We report a multi-sensory illusion observed in a tram climbing up the Peak of Hong Kong. The skyscrapers nearby appear to spectacularly fall towards the Peak at night as the tram ascends or descends the slope. We investigated the contribution to this illusion from the vestibular system.

Five observers sat in a row of a compartment and marked the perceived tilt of the skyscrapers with a rotary pitch (which reads the angle relative to gravity). The slope of the mountain was measured simultaneously with the observers’ report by an assistant with an identical pitch. In two separate trips, observers sat with a wedge of 18.7 degrees at their back or stood up thereby changing the self-position information from the otolith system. The skyscrapers appeared to tilt away by an average of 12 degrees during normal seating in the tram. This subjective tilt illusion was reduced by an average of 20% when the wedge was inserted and by more than 50% when observers stood up.

Our results show that self-position information profoundly modulates the perceived tilt of buildings observed from a moving tram. The noticeable individual differences we observed also suggested the weighting from each sensory modality may differ for different observers.