Unraveling the Hong Kong peak tram illusion

Abstract

The Hong Kong Peak Tram Illusion is the perception of an exaggerated tilt to vertical buildings while ascending or descending Victoria Peak on the Peak Tram in Hong Kong. We have previously reported this effect to be multisensory in origin and based upon the confluence of input from positional, visual, and passive motion cues. We elaborate upon earlier investigations by introducing new apparatus to isolate contributions to the effect from three sensory modalities: vision, proprioception, and the vestibular system. Using a new tilt-matching task, four observers chose the tilt line which best matched their perceived tilt while the objective slope was simultaneously measured. The use of under-foot, under-buttocks, and behind-back wedges to compensate for body slant and return the torso to true vertical weakened the illusion, but only when used in combination, suggesting that proprioceptive and vestibular systems are co-dependent in their contribution to the effect. Perception of the illusion is further lessened by standing, which eliminates bodily cues and requires active balancing. Removal of some visual cues, such as the rod-and-frame effect, by use of a viewing box weakened the illusion independently of objective slope. Individual differences in observer data suggest variations in the way sensory information is integrated. Further investigation into causes of the effect will involve elimination of confounding visual factors, comparison of observations of particular stimuli, and may be facilitated by an offsite simulation of the illusion. The Hong Kong Peak Tram Illusion is a real-life phenomenon which strikingly reveals how the labyrinthine complexity of sensory integration may lead to a dramatic misperception of the world when conflicting cues interact.