Does depth-cue combination yield identical biases in perception and grasping?

Abstract

Grasping critically depends on stereo information. We previously found that binocular disparities yield a distorted visual space, in which objects close to the observer are grasped and perceived as if they were more elongated than farther objects. Such lack of shape constancy results from the inaccurate estimate of the viewing distance, which affects the estimated depth-to-width ratio of an object. This is because (1) depth from binocular disparities scales with the square of the distance and (2) width from retinal size scales linearly with distance. Conversely, depth from monocular cues (i.e., motion and texture gradients) scales linearly with distance, hence the overall shape from these signals should not be affected by errors in egocentric estimates of object location. We therefore reasoned that adding these cues to stereo information should improve shape constancy. Contrary to expectations, in four experiments we found that stereo-texture and stereo-motion stimuli appeared even more distorted than stereo stimuli. More remarkably, results revealed that grasping execution showed identical biases, which were corrected only late in the movement through online control mechanisms, but only if both grasping digits could be visually guided on their respective contact locations. On the contrary, when the index was occluded by the object, biases in shape estimation continued to affect grasping execution until movement completion. Moreover, while the initial part of the grasp showed evidence of collision avoidance, a control experiment suggested that the above biases could have emerged as early as at movement planning, consistent with previous evidence.

Explicit and implicit depth-cue integration: evidence of systematic biases with real objects

“…First, the same prediction method was successfully applied independent of the level of conscious processing of depth information, as the model predicted both the perceptual and the motor results. Second, the symmetry of the results from this study with the results of the previous experiments in VR (Campagnoli et al, 2019), demonstrate that our combination rule well describes how depth signals are translated into estimates in the visual system, and is independent of the medium (virtual versus real objects) conveying depth information to the brain.…”

“…If, on the other hand, superadditivity relates directly to the readout of the internal depth estimates, we should expect it to equally affect non-explicit tasks such as reaching towards a 3D object with the hand to pick it up. Converging evidence in support of the latter possibility has been documented in previous VR investigations (Foster et al, 2011; Campagnoli & Domini, 2016, 2019). However, the artificial setups used in those studies may have introduced yet another potential source of explicit bias in the responses, because participants immersed in a virtual scene tend to supervise their behavior and exert top-down control over their own actions (Chessa et al, 2019).…”

“…IC is therefore sufficient to explain the phenomenon of superadditivitydepth estimates increase with the amount of depth informationfound both in this study and in VR (Campagnoli & Domini, 2019). Moreover, it can do so without the necessity of further ad-hoc assumptions such as prior-to-flatness (Di Luca et al, 2010;Domini and Caudek, 2003, 2009Domini et al, 2006;Kemp et al, 2018).…”

“…In previous studies we showed that, when sources of depth information (cues) are added to the virtual rendering of a 3D shape, the perceived magnitude of a given 3D property of that shape (e.g., its depth, slant or curvature) increases. For instance, when viewing a cylindrical surface from a close distance (50 cm), its overall depth is consistently underestimated when the shape is only specified by binocular disparity, compared to when specified by a mixture of disparity signals and texture gradient (Campagnoli & Domini, 2019).…”

“…Under these visual conditions, it is possible to study a true disparity-only (that is, motion has strength k M = 0) condition and test whether adding motion to the display (k M ≠ 0) yields larger magnitudes of perceived depth. This was done in our past study (Campagnoli and Domini, 2019). Consider now a real object viewed at a distance yielding sizable focus cues, like a reachable distance.…”