Abstract
Under normal viewing conditions, adjustments in body posture and involuntary head movements continually shift the eyes in space. Like all translations, these movements may yield depth information in the form of motion parallax, the differential motion on the retina of objects at different distances from the observer. However, studies on depth perception rarely consider the possible contribution of this cue, as the resulting changes in viewpoint appear too small to be of perceptual significance. Here, we quantified the parallax present during fixation in normally standing observers. We measured the trajectories followed by the eyes in space by means of a high-resolution head-tracking system and used an optical model of the eye to reconstruct the stimulus on the observer's retina. We show that, within several meters from the observer, relatively small changes in depth yield changes in the velocity of the retinal stimulus that are well above perceivable thresholds. Furthermore, relative velocities are little influenced by fixation distance, target eccentricity, and the precise oculomotor strategy followed by the observer to maintain fixation. These results demonstrate that the parallax available during normal head-free fixation is a reliable source of depth information, which the visual system may use in a variety of tasks.