It is widely believed that excessive binocular disparity produces diplopia without clear depth perception. However, recent studies have reported that diplopic images with a very large disparity appear clearly in depth when they move. It is currently unclear whether this facilitation of stereoscopic depth caused by target movement in diplopic images requires the involvement of both eyes. A monocular target stimulating the nasal or temporal retina of either eye appears in depth as if it has uncrossed or crossed disparity, respectively (i.e. monoptic depth). In the present study we examined the dynamic properties of monoptic depth and binocular stereopsis. Two small circular targets were presented 5 degrees above and below a fixation point and oscillated horizontally in counter phase. With binocular stereopsis, disparities with the same magnitude and opposite polarity were applied to the two targets. With monoptic depth, targets were removed for either eye. The results revealed that target motion facilitated binocular stereopsis but not monoptic depth. These findings suggest that corresponding target images stimulating both eyes are necessary for a depth of large magnitude to be perceived in motion, in spite of diplopia.
