Vergence modulation as a cue to movement in depth.

It is known that the vergence state of the eyes can serve as a cue to distance. However, it has been claimed that changes in vergence induced by modulations of the binocular disparity of a random-dot display do not create a sensation of motion in depth. The present experiment tests the hypothesis that modulation of binocular disparity in a random-dot display does not create a sensation of motion in depth because, in such a display, looming of the image that normally accompanies motion in depth is absent. The image of a radial pattern remains self-similar when it moves in depth, so the absence of looming in such an image should not inhibit the effects of modulation of disparity. Subjects tracked with unseen hand the perceived motion in depth created by modulations of the disparity of a display of random spots, a single spot, and a radial pattern. As previously reported, the random-spot display produced almost no motion in depth. However, the single spot and the radial pattern produced motion in depth. It is concluded that modulations of vergence and/or of absolute disparity can create a sensation of motion in depth when the effects of looming are weakened or removed. However, strong sensations of motion in depth of isolated objects require the presence of looming.