A set of 4 experiments evaluated observers' sensitivity to three-dimensional (3-D) length, using both discrimination and adjustment paradigms with computer-generated optical patterns and real objects viewed directly in a natural environment. Although observers were highly sensitive to small differences in two-dimensional length for line segments presented in the frontoparallel plane, their discrimination thresholds increased by an order of magnitude when the line segments were presented at random orientations in 3-D space. There were also large failures of constancy, such that the perception of 3-D length varied systematically with viewing distance, even under full-cue conditions.

The intraparietal cortex is involved in the control of visually guided actions, like reach-to-grasp movements, which require extracting the 3D shape and position of objects from 2D retinal images. Using fMRI in behaving monkeys, we investigated the role of the intraparietal cortex in processing stereoscopic information for recovering the depth structure and the position in depth of objects. We found that while several areas (CIP, LIP, and AIP on the lateral bank; PIP and MIP on the medial bank) are activated by stereoscopic stimuli, AIP and an adjoining portion of LIP are sensitive only to depth structure. Furthermore, only these two regions are sensitive to both the depth structure and the 2D shape of small objects. These results indicate that extracting 3D spatial information from stereo involves several intraparietal areas, among which AIP and anterior LIP are more specifically engaged in extracting the 3D shape of objects.

A series of four experiments was designed to investigate the minimal amounts of information required to perceive the structure of a smoothly curved surface from its pattern of projected motion. In Experiments 1 and 2, observers estimated the amplitudes of sinusoidally corrugated surfaces relative to their periods. Observers' judgments varied linearly with the depicted surface amplitudes, but the amount of perceived relative depth was systematically overestimated by approximately 30%. The observers' amplitude judgments were also influenced to a lesser extent by the amount of rotary displacement of a surface at each frame transition, and by increasing the length of the apparent motion sequences from two to eight frames. The latter effect of sequence length was quite small, however, accounting for less than 3% of the variance in the observers' judgments. Experiments 3 and 4 examined observers' discrimination thresholds for sinusoidally corrugated surfaces of variable amplitude and for ellipsoid surfaces of variable eccentricity. The results revealed that observers could reliably detect differences of surface structure as small as 5%. The length of the apparent motion sequences had no detectable effect on these tasks, although there were significant effects of angular displacement and surface orientation. These results are considered with respect to the analysis of affine structure from motion proposed by Todd and Bressan (1990).One of the most extensively investigated phenomena in human vision is the perception of three-dimensional structure from motion, often referred to as the kinetic depth ef feet (Wallach & O'Connell, 1953). A particularly compelling method of demonstrating this phenomenon is to present observers with apparent motion sequences of random-dot surfaces rotating in depth under orthographic projection. When any individual image from such a sequence is viewed in isolation, it appears as nothing more than a pattern of random dots in the picture plane. When several such images are presented in rapid succession, however, the depicted pattern is almost always perceived as a coherent surface rotating rigidly in three-dimensional space.What type of perceptual mechanism would allow observers to determine the three-dimensional structure of an object solely on the basis of its pattern of projected motion? In attempting to develop a computational analysis of this phenomenon, numerous investigators have

The geometric relation between physical and perceived space as specified by binocular stereopsis and structure from motion was investigated. Four experimental tasks were used, each of which required a different aspect of three-dimensional (3-D) structure to be performed accurately. To examine whether the transformation between physical and perceptual space preserved the 3-D structural properties required to perform each of our tasks, the constancy of judged shape over changes in a depicted object's viewing distance or orientation was examined. Our results reveal that observers' judgments of 3-D shape from binocular stereopsis and motion contained systematic distortions: Perceived 3-D shape from motion was not invariant over orientation change and perceived 3-D structure from stereo, and motion and stereo in combination was not invariant over changes in viewing distance.

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& We used fMRI to directly compare the neural substrates of three-dimensional (3-D) shape and motion processing for realistic textured objects rotating in depth. Subjects made judgments about several different attributes of these objects, including 3-D shape, the 3-D motion, and the scale of surface texture. For all of these tasks, we equated visual input, motor output, and task difficulty, and we controlled for differences in spatial attention. Judgments about 3-D shape from motion involve both parietal and occipito-temporal regions. The processing of 3-D shape is associated with the analysis of 3-D motion in parietal regions and the analysis of surface texture in occipito-temporal regions, which is consistent with the different behavioral roles that are typically attributed to the dorsal and ventral processing streams. &

Three experiments are reported in which observers judged the three-dimensional (3-D) structures of virtual or real objects defined by various combinations of texture, motion, and binocular disparity under a wide variety of conditions. The tasks employed in these studies involved adjusting the depth of an object to match its width, adjusting the planes of a dihedral angle so that they appeared orthogonal, and adjusting the shape of an object so that it appeared to match another at a different viewing distance. The results obtained on all of these tasks revealed large constant errors and large individual differences among observers. There were also systematic failures of constancy over changes in viewing distance, orientation, or response task. When considered in conjunction with other, similar reports in the literature, these findings provide strong evidence that human observers do not have accurate perceptions of 3-D metric structure.

In this study, we evaluated observers' ability to compare naturally shaped three-dimensional (3-D) objects, using their senses of vision and touch. In one experiment, the observers haptically manipulated 1 object and then indicated which of 12 visible objects possessed the same shape. In the second experiment, pairs of objects were presented, and the observers indicated whether their 3-D shape was the same or different. The 2 objects were presented either unimodally (vision-vision or haptic-haptic) or cross-modally (vision-haptic or haptic-vision). In both experiments, the observers were able to compare 3-D shape across modalities with reasonably high levels of accuracy. In Experiment 1, for example, the observers' matching performance rose to 72% correct (chance performance was 8.3%) after five experimental sessions. In Experiment 2, small (but significant) differences in performance were obtained between the unimodal vision-vision condition and the two cross-modal conditions. Taken together, the results suggest that vision and touch have functionally overlapping, but not necessarily equivalent, representations of 3-D shape.