Delineating the perceived ground surface from a direction constancy rule

Perceived direction is largely veridical; for example, a target in the dark is seen along its projection line from the observer (Ooi et. al., 2001). This direction constancy rule is used by the visual system to localize an object. Adherence to the direction constancy rule can also reveal how we perceive the ground surface. Namely, it predicts that when depth is underestimated due to insufficient depth cues, the perceived ground surface is slant with its far end upward. Only in violation of the direction constancy rule, is the ground surface perceived as compressed with a zero degree slant. This prediction is consistent with our earlier observation in a reduced cue (dark) condition, where we found that small light targets (1.5-7.5 m) on the floor, or 0.5 m above, were located as if they were on an implicit ground surface with its far end slant upward. For targets at 0.5 m above the floor, the apparent slant was about 15 deg (Wu et. al., 2000). Here, we provide an additional proof for the implicit representation of the ground surface in the dark, by applying the direction constancy rule to the depth foreshortening effect (e.g., Loomis & Philbeck, 1999). Our observers matched the vertical length (in depth) of a fluorescent L-shaped stimulus to its horizontal length (fixed at 30 cm). The stimulus was viewed from five different distances (2.5-7.5 m) and placed flatly either on the floor or 0.5 m above. Arguably, if the flat L-shaped stimulus is perceived as if on an implicit slant surface of about 15 deg, the aspect ratio measured should reflect this implicit slant. Indeed, this is what we found. Thus, the foreshortening effect in the dark is related to the subjective slant of the implicit ground surface, in which the direction constancy rule is a major player.