Stereovision for action reflects our perceptual experience of distance and depth.

Abstract

Binocular vision is widely recognized as the most reliable source of 3D information within the peripersonal space, where grasping takes place. Since grasping is normally successful, it is often assumed that stereovision for action is accurate. This claim contradicts psychophysical studies showing that observers cannot estimate the 3D properties of an object veridically from binocular information. In two experiments, we compared a front-to-back grasp with a perceptual depth estimation task and found that in both conditions participants consistently relied on the same distorted 3D representation. The subjects experienced (a) compression of egocentric distances: objects looked closer to each other along the z-axis than they were, and (b) underconstancy of relative depth: closer objects looked deeper than farther objects. These biases, which stem from the same mechanism, varied in magnitude across observers, but they equally affected the perceptual and grasping task of each subject. In a third experiment, we found that the visuomotor system compensates for these systematic errors, which are present at planning, through online corrections allowed by visual and haptic feedback of the hand. Furthermore, we hypothesized that the two phenomena would give rise to estimates of the same depth interval that are geometrically inconsistent. Indeed, in a fourth experiment, we show that the landing positions of the grasping digits differ systematically depending on whether they result from absolute distance estimates or relative depth estimates, even when the targeted spatial locations are identical.