Only a few perceptual studies on motion-in-depth have used aIOVD stimuli [8,35]. Most other studies have used dIOVD stimuli, but the specific design of these stimuli have varied from standard random-dot stereograms to modified stereograms in which lines of dots alternated with uniform grey bands in counterphase in the two eyes [7] or sparse plaids of drifting Gabors [31]. Shioiri et al. [7] claimed that motion-in-depth only can reliably be discriminated when there is opposing motion in both eyes. One way to achieve this is to present to each eye two random-dot stereograms, one located vertically above the other, where corresponding pairs of dots in the two eyes move in opposite directions, e.g., dots in the upper stereogram move towards the participant while those in the lower stereogram move away, and vice versa. These differences in the stimuli might account for some of the variability in the findings regarding the IOVD mechanism. One converging result seems to be that the IOVD mechanism is more sensitive to higher temporal frequencies and velocities while the CD mechanism prefers lower temporal frequencies and velocities [34,35,37].