Figure 1.

Hypothetical asymmetries in sensitivity to motion direction in a cartesian and polar reference frame. Arrow length indicates predicted sensitivity. Left to right: Cardinal (green) and oblique (purple) directions in a cartesian reference frame. The cardinal arrows are longer to depict a predicted cartesian cardinal advantage. Cardinal (brown) and oblique (teal) directions in a polar reference frame. The longer arrows depict a predicted polar cardinal advantage. Radial (blue) and tangential (red) directions in a polar reference frame. The longer blue arrows depict a predicted radial advantage. Combined advantages (gray) in both reference frames assuming an equal weight of cartesian cardinal, polar cardinal and radial advantages. The largest asymmetry effects are predicted along the primary meridians of the visual field (shown as difference in grayscale value). In principle, an unequal combination of these three factors might be observed.