A Multi-Sensory Illusion: Hong Kong Peak Tram Illusion (II) – Subjective Vertical

Abstract

Our ability to maintain orientation to the upright is a result of multi-sensory integration, but little is known about how our subjective vertical interacts with our perceived world. We measured observers' subjective vertical in the same environment where we observed the Hong Kong Peak Tram Illusion. We tested whether the perceived tilt of a physically vertical building is a result of observers' misjudgment of their subjective vertical position. Six observers sat in the tram and held a stick on the bench to indicate their subjective vertical on both upward and downward trips. In a separate trip, they did the same measurement with eyes closed. An assistant marked the angle of the stick relative to gravity with a rotary pitch. Observers' reclining position, which matched with the slope of the mountain, was measured simultaneously by another pitch. We found observers' subjective vertical was biased away from gravitational vertical, which linearly increased with mountain slope. A blind-fold reduced this bias to half, suggesting visual information is crucial for our judgment of subjective vertical. However, observers' illusory tilt of buildings did not reflect the same trend, implying the illusion cannot be fully accounted for by observers' misjudgment of subjective vertical.