Effects of the gravitational vertical on the visual perception of reversible figures.

We examined the effects of the gravitational vertical on the perception of reversible figures. Each reversible figure had two interpretations depending on whether it was viewed vertically in one orientation or upside down. When it was presented horizontally, the two interpretations alternated. Subjects were required to indicate which interpretation was perceived after briefly viewing the horizontal presentation on a head-mounted display. When the subject was upright, each interpretation occurred by chance. When the subject was lying on one side, the horizontal figure was generally perceived as if it was presented vertically with the side down. The results suggest that the perception of reversible figures is influenced by the gravitational vertical, which might be reconstructed by multimodal integration of vestibular, proprioceptive, and tactile inputs.