An interaction between body orientation and gravity produces a novel illusion in visual distance perception

Does perceived distance depend on gravity? The moon illusion, in which the moon looks smaller when viewed overhead, suggests that it might, although so many factors are involved in this complex illusion. Prior research assessing perceived distance had blindfolded observers walk to a remembered location. However, this precludes altering an observer’s orientation relative to gravity. Here, a crossmodal comparison of a visual line and a standard-length tactile rod provided a novel means for assessing the perception of distance by exploiting size constancy — a shorter perceptual visual length (measured in rod lengths) corresponds to a shorter perceived distance from the observer. Experiments were conducted in a fully decorated room that was oriented at 90° with respect to gravity (the York University tumbled room) and in an identical normally oriented control room. Observers maintained an upright orientation with respect to the rooms. Observers judged the length of a variable visual line, projected with a laser and a pair of galvanometers, and varied by a QUEST procedure, relative to a fixed length tactile rod. The length of the visual line matched to the reference length was significantly longer — compatible with being perceived as further away — in the tumbled room than in the control room. A separate experiment failed to replicate this change in distance perception when the observer’s orientation relative to gravity was changed outside the tumbled room, suggesting that the effect is due to a conflict between visually- and gravitationally-defined reference frames.