Figure 1.

Schematic top-down view illustrating how uncertainty in retinal velocity propagates asymmetrically to motion trajectories in the world. (A) Two orthogonal motion vectors with the same speed in the world (motion in depth in green and lateral motion in orange) project to different angular speeds on the retina. (B) A fixed retinal speed projects to a longer vector for motion in depth than for lateral motion. The same geometry applies to the transformation of uncertainty. (C) This difference is much reduced at near viewing distances. (D) This relationship can invert for trajectories that occur off of the midsagittal plane. (E) Illustration of how the tangent line of a circle determines the vector direction with the minimum length for a given angle and distance. Note that when motion is directly toward either eye, this will project to zero retinal velocity in one eye (ignoring looming/optical expansion) and nonzero velocity in the other.