The Hong Kong Peak Tram Illusion

Abstract

Hong Kong Peak Tram Illusion is the perceived slant of buildings towards the peak away from the vertical while observers travel on the Hong Kong peak tram. We measured the perceived tilt of the buildings from true vertical (illusion size) using a rotary pitch while an identical pitch read the slope of the hill. The illusion was hypothesized to be jointly determined by at least four factors: (i) the reclining position of the observer, (ii) the frame of the tram window, (iii) the direction of motion, and (iv) additional reference cues from outside the window. Our results showed that the illusion: (i) was reduced by up to 20% when observers sat with a wedge on their back and up to 40% when they stood up. (ii) remained even when observers moved closer to the window to avoid the effect of the frame. (iii) was 20% larger when the tram was descending, as opposed to ascending. (iv) was less apparent during the day. The illusion appears to be due to integration of the above sensory information as it cannot be accounted for by each factor alone. The illusion provides a unique venue to study cross-modal sensory interaction in the nature setting.