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Our ability to perceive what is truly vertical is crucial. Without it, we would struggle to perform simple tasks such as holding a cup of coffee without spilling it and maintaining appropriate body posture.

Now, an international team of scientists from Japan, Canada and Germany have discovered a new situation from our everyday environment where this ability is compromised – when our body pitches (body pitch) and moves at the same time (body motion).

The scientists’ observations, which were published in the journal Multisensory Research, were based on real-world scenarios, revealing factors long overlooked by past laboratory studies.

„Unlike typical studies of this kind, which happen in well-controlled laboratories, our team traveled to Hong Kong to conduct field research on the world-renowned Peak Tram,” said Dr Chia-huei Tseng, who led the research and is associate professor at Tohoku University’s Research Institute of Electrical Communication (RIEC). „We performed experiments on hundreds of commuters making their way up Hong Kong’s highest hill, Victoria Peak.”

Although body pitch and body motion have been studied in previous studies, the phenomena were isolated. This study combined both aspects to determine a new situation where verticality becomes distorted.

The team enlisted the help of an architect to build a device that participants could hold to indicate their perceived verticality (i.e., subjective haptic vertical or SHV).

In most laboratory setting, humans are capable of perceiving SHV with an error of less than 1 degree. However, when measurements were taken on the moving Peak Tram, a misjudgment of 10 degrees was observed, something never previously reported.

To investigate the possible sources of errors, the researchers devised additional experiments. First, they had participants close their eyes to rule out visual influences. Then, to eliminate the effects of the vestibular system – the sensory system that creates a sense of balance and spatial orientation – they inserted a back wedge between the participants and their chairs. None of these alternations had much bearing on diminishing the SHV error, ruling out visual and vestibular systems as the primary cause.

Additionally, they ran their tests on Hong Kong’s Ding Ding Tram, which runs at similar speeds to the Peak Tram but on a relatively flat surface. In both situations, no bias was detected.

Psychology Professor Kenzo Sakurai from Tohoku Gakuin University, an expert on self-motion and co-author of the study, points out what a major discovery their findings are. „The failure to accurately sense our body orientation relative to gravity may result in inappropriate movements or falls, as well as impair daily functions such as walking, climbing a staircase, and carrying a tray. In specialized tasks such as driving a car or piloting a plane, this failure could be disastrous.”

Dr Tseng thinks that their study demonstrates a great example of scientific inquiry generalizing the actual world we live in. „On top of its pure scientific contribution, this study challenges the misconception that research is limited to scientific cubicles indoors. In this case, the natural environment provided a well-designed puzzle for scientists to solve.”

With international travel restriction gradually lifting, the paper’s authors encourage those to take a ride on Hong Kong’s Peak Tram to feel this fascinating perceptual phenomenon. But maybe wait until after the ride to get your coffee.

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