Differential motion hyperacuity under conditions of common image motion

Sensitivity to horizontal shearing motion in random dots was measured as a function of common image motion amplitude. Without common image motion, thresholds for differential motion are comparable or better than vernier acuity. Above about 2 arc min of common image motion there is a proportionate increase in motion parallax thresholds such that for 20 arc min of common image motion, differential motion thresholds have risen by an order of magnitude. This differs from a comparable lack of threshold elevation for vernier acuity targets under similar conditions of movement (Westheimer and McKee, 1975). By varying movement duration, it can be shown that common image motion amplitude rather than common image velocity is the primary determinant of the effect. Furthermore this degradation of performance is not affected by changes in random dot size. Under favorable circumstances the phenomenon can also be seen with differentially moving vernier lines. The effect shows directional selectivity such that common motion directions closest to the horizontal leads to the greatest interference. The results add weight to the view that differential motion hyperacuity is mediated by a separate mechanism than differential position hyperacuity.