Differential intrinsic bias of the 3-D perceptual environment and its role in shape constancy.

In a three-dimensional (3-D) environment, sensory information is projected on a 2-D retina with the consequence that the visual system needs space information for accurately reconstructing the visual world. However, the 3-D environment is not accurately represented in the brain; in particular, the perception of distances in depth is imprecise. It has been argued that the visual system has an intrinsic bias of visual space where targets located on the ground floor are perceived on an implicit elevated surface. We studied how such an intrinsic bias of visual space affects shape constancy. We found that the projected shape of a semicircle can be explained taking into account a differential implicit slant surface. The depth/width ratio, which is a measure for the shape of the stimulus, is overestimated for angular declination smaller than ~60°, while it is underestimated for larger angular declinations. Our results are important for explaining shape constancy and may be important for understanding some perceptual illusions.