Perceived distance, shape and size

Abstract

If distance, shape and size are judged independently from the retinal and extra-retinal information at hand, different kinds of information can be expected to dominate each judgement, so that errors in one judgement need not be consistent with errors in other judgements. In order to evaluate how independent these three judgments are, we examined how adding information that improves one judgement influences the others. Subjects adjusted the size and the global shape of a computer-simulated ellipsoid to match a tennis ball. They then indicated manually where they judged the simulated ball to be. Adding information about distance improved the three judgements in a consistent manner, demonstrating that a considerable part of the errors in all three judgements were due to misestimating the distance. Adding information about shape that is independent of distance improved subjects' judgements of shape, but did not influence the set size or the manually indicated distance. Thus, subjects ignored conflicts between the cues when judging the shape, rather than using the conflicts to improve their estimate of the ellipsoid's distance. We conclude that the judgements are quite independent, in the sense that no attempt is made to attain consistency, but that they do rely on some common measures, such as that of distance.

Merging the senses into a robust percept

“…For example, Johnston, Cumming and Landy [17] found that people form a weighted average of motion and disparity signals when asked to report an object's shape. The same is true for texture and disparity signals to depth [18], for the visual perception of slant [19], for the judgment of texture-defined edges [20], and for the estimate of distance [21]. More importantly, Young et al [18], among others, showed that the weights change in the predicted direction as signal reliability is manipulated.…”

Grasping reveals visual misjudgements of shape

“…For the perceived shape of the cylinders, the errors that we expect to see are compressions or expansions of depth extents (Brenner and Van Damme 1999;Johnston 1991;Todd et al 1995). The visual information about the shape and orientation of the cylinders is primarily contained in the contours of the top and bottom edges, stereopsis and motion parallax.…”

“…We suspected that the systematic deviations were due partly to a deformation of perceived shape. If so, we could expect an interaction between the distance of the cylinder from the observer and its shape, because the perceived depth (along the line of sight) is a non-linear function of distance (Brenner and Van Damme 1999;Johnston 1991;Todd et al 1995). However, the systematic deviations did not depend on either the cylinder's aspect ratio or its distance from the observer.…”

“…Vision is a rich source of information about the shape of an object, but it does not always lead to a veridical percept (Todd et al 1995;Brenner and Van Damme 1999;Johnston 1991), so visual errors can influence the way that people grasp objects (Hibbard and Bradshaw 2003;Watt and Bradshaw 2003). Similarly, misperceiving an object's size can influence grasping (Franz et al 2003), but the evidence is still under debate (Aglioti et al 1995;Smeets and Brenner 1999).…”

Direct perception of global invariants is not a fruitful notion

“…Separate independent processing for different judgments can result in substantial conf1icts between them (Abrams & Landgraf 1990;Glennerster et al 1996;Mack et al 1985). Nevertheless, the separation seems be so complete that we even fail to notice conflicts between attributes when the conflicts themselves could give us valuable additional information (Brenner & Damme 1999;Brenner & Landy 1999). The main reason for our judgments normally being approximately consistent is presumably the consistency in the world "outside," in what S&B call the "global array.…”