Disparity-defined objects moving in depth do not elicit three-dimensional shape constancy

Abstract

Observers generally fail to recover three-dimensional shape accurately from binocular disparity. Typically, depth is overestimated at near distances and underestimated at far distances [Johnston, E. B. (1991). Systematic distortions of shape from stereopsis. Vision Research, 31, 1351-1360]. A simple prediction from this is that disparity-defined objects should appear to expand in depth when moving towards the observer, and compress in depth when moving away. However, additional information is provided when an object moves from which 3D Euclidean shape can be recovered, be this through the addition of structure from motion information [Richards, W. (1985). Structure from stereo and motion. Journal of the Optical Society of America A, 2, 343-349], or the use of non-generic strategies [Todd, J. T., & Norman, J. F. (2003). The visual perception of 3-D shape from multiple cues: Are observers capable of perceiving metric structure? Perception and Psychophysics, 65, 31-47]. Here, we investigated shape constancy for objects moving in depth. We found that to be perceived as constant in shape, objects needed to contract in depth when moving toward the observer, and expand in depth when moving away, countering the effects of incorrect distance scaling (Johnston, 1991). This is a striking example of the failure of shape constancy, but one that is predicted if observers neither accurately estimate object distance in order to recover Euclidean shape, nor are able to base their responses on a simpler processing strategy.

Numerosity and density judgments: Biases for area but not for volume

“…Figure 5. A schematic representation of the geometry of the depth added to a stimulus image by creating a disparity between a dot in the right eye (RE) and the left eye (LE), adapted from Scarfe and Hibbard (2006). The observer is at a distance Z from the point of fixation P, which subtends different angles from his two eyes.…”

Can appearance be so deceptive? Representationalism and binocular vision

“…While such examples do not provide evidence against the existence of a hierarchy of depth representations, they do question whether the lower levels of the hierarchy are readily available for solving simple perceptual tasks. Additionally, there have been a number of failures to demonstrate good performance on shape matching tasks (Scarfe and Hibbard, 2006;Todd and Norman, 2003), which might be expected to produce more veridical responses than shape judgment tasks requiring full metric structure. For example, while Glennerster et al (1996) argue that observers can match the depth of two objects if they are able to estimate the ratio of the distances to the two objects, they appear unable to match threedimensional shape, which would require a comparison between the (unscaled) width and disparity of the two objects.…”

Large continuous perspective transformations are necessary and sufficient for accurate perception of metric shape

“…On the other hand, metric shape can be measured by curvedness (Koenderink, 1990) or, alternatively, the ratio of an object's width to its depth. As has been reviewed by Todd, Tittle, and Norman (1995) and shown by Perotti et al (1998), among many others (e.g., Brenner & van Damme, 1999;Lappin & Ahlström, 1994;Scarfe & Hibbard, 2006;Tittle, Todd, Perotti, & Norman, 1995;Todd & Norman, 1991), observers do not appear to be able to perceive metric shape, or the relative depth of objects, very accurately. Perotti et al (1998), in particular, found that observers were well able to judge the shape characteristic but that judgments of curvedness were biased and highly variable.…”