How various aspects of motion parallax influence distance judgments, even when we think we are standing still

It is well known that when we intentionally make large head movements, the resulting motion parallax helps us judge objects' distances. The information about distance could be obtained in various ways: from the changes in the object's position with respect to ourselves, from the changes in its orientation relative to the line of sight, and from the relative retinal motion between the target's image and that of the background. We explore here whether these motion parallax cues are used when we think we are standing still. To answer this question we asked subjects to indicate the position of a virtual target with their unseen finger. The position and the size of the target changed across trials. There were pairs of trials in which the same target was presented at the same location, except that one or more of the three motion parallax cues indicated that the target was either 10 cm closer or 10 cm farther away than the 'true' distance. Any systematic difference between the positions indicated for the closer and further targets of such pairs indicates that the cues in question influence subjects' judgments. The results show that motion parallax cues have a detectable influence on our judgments, even when the head only moves a few millimeters. Relative retinal image motion has the clearest effect. Subjects did not move their head differently when we presented the targets to only one eye in order to increase the benefit of considering motion parallax.