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Fig. 7. Diagram illustrating the physical basis for the differences in the probability distributions of the sources of Müller-Lyer stimuli as a function of arrow tail versus arrowhead adornments. Given the presence of a conditional adornment (black) extending to the left of a starting position (vertical line), as in Upper, the physical points in 3D space corresponding to the complementary adornment (gray) would, on average, "fall off" the planar surface underlying the conditional adornment (e.g., the dashed area) sooner when moving away to the right (forming an arrow-tails figure) than when moving away to the left (forming an arrow-heads figure). Conversely, if the conditional adornment extends to the right of a starting position, as in Lower, the physical points corresponding to the complementary adornment would fall off the planar surface underlying the conditional adornment sooner when moving away to the left (forming an arrow-tails figure) than when moving away to the right (forming an arrow-heads figure).