Minimization of modal contours: An instance of an evolutionary internalized geometric regularity?

The stratification in depth of chromatically homogeneous overlapping figures depends on a minimization rule which assigns the status of being “in front” to the figure that requires the formation of shorter modal contours. This rule has been proven valid also in birds, whose visual neuroanatomy is radically different from that of other mammals, thus suggesting an example of evolutionary convergence toward a perceptual universal. [Shepard].