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PubMed1998

Local and global factors affecting the coherent motion of gratings presented in multiple apertures.

Using stimuli composed of two independent gratings viewed through multiple apertures, we investigate a number of parameters affecting the integration of locally ambiguous motions into globally coherent motion. In four experiments, we varied local factors (grating spatial frequency, speed, contrast, duty cycle, orientation) and global factors (degree of similarity and common fate between the gratings, and symmetry in the configuration of the grating pattern) and examined their effects on global motion coherence. Our results, confirming accounts offered by previous investigators, indicate that local competition between motion signals generated by contours (ambiguous) and their line terminations (unambiguous) is important in determining global motion coherence in multiple-aperture stimuli. Our results also indicate that global factors can affect perceived coherence independently of local motion signals, suggesting the involvement of higher-level motion areas and a role for non-motion processes such as those involved in pattern and form perception. Comparing motion coherence with other two-dimensional (2-D) stimuli (plaids) shows that 2-D multiple-aperture stimuli are not analogous and that coherence models derived from plaid stimuli do not account for the data.

Humans↗
PubMed1998

Shape from shading: estimation of reflectance map.

The reflectance map used by the visual system for perception of shape from shading was estimated. In Experiment 1, an image of a cylinder or a sphere illuminated from the viewer's direction was presented, and subjects estimated the cross-section of perceived 3D-shape. The reflectance map was estimated from the relationship between the stimulus image intensities and the slants of the measured cross-section. The estimated reflectance maps were not the ones based on Lambertian reflectance properties. In Experiment 2, whether perceived shapes could be predicted based on the reflectance maps obtained in Experiment 1 was examined. Subjects performed the same shape estimation task with images of cylinders generated by the reflectance map obtained in Experiment 1. The perceived shapes coincided well with the shapes used for stimulus image generation. These results indicate that the visual system's estimation of shape from shading can be fully understood based on empirically obtained reflectance maps without mentioning its inaccurate nature which has been claimed by past studies.

Algorithms↗
PubMed1998

Large-scale tests of a keyed, appearance-based 3-D object recognition system.

We describe and analyze an appearance-based 3-D object recognition system that avoids some of the problems of previous appearance-based schemes. We describe various large-scale performance tests and report good performance for full-sphere/hemisphere recognition of up to 24 complex, curved objects, robustness against clutter and occlusion, and some intriguing generic recognition behavior. We also establish a protocol that permits performance in the presence of quantifiable amounts of clutter and occlusion to be predicted on the basis of simple score statistics derived from clean test images and pure clutter images.

Computer Simulation↗
PubMed1999

Quantitative depth for a phantom surface can be based on cyclopean occlusion cues alone.

Liu, L., Stevenson, S.B., and Schor, C.M. (1994, Nature, 367, 66-669) reported quantitative stereoscopic depth in a phantom rectangle which appeared to lack conventional matching elements. Later, Gillam, B.J. (1995, Nature, 373, 202-203) and Liu, L., Stevenson, S.B., and Schor, C.M. (1995, Nature, 373, 203) and Liu, L., Stevenson, S.B., and Schor, C.M. (1997, Vision Research, 37(5), 633-644) indicated that the varying depth of the phantom rectangle could be based on stereoscopic matching. To remove the contaminating effects of conventional stereopsis from the Liu et al. (1994) original example, we presented a pair of parallel vertical lines to each eye where there is a central gap in the right line for the left eye's view and in the left line for the right eye's view. Observers saw a phantom rectangle bounded by subjective contours whose depth increased with the thickness of the lines. We attribute the quantitative variation of depth to a purely cyclopean (binocular) process sensitive to the pattern of contour presence and absence in the two eye's view.

Cues↗
PubMed1998

2D observers for human 3D object recognition?

In human object recognition, converging evidence has shown that subjects' performance depends on their familiarity with an object's appearance. The extent of such dependence is a function of the inter-object similarity. The more similar the objects are, the stronger this dependence will be and the more dominant the two-dimensional (2D) image-based information will be. However, the degree to which three-dimensional (3D) model-based information is used remains an area of strong debate. Previously the authors showed that all models with independent 2D templates that allowed 2D rotations in the image plane cannot account for human performance in discriminating novel object views. Here the authors derive an analytic formulation of a Bayesian model that gives rise to the best possible performance under 2D affine transformations and demonstrate that this model cannot account for human performance in 3D object discrimination. Relative to this model, human statistical efficiency is higher for novel views than for learned views, suggesting that human observers have used some 3D structural information.

Computer Simulation↗
PubMed1999

Stereopsis based on monocular gaps: metrical encoding of depth and slant without matching contours.

It is often the case in binocular vision that one eye can see between two objects lying at different distances but the other eye cannot. We have found that the visual system is able to correctly interpret images produced this way in which a single solid rectangle in one eye is fused with two half-sized rectangles in the other eye separated by a vertical gap comprising the background. Two rectangles in depth are seen. It is as if the solid rectangle is treated as two components which each match one of the physically separated rectangles in the contralateral eye. The sign of the depth depends on which eye's view has the gap and its magnitude increases with gap width. Measured depth is found to be equivalent to real stereoscopic depth with a relative disparity equal to the monocular gap. If overall disparity differences are eliminated, between the left and the right images, variations in perceived slant of the two rectangles are still seen with increasing gap size. That two surfaces can be seen in metric binocular depth despite complete camouflage of their separation in one eye's view, suggests that stereopsis be regarded as a broad process of surface recovery not necessarily requiring image disparity at the location of the depth step.

Depth Perception↗
PubMed1999

Categorical learning in pigeons: the role of texture and shape in complex static stimuli.

Pigeons are known to be able to categorize a wide variety of visual stimulus classes. However, it remains unclear which are the characteristics of the perceptually relevant features employed to reach such good performance. Here, we investigate the relative contributions of texture and shape information to categorization decisions about complex natural classes. We trained three groups of pigeons to discriminate between sets of photorealistic frontal images of human faces according to sex and subsequently, tested them on different stimulus sets. Only the pigeons that were presented with texture information were successful at the discrimination task. Pigeons seem to possess a sophisticated texture processing system but are less capable in discriminating shapes. The results are discussed in terms of the possible evolutionary advantages of utilizing texture as a very general and potent perceptual dimension in the birds' visual environment.

Animals↗
PubMed1999

Perceived distance, shape and size.

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.

Cues↗
PubMed1999

Feature specific segmentation in perceived structure-from-motion.

Motion information is important to vision for extracting the 3-D (three-dimensional) structure of an object, as evidenced by the compelling percept of three-dimensionality attainable in displays which are purely motion-defined. It has recently been shown that when subjects view a rotating transparent cylinder of dots simulated with parallel projection, they rarely perceive rotation reversals which are physically introduced (Treue, Andersen, Ando & Hildreth, Vision Research, 35;1995:139-148). We show however that when the elements defining the cylinder are oriented, the number of perceived reversals increases systematically to near maximum as the difference between element orientations on the two surfaces increases. These results imply that structure-from-motion mechanisms are capable of exploiting local feature differences between the different surfaces of a moving object.

Depth Perception↗
PubMed1999

One-shot viewpoint invariance in matching novel objects.

Humans often evidence little difficulty at recognizing objects from arbitrary orientations in depth. According to one class of theories, this competence is based on generalization from templates specified by metric properties (MPs), that were learned for the various orientations. An alternative class of theories assumes that non-accidental properties (NAPs) might be exploited so that even novel objects can be recognized under depth rotation. After scaling MP and NAP differences so that they were equally detectable when the objects were at the same orientation in depth, the present investigation assessed the effects of rotation on same-different judgments for matching novel objects. Judgments of a sequential pair of images of novel objects, when rendered from different viewpoints, revealed relatively low costs when the objects differed in a NAP of a single part, i.e. a geon. However, rotation dramatically reduced the detectability of MP differences to a level well below that expected by chance. NAPs offer a striking advantage over MPs for object classification and are therefore more likely to play a central role in the representation of objects.

Adolescent↗
PubMed1999

Interaction between the perceived shape of two objects.

The difference between the way in which binocular disparity scales with viewing distance and the way in which motion parallax scales with viewing distance introduces a potential indirect cue for viewing distance: the viewing distance is the only distance at which disparity and motion specify the same depth. The present study examines whether this information is used. Two simulated ellipsoids were presented on a computer screen in complete darkness. The two ellipsoids were 6 degrees to the left and right of straight ahead. Subjects set the width and depth of each ellipsoid to match a tennis ball, and set the distance of the one on the right to half that of the one on the left. The distance of the left ellipsoid varied between trials. On half of the trials it was static. On the other half it was rotating up and down around its frontal horizontal axis. Rotating the left ellipsoid influenced its set depth: rotating ellipsoids were set to be much more spherical. There was no influence on the set depth of the other ellipsoid, or on the set width of either. The set distance of the right ellipsoid was also unaffected. We conclude that subjects do not combine binocular disparity and motion parallax to obtain more veridical information about viewing distance.

Distance Perception↗
PubMed2000

Perception of three-dimensional shape from texture is based on patterns of oriented energy.

This paper presents empirical support for a new observer model of inferring three-dimensional shape from monocular texture cues. By measuring observers' abilities to estimate the relative three-dimensional curvature along a textured surface from two-dimensional projected images, and concurrently examining the local spectral changes occurring in the projected image for various texture patterns, we have found that correlated changes in oriented energy along lines corresponding to the lines of maximum and minimum curvature of the surface are crucial for conveying the three-dimensional shape of the surface. Energy along these lines of maximum and minimum curvature can be used to compute the orientation of local surface patches. Texture patterns consisting of simple and complex sinusoidal gratings and plaids, and filtered noise were drawn onto a surface that was corrugated sinusoidally in depth about the horizontal axis and projected in perspective onto an image plane. The perceived relative surface curvature was reconstructed from measurements of local ordinal depth around a central fixation point at 12 different phases of the corrugation. Our results show that: (1) it is neither necessary nor sufficient to identify individual texture elements or texture gradients in order to extract the shape of the surface; (2) one-dimensional frequency modulation is insufficient for conveying complex three-dimensional shape. (3) Veridical ordinal depth is seen only when the projected pattern contains changes in oriented energy along lines corresponding to projected lines of maximum curvature of the surface. (4) For a surface corrugated in depth about the horizontal axis, this pattern of oriented energy arises from energy along the vertical direction in the global Fourier transform of the pre-corrugated pattern. (5) Local orientation changes across lines of minimum curvature can be also critical for conveying shape. (6) These correlated orientation changes along lines of maximum and minimum curvature are entirely lost in parallel projection. Hence texture is a useful cue for shape if the image is a perspective projection. (7) Only some natural textures will provide sufficient monocular cues to support veridical shape inferences, and this can be predicted from their global Fourier transforms.

Computer Simulation↗
PubMed2000

Contour integration in color vision: a common process for the blue-yellow, red-green and luminance mechanisms?

We compare the performance of the red-green, blue-yellow and luminance postreceptoral mechanisms on a contour integration task requiring the linking of oriented Gabor elements across space to extract a winding 'path' or contour. We first establish that for all three mechanisms curvature and contrast are independent; losses in performance due to one cannot be compensated by changes in the other. We then compare contour integration by the three mechanisms using a method that controls for their differences in cone contrast thresholds. Our results show that despite the poor orientation discrimination thresholds and poor spatial sampling found for the blue-yellow mechanism, all three mechanisms perform similarly on contour integration over a wide range of curvatures. Furthermore, all three mechanisms have the same dependence on path curvature. We also investigate the effects of adding external orientation noise. Our results imply that the internal orientation noise for extracting 'aligned' path elements is similar in the three mechanisms and for all path curvatures, and the relative efficiencies are also similar for the three mechanisms. To account for our results, we propose that the three postreceptoral mechanisms use a common contour integration process. This linking process, however, cannot be color-blind; our last experiment shows that linking between different chromatic mechanisms or between opposite spatial phases disrupts contour integration. We thus propose that the common integration process remains sensitive to the color contrast and phase of its inputs.

Color Perception↗
PubMed2000

Neon colour spreading in three-dimensional illusory objects in humans.

We studied whether neon spreading can be induced within three-dimensional illusory triangles. Kanizsa triangles were induced by black pacman disks consisting of red sectors with curved sides. Viewing our stimuli monocularly produced two-dimensional illusory contours and surfaces as well as neon spreading in each figure. Triangles appeared concave or convex under stereoscopical viewing. Neon colour spreading was induced within illusory figures bending in three-dimensional space, suggesting that neural contour completion and surface filling-in interact across depth. Surprisingly, neon spreading was induced above the intervening surface even when the inducers were below the surface. Neon colour and illusory configuration were preserved behind the intervening surface only when it appeared transparent.

Adolescent↗
PubMed2001

Neural response to perception of volume in the lateral occipital complex.

Projection of a 3D scene onto the 2D retina necessarily entails a loss of information, yet perceivers experience a world populated with volumetric objects. Using simultaneous behavioral and neural (fMRI) measures, we identify neural bases of volume perception. Neural activity in the lateral occipital cortex increased with presentation of 3D volumes relative to presentation of 2D shapes. Neural activity also modulated with perceived volume, independent of image information. When behavioral responses indicated that observers saw ambiguous images as 3D volumes, neural response increased; when behavioral data revealed a 2D interpretation, neural response waned. Crucially, the physical stimulus was identical under both interpretations; only the percept of volume can account for the increased neural activity.

Adult↗
PubMedDate not supplied

Cortical projections to anterior inferior temporal cortex in infant macaque monkeys.

Inferior temporal (IT) cortex is a "high-order" region of extrastriate visual cortex important for visual form perception and recognition in adult primates. The pattern of cortical afferents from both ipsilateral and contralateral hemispheres to anterior IT cortex was determined in infant macaque monkeys 7-18 weeks of age following injections of wheat-germ agglutinin-HRP. Within the ipsilateral hemisphere, the locations and laminar distribution of labeled cells were similar to those observed after comparable injections in adult monkeys. Specifically, ipsilateral afferents derived from visual areas V4, TEO, anterior and posterior IT, and STP, from parahippocampal, perirhinal, and parietal zones, and from several anterior zones including lateral and ventral frontal cortex, the insula, and cingulate cortex. Within the contralateral hemisphere, we observed labeled cells in homotopic regions of IT and in parahippocampal and perirhinal areas, as has been reported for adult monkeys. However, we also identified additional contralateral regions not previously known to provide input to anterior IT, including lateral and ventral frontal cortex, cingulate cortex, and STP. Overall, the strongest and most widespread projections from outside the temporal lobe were found in the youngest monkey, suggesting that some of these projections may represent transient circuitry necessary for the development of complex visual response properties in anterior IT.

Afferent Pathways↗
PubMedDate not supplied

Simulation of neuronal responses defining depth order and contrast polarity at illusory contours in monkey area V2.

Neurophysiological, brain imaging, and perceptual studies in animals and humans suggest that illusory (occluding) contours are represented at an early level of visual cortical processing. Comparatively little is known about the mechanisms defining the depth order and the brightness illusion associated with such contours. Baumann et al. (1997) found neurons in area V2 of the alert monkey that signaled not only illusory contours but also the figure-ground direction that human observers perceive at such contours. The majority of these neurons showed this property independent stimulus contrast; a small minority preferred a certain combination of figure-ground direction and contrast polarity at these contours. In this article, we simulate the responses of these neurons by means of a grouping mechanism that uses occlusion cues (line-ends, corners) to define figure-ground direction and contrast polarity at such contours.

Animals↗