For this reason, we decided not to manipulate the changes in image size that occur when observers move backwards and forwards. However, since both retinal image size (e.g., Sousa et al., 2010, 2011a, 2011b) and change in image size (e.g., Brenner, van den Berg, & van Damme, 1996) are known to influence the perceived distance, it would be difficult to distinguish between such direct influences and the effects of changing size in the context of motion parallax. The way in which an object's retinal image changes when the observer moves toward or away from an object might also provide information about the object's distance. We only manipulated changes that occur when the observer moves laterally or vertically. In the present study we independently manipulated the three above-mentioned cues. Beside relying on such retinal cues to judge objects' distances from the change in vantage point, observers might also register changes in the object's position relative to themselves from the extent to which they have to turn their head and eyes in order to keep looking at the object. Although changes in an object's orientation with respect to the line of sight and changes in two objects' relative retinal positions are equivalent in terms of the relative motion involved, only differing in whether the comparison is made between parts of the same object or between separate objects, they might be obtained differently (through changing orientation or changing relative position respectively). Consequently, the retinal images of objects at different distances move relative to each other. The extent to which the view changes depends on the object's distance, as well as on how much the observer moved. This sometimes even means that they see different parts of an object at different times. As observers move around, they perceive objects from different vantage points. We use the term motion parallax to refer to any information about structures' distances that could be obtained by an observer changing his or her viewing position. A moving observer can also consider information from motion parallax (e.g., J. Static observers consider binocular disparities (e.g., Rogers & Graham, 1982 Johnston, Cumming, & Landy, 1994 Bradshaw, Parton, & Eagle, 1998 Bradshaw, Parton, & Glennerster, 2000 Sousa, Brenner, & Smeets, 2010, 2011a), the object's retinal image size (e.g., Gillam, 1995 McIntosh & Lashley, 2008 Lugtigheid & Welchman, 2010 Sousa et al., 2010, 2011a, 2011b Sousa, Smeets, & Brenner, 2012a, 2012b), accommodation (e.g., Wallach & Floor, 1971 Leibowitz & Moore, 1966), and vergence (e.g., Gogel, 1961, 1977 Brenner & van Damme, 1998). It has been shown that people consider various sources of information when judging objects' distances. We live in a three-dimensional (3-D) world, so most of the tasks that people perform in daily life require judgments of distance as well as of elevation and azimuth. 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. Relative retinal image motion has the clearest effect. The results show that motion parallax cues have a detectable influence on our judgments, even when the head only moves a few millimeters. 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. 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. The position and the size of the target changed across trials. To answer this question we asked subjects to indicate the position of a virtual target with their unseen finger. We explore here whether these motion parallax cues are used when we think we are standing still. 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. It is well known that when we intentionally make large head movements, the resulting motion parallax helps us judge objects' distances.
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