Motion-induced spatial shifts and motion-based contrast enhancement are linked.

Two lines of research on the effects of motion on spatial vision have developed largely independently. Motion induces a forward shift in the apparent spatial location of moving pattern (Ramachandran & Anstis, 1990; De Valois & De Valois, 1991). Also, moving pattern appears less blurred than might be expected from temporal summation or visual persistence (Burr, 1980). This might be explained by spatial summation along the motion trajectory (Nishida et al., 2007). Here we describe evidence that the two phenomena may be connected. We constructed a tripartite stimulus consisting of an array of square binary blocks (2 pixel; 0.039 × 0.039 deg) in a fixed window on a mean luminance grey background. The central rows translated by one or more blocks while the rows in the upper and lower third of the array randomly updated from frame to frame. Thus each frame appeared as an undifferentiated random block array and the temporal properties of the translating and updating blocks were identical. Participants reported whether the moving pattern or the flanking updating regions appeared to have the higher contrast. The contrast of the randomly updating pattern was varied by a staircase procedure to determine the matching contrast. We found the apparent contrast of the moving pattern was generally higher than the updating pattern. The effect was greater for slower speeds and for lower contrasts. We also measured the apparent spatial shift of the translating section relative to the updating section using a staircase method while varying the drifting section position. The spatial shift was greater for lower contrasts. Since the same participants contributed to both experiments, we used repeated measures correlation to compare scores over participants and conditions. We found a high correlation (0.69, p<0.001) between the spatial shift as a proportion of the pattern displacement and the apparent contrast enhancement.