Do We See One Percept Or Two? A Response to Morales et al. (2020)

Morales et al. show that a front-facing ellipse is more quickly identified amongst front-facing circles than rotated circles, leading them to conclude that vision has a dual character: a rotated circle is perceived both as a perspectival 2D ellipse and as a 3D circle. We have two concerns. First, are they testing perceived shape? Second, is this a failure of depth constancy?

5. Perception of a 3-D world

The ability to perceive the 3-D world has often been regarded as a task that poses particular problems for the visual system. However, ‘Perception of a 3-D world’ argues that we are particularly fortunate because there are multiple sources of information to tell us about the different aspects of the 3-D structure of objects. It discusses three of these sources of information—perspective, occlusion, and shading—and then explains motion parallax, optic flow, binocular stereopsis, eye vergence and depth constancy, vertical disparities and differential perspective, and primary and secondary depth cues. The effectiveness of these different sources of 3-D information is considered along with how they are all brought together.

Stereoscopic depth constancy

Depth constancy is the ability to perceive a fixed depth interval in the world as constant despite changes in viewing distance and the spatial scale of depth variation. It is well known that the spatial frequency of depth variation has a large effect on threshold. In the first experiment, we determined that the visual system compensates for this differential sensitivity when the change in disparity is suprathreshold, thereby attaining constancy similar to contrast constancy in the luminance domain. In a second experiment, we examined the ability to perceive constant depth when the spatial frequency and viewing distance both changed. To attain constancy in this situation, the visual system has to estimate distance. We investigated this ability when vergence, accommodation and vertical disparity are all presented accurately and therefore provided veridical information about viewing distance. We found that constancy is nearly complete across changes in viewing distance. Depth constancy is most complete when the scale of the depth relief is constant in the world rather than when it is constant in angular units at the retina. These results bear on the efficacy of algorithms for creating stereo content. This article is part of the themed issue ‘Vision in our three-dimensional world’.

Lack of depth constancy for grasping movements in both virtual and real environments

Recent studies on visuomotor processes using virtual setups have suggested that actions are affected by similar biases as perceptual tasks. In particular, a strong lack of depth constancy is revealed, resembling biases in perceptual estimates of relative depth. With this study we aim to understand whether these findings are mostly caused by a lack of metric accuracy of the visuomotor system or by the limited cues provided by the use of virtual reality. We addressed this issue by comparing grasping movements towards a spherical object located at four distances (420, 450, 480, and 510 mm) performed in three conditions: 1) virtual, in which the target was a virtual object defined by binocular cues, 2) glow-in-the-dark, in which the object was painted with luminous paint but no other cue was provided, and 3) full-cue, in which the movement was performed with the lights on and all the environmental information was available. Results revealed a striking effect of object distance on grip aperture equally in all three conditions. Specifically, grip aperture gradually decreased with increase in object distance, proving a consistent lack of depth constancy. These findings clearly demonstrate that systematic biases in grasping actions are not induced by the use of virtual environments and that action and perception may involve the same visual information, which does not engage a metric reconstruction of the scene.