditions, such as when the eyes were closed, when the angle of the upper body was adjusted, when the backrest was reclined and the seat was not moving, and when the tram was running on a flat road.As a result, we found that two factors, the fore-and-aft tilt of the body and the linear movement, occur when combined simultaneously.

　This finding is of great significance, as the inability to perceive the vertical correctly causes various problems in daily activities, and in some cases can lead to serious accidents.Furthermore, it will lead to technology that distorts vertical perception in real space, and is expected to be applied to entertainment and information communication technology.

Paper information:[Multisensory Research] Body Pitch Together with Translational Body Motion Biases the Subjective Haptic Vertical