Alignment with the horizontal plane: evidence for an orientation constraint in the perception of shape

In order to convert a 2-D image to a 3-D percept, the visual system must apply constraints that maximize the chances that the result will be unique and veridical. Previously proposed constraints include one that maximizes the symmetry of the percept, and one that maximizes its compactness (Li et al, 2009 Vision Research 49 979-991). Analysis of the 3-D percepts elicited by certain 2-D forms suggests the action of an additional constraint, favoring the alignment of the surfaces and/or axis of symmetry of the perceived object with the horizontal plane.