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.

Subjects misjudge distances considerably when forced to rely on extra-retinal information. Nevertheless, they can reproducibly set a target to the same distance as a reference, or to double or half that distance, even when they have to look back and forth between them because they are prevented from seeing one when looking at the other. Our explanation for this apparent discrepancy is that people have access to reasonably accurate extra-retinal information on changes in ocular convergence, but can only use this information to judge distances if they had reliable information about the orientation of the eyes before the convergence changed.

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We examined the ability of human observers to discriminate between different 3-D quadratic surfaces defined by motion, and with head position fed back to the stimulus to provide an up-todate dynamical perspective view. We tested whether 3-D shape or 3-D curvature would affect discrimination performance. It appeared that discrimination of 3-D quadratic shape clearly depended on shape but not on the amount of curvature. Even when the amount of curvature was randomized, subjects' performance was not altered. On the other hand, the discrimination of 3-D curvature clearly depended linearly on curvature with Weber fractions of 20% on the average and, to a small degree, on 3-D shape. The experiment shows that observers can easily separate 3-D shape and 3-D curvature, and that Koenderink's shape index and curvedness provide a convenient way to specify shape. These results warn us against using just any arbitrary 3-D shape in 3-D shape perception tasks and indicate, for example, that emphasizing 3-D shape in computer displays by exaggerating curvature does not have any effect.Since Wallach and O'Connell (1953) demonstrated that three-dimensional (3-D) shape could be extracted from orthographic projections of rotating objects, there have been numerous studies of this phenomenon, which is commonly known as the kinetic depth effect. Such psychophysical studies include, for example, those on object rigidity (Todd, 1984), perceived depth or curvature (Dosher, Landy, & Sperling, 1989;Norman & Lappin, 1992;Saidpour, Braunstein, & Hoffman, 1992), minimal conditions for the perception of 3-D shape from motion in multidot displays (Sperling, Landy, Dosher, & Perkins, 1989;Todd & Norman, 1991;Treue, Husain, & Andersen, 1991), projection methods (Todd, 1984), and token dependency (Landy, Dosher, Sperling, & Perkins, 1991).The most common way to study the phenomenon of 3-D shape from motion is to use random dot cinematograms of moving 3-D objects. The objects under study are mostly cylinders, planes, or spheres. There is no reason to assume that the results of these studies hold for all kinds of 3-D shapes, but most studies have not included a wide range of shapes or curvature values. On the other hand, the importance of a multiobject study has been pointed out by several authors. Norman and Lappin (1992) reported differences in discriminability between cylinders and spheres, spheres and planes, and cylinders and planes. for example, might have certain heuristic topological characteristics that would make them special objects, not suitable for a generalization to other 3-D objects. It has to be examined yet what exactly these "heuristic" properties are. It is well possible that a certain systematic 3-D shape dependency in structure from motion is responsible for this effect. A quantitative examination of 3-D shape perception obviously requires a mathematical defInition on-D shape. On the other hand, if we want to study 3-D shape perception in psychophysical experiments, this definition must be in some way ecologically valid so that ...

We examined whether non-visual signals improve visual perception of three-dimensional structure-from-motion. Observers discriminated curvature in quadratic surfaces defined by random dot cinematograms with limited lifetime. They either explored visually a static surface by making head movements that were fed back to the display (HM condition) or they viewed statically the same surface which now rotated (NHM condition). Both conditions showed a clear build-up of performance as lifetime increases, but with different time constants for the HM and NHM condition. A second experiment showed that these differences could not be caused by differences in motion detection for the HM and NHM conditions. We suggest that non-visual information is combined with visual information at a high stage of visual processing, and that it does not mainly serve as input for a retinal stabilization process.