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PubMed1989

Kinetic depth effect and optic flow--I. 3D shape from Fourier motion.

Fifty-three different 3D shapes were defined by sequences of 2D views (frames) of dots on a rotating 3D surface. (1) Subjects' accuracy of shape identifications dropped from over 90% to less than 10% when either the polarity of the stimulus dots was alternated from light-on-gray to dark-on-gray on successive frames or when neutral gray interframe intervals were interposed. Both manipulations interfere with motion extraction by spatio-temporal (Fourier) and gradient first-order detectors. Second-order (non-Fourier) detectors that use full-wave rectification are unaffected by alternating-polarity but disrupted by interposed gray frames. (2) To equate the accuracy of two-alternative forced-choice (2AFC) planar direction-of-motion discrimination in standard and polarity-alternated stimuli, standard contrast was reduced. 3D shape discrimination survived contrast reduction in standard stimuli whereas it failed completely with polarity-alternation even at full contrast. (3) When individual dots were permitted to remain in the image sequence for only two frames, performance showed little loss compared to standard displays where individual dots had an expected lifetime of 20 frames, showing that 3D shape identification does not require continuity of stimulus tokens. (4) Performance in all discrimination tasks is predicted (up to a monotone transformation) by considering the quality of first-order information (as given by a simple computation on Fourier power) and the number of locations at which motion information is required. Perceptual first-order analysis of optic flow is the primary substrate for structure-from-motion computations in random dot displays because only it offers sufficient quality of perceptual motion at a sufficient number of locations.

Depth Perception↗
PubMed1990

"Colour constancy" in Mondrian patterns: a partial cancellation of physical chromaticity shifts by simultaneous contrast.

Edwin Land's Mondrian demonstrations (Land 1977, 1983, 1986a) are striking examples that the perceived colours of objects are largely independent of the chromaticity of the light incident upon them. Attempts to implement this independence in artificial vision systems have renewed interest in colour constancy and contrast, and the explanation of these phenomena in the Retinex theory. We use colour matches to demonstrate that departures from "colour constancy" are large and that it is possible to obtain the same colour shifts when the complex Mondrian pattern is replaced by a homogeneous grey field surrounding a test patch. A given patch has the same colour when surrounded by the Mondrian as when set in a grey background, provided that the grey represents the spatially weighted average of the Mondrian. Neither the colour shifts nor the equivalence of this neutral surround are correctly predicted by the Retinex theory. The phenomenon of partial cancellation of physical chromaticity shifts with changes of illuminant thus reduces to one of simultaneous contrast and adaptation where a spatio-chromatic and luminance average over a Mondrian pattern is the same as for a grey surround. Experiments with simultaneous contrast demonstrate that spatial weighting factors need to be applied in computations of the effect of the separate areas of a complex Mondrian pattern.

Algorithms↗
PubMed1990

da Vinci stereopsis: depth and subjective occluding contours from unpaired image points.

Distant surfaces are occluded by nearer surfaces to different extents in the two eyes, leading to the existence of unpaired image points visible in one eye and not the other. An ecological analysis of the real world situation that could have given rise to such unpaired points indicates the presence of a depth constraint zone, defined by visibility lines between which possible real world points must lie. The leading edge of this zone starts at the edge of a fused binocular occluding surface and recedes linearly with increases in horizontal distance to the unpaired point. Psychophysical evidence indicates that the human visual system makes use of this unpaired information in a remarkably adaptive manner, showing an increase in perceived depth for increasing horizontal separations between the unpaired target and fused edge, at least over a significant angular range (approx. 25-40 min arc). We also show that unpaired points in binocular images can lead to the formation of subjective occluding contours and surface having the qualitatively appropriate sign of depth. Furthermore, we show that the visual system could not recover depth of unpaired points camouflaged from the other eye against silhouettes. Our findings indicate that the visual system makes use of occlusive relations in the real world to recover depth, contour, and surface from unpaired points. The fact that such processes must utilize eye-of-origin information implies that they share this essential characteristic with classical or Wheatstone stereopsis. The necessity of eye-of-origin information also suggests that the processing may begin relatively early in cortical visual processing, possibly as early as V1. Finally, the novel emergence of subjective occluding contours from unpaired monocular stimuli raises the possibility that this process is mediated by visual experience, built up by the association of unpaired points and occluding contours.

Depth Perception↗
PubMed1995

Perception of three-dimensional shape from ego- and object-motion: comparison between small- and large-field stimuli.

We compare the performance in the detection of the shape of concave, planar and convex surfaces for small-field (8 deg) and large-field (90 deg) stimuli. Shape is perceived from head translations, object translations and object rotations. We find large differences between small-field and large-field stimulation. For small-field stimulation performance is best for object rotation, intermediate for self-motion and worst for object translation. For large-field stimulation performance is similar across conditions. Few errors on the sign of the curvature are found for self-motion for both field sizes, indicating that self-motion information disambiguates the curvature sign. For object rotation with small-field stimulation, the concave-convex ambiguity is strong with many apparent deformations. In contrast, large-field curvature signs are always accurately reported, suggesting that the weight of the rigidity hypothesis depends on field size.

Depth Perception↗
PubMed1995

Measurement and modeling of depth cue combination: in defense of weak fusion.

Various visual cues provide information about depth and shape in a scene. When several of these cues are simultaneously available in a single location in the scene, the visual system attempts to combine them. In this paper, we discuss three key issues relevant to the experimental analysis of depth cue combination in human vision: cue promotion, dynamic weighting of cues, and robustness of cue combination. We review recent psychophysical studies of human depth cue combination in light of these issues. We organize the discussion and review as the development of a model of the depth cue combination process termed modified weak fusion (MWF). We relate the MWF framework to Bayesian theories of cue combination. We argue that the MWF model is consistent with previous experimental results and is a parsimonious summary of these results. While the MWF model is motivated by normative considerations, it is primarily intended to guide experimental analysis of depth cue combination in human vision. We describe experimental methods, analogous to perturbation analysis, that permit us to analyze depth cue combination in novel ways. In particular these methods allow us to investigate the key issues we have raised. We summarize recent experimental tests of the MWF framework that use these methods.

Bayes Theorem↗
PubMed1995

Contour integration across depth.

In order to investigate the extent of the local connections subserving contour integration across depth, we measured performance for detecting the continuity of a path of Gabor elements distributed in depth and embedded in a three-dimensional field of random background elements. The results show that performance cannot be explained in terms of monocular performance and that contour information is not limited to single disparity planes. Path detection does indeed involve the integration of information across different, very disparate depth planes. The rules which emerge are in general similar to that already described in the two-dimensional case in as far as orientation and disparity are important. Unlike the two-dimensional case, three-dimensional integration operates over relatively large three-dimensional distances.

Depth Perception↗
PubMed1994

A theory of shape constancy based on perspective invariants.

Shape constancy refers to the phenomenon in which the percept of the shape of a given object remains constant despite changes in the shape of the object's retinal image. The phenomenon of shape constancy is considered from historical, theoretical and empirical perspectives in this paper. First, four prior theories are discussed; specifically, (1) Helmholtzian theory, which assumes that shape constancy is achieved by taking an object's orientation into account, (2) Gestalt theory, which assumes that shape constancy involves a relationship between the perceived shape and perceived orientation of an object, (3) Gibsonian theory, which assumes that shape constancy is based on projective invariants and (4) multiple view theory, which assumes that shape constancy is achieved by memorizing a large set of different views of the object. It is shown, by an analysis of the prior literature, that none of these theories can actually explain the phenomenon of shape constancy. A new theory, which is based on new perspective invariants of a flat shape, is then proposed. The new Perspective Invariants Theory can account for all prior shape constancy experiments. New experiments, testing predictions of the Perspective Invariants Theory are then described. These experiments showed that: (1) a novel shape can be matched with its single perspective image in the absence of depth cues, (2) perceptual processing of shape is impaired when the range of possible values of tilt is wide, (3) perceptual processing of shape is not affected by the width of the range of possible values of slant. These results support predictions of Perspective Invariants Theory.

Algorithms↗
PubMed1995

Shape from shaded random surfaces.

The perception of surface relief from random shading patterns is measured by having observers adjust three-dimensional local probes, the projections of which are superimposed on the image. Three observers perform four settings of 91 probes on each of 14 images. These images are generated by calculating the Lambertian reflectance of a random superposition of elliptical Gaussian hills and valleys illuminated by a single distant light source as well as by ambient light. Neither the surface reflectance equation nor the light source direction is conveyed to our observers in any way. Mathematically, this "pure" shape-from-shading problem has highly non-unique solutions. Perception of a well-defined, stable shape therefore implies that the ambiguity is resolved, i.e. a gauge is fixed. We analyse the surface ambiguity or gauge freedom which is left unconstrained by pure shading information and we investigate possible ways of restricting it. Statistical analysis of the curl component of the field of probe settings reveals that the settings are significantly consistent with an underlying perceived surface. In spite of the large theoretical ambiguity in the stimuli, the settings are reproducible and show considerable inter-observer agreement. Even the correlation of the settings with the real surfaces is surprisingly large. If the settings are compared to the real surface normals, one finds a series of biases, the strongest of which is that the global surface slant is systematically underestimated, even in those cases where ending occluding contours or high-contrast luminance ridges, indicative of "almost" contours, are present in the image. Another bias then is that the corresponding rims on the surface are seen as roughly parallel to the picture plane.

Cues↗
PubMed1996

The effect of illuminant position on perceived curvature.

In shaded scenes surface features can appear either concave or convex, depending upon the viewer's judgement about the direction of the prevailing illumination. If other curvature cues are added to the image this ambiguity can be removed. However, it is not clear to what extent, if any, illuminant position exerts an influence on the perceived magnitude of surface curvature. Subjects were presented with pairs of spherical surface patches in a curvature matching task. The patches were defined by shading and texture cues. The perceived curvature of a standard patch was measured as a function of light source position. We found a clear effect of light source position on apparent curvature. Perceived curvature decreased as light source tilt increased and as light source slant decreased. We also found that the strength of this effect is determined partly by a surface's reflectance function and partly by the relative weight of the texture cue. When a specular component was added to the stimuli, the effect of light source orientation was weakened. The weight of the texture cue was manipulated by disrupting the regular distribution of texture elements. We found an inverse relationship between the strength of the effect and the weight of the texture cue: lowering the texture cue weight resulted in an enhancement of the illuminant position effect.

Cues↗
PubMed1996

Metamerisms in Structure-from-motion perception.

As a three-dimensional object is moving through our world, we generally obtain a vivid impression of both its structure and its motion through space. The time-course of two-dimensional projections of the scene (optic flow) is important in conveying this three-dimensional information to us. The extent to which we can solve this specific inverse problem, i.e. infer a three-dimensional scene from two-dimensional flow, depends on the accuracy with which the required flow characteristics are processed by our visual system. In adequate two-dimensional processing can lead to incomplete representations of the three-dimensional world (three-dimensional metric information is lost). Then the motion and structure of objects can no longer be recovered uniquely. Consequently, metameric classes of three-dimensional representations arise (e.g. only affine properties are conserved). this study investigates under what conditions we find metameric combinations of the perceived attitude and perceived rotation of a plane. Our subjects are presented with stimuli consisting of two horizontally separated planar patches rotating back and forth in depth about vertical axes. Subjects are required to match both the attitude and the rotation magnitude of these two patches. We vary the attitude from 15 to 60 deg vertical slant, and the rotation magnitude from 28 to 98 deg. We find that the matched slant and rotation settings vary widely. For high slant values and for small rotations, attitude and rotation settings become highly correlated, suggesting metamery. For low slant values and for large rotations, the correlation almost disappears, suggesting that both quantities are estimated independently and uniquely. Our paradigm reveals that with one task and one type of stimulus a gradual transition occurs from unique settings (metric representations) to metameric classes of settings (e.g. affine representations).

Depth Perception↗
PubMed1996

Investigating shape-from-shading illusions using solid objects.

Recent growth in the shape-from-shading psychophysics literature has been paralleled by an increasing availability of computer graphics hardware and software, to the extent that most psychophysical studies in this area now employ computer lighting algorithms. The most widely used of these algorithms in shape-from-shading psychophysics is the Phong lighting model. This model, and other shading models of its genre, produce readily interpretable images of three-dimensional scenes. However, such algorithms are only approximations of how light interacts with real objects in the natural environment. Nevertheless, the results from psychophysical experiments using these techniques have been used to infer the processes underlying the perception of shape-from-shading in natural environments. It is important to establish whether this substitution is ever valid. We report a series of experiments investigating whether two recently reported illusions seen in computer-generated, Phong shaded images occur for solid objects under real illuminants. The two illusions investigated are three-dimensional curvature contrast and the illuminant-position effect on perceived curvature. We show that both effects do occur for solid objects, and that the magnitude of these effects are equivalent regardless of whether subjects are presented with ray traced or solid objects.

Algorithms↗
PubMed1996

Stereoscopic depth constancy depends on the subject's task.

Under identical viewing conditions, observers made two types of judgement about the shape of stereoscopically defined surfaces: one required an estimate of viewing distance for correct performance (e.g. setting the depth of a hemi-cylinder to equal its half-height or a dihedral angle to 90 deg), the other did not (matching the depth of, for example, sinusoidal corrugations or hemi-cylinders presented at two distances). Depth constancy for the two types of task was about 75% and 100%, respectively. We argue that observers may use a simple "direct" strategy to perform the depth matching task rather than constructing and comparing a metric representation of each surface.

Convergence, Ocular↗
PubMed1995

Words interact with colors in a globally aphasic patient: evidence from a Stroop-like task.

Stroop-like manual reaction time tasks were administered to 16 controls and one globally aphasic patient who was unable to match written color words with the corresponding color. Although the patient could not explicitly access the meaning of written color words, he showed a similar pattern of performance in the reaction time tasks as the controls: written words were processed automatically and they interacted with the processing of the color of the letters. Thus, the results of this study suggested that it is possible to process unrecognized words implicitly to a cross-domain semantic level where the processing of words interacts with the processing of colors. The results from this and other studies on implicit language processing indicate that linguistic stimuli can be analysed to relatively high levels without any awareness.

Aphasia↗
PubMed2000

The viewpoint-dependency of veridicality: psychophysics and modelling.

Human observers were shown projected angles, embedded in solid cross-like figures and were asked whether these projected angles could be the projection of an orthogonal angle in 3-D space (i.e. whether the two legs of the cross were orthogonal to each other). We found that performance depended on the viewpoint at which the angle was viewed: Both slant (i.e. the angle between the normal of the target angle relative to the plane of projection) and roll (i.e. the rotation around the normal of the target angle) had a systematic effect on the proportion of errors when observers were shown non-orthogonal angles. With orthogonal angles, however, this effect was absent (i.e. very low error rate with no systematic effect of slant and roll). Instead of assuming a viewpoint-dependent bias towards orthogonality, a computational analysis of the task, using a Bayesian approach, and a computer simulation showed that the viewpoint-dependency can be modelled by a fixed set of biases in order to constrain the set of possible scenes that could give rise to the projection.

Computer Graphics↗
PubMed2000

Electrophysiological correlates of visual impairments after traumatic brain injury.

Our aims were to investigate: (i) the VEP correlates of functional visual impairments following traumatic brain injury (TBI), in particular of the reduced spatial form perception; and (ii) the VEP correlates of visual sustained arousal in TBI patients. We used two approaches: (i) the analysis of latency and amplitude of the peaks; and (ii) the study of the correlations among the latencies of the peaks as a label of temporal synchronization. Thirty-five severe TBI outcome inpatients and 35 matching controls were studied. Pattern-reversal VEPs were recorded at Oz-Fz and Cz-A1, first without counting, then with counting of the reversals. Seven peaks of the waveform at Oz and eight peaks at Cz were measured. We found several differences in amplitude and latency between patients and controls, and between nocount/count. The temporal binding of the peaks within each channel and between the two channels was calculated by correlation matrices, and tested by factor analysis. Results indicated that the synchronization of the peaks within each channel did not differ between patients and controls. The temporal covariation between peaks occurring at Oz and Cz, however, was highly significantly altered in patients. This suggests that visual impairments in TBI patients may be due to a deranged synchronization of the activity of different brain regions.

Adolescent↗
PubMed2001

The lateral occipital complex and its role in object recognition.

Here we review recent findings that reveal the functional properties of extra-striate regions in the human visual cortex that are involved in the representation and perception of objects. We characterize both the invariant and non-invariant properties of these regions and we discuss the correlation between activation of these regions and recognition. Overall, these results indicate that the lateral occipital complex plays an important role in human object recognition.

Brain Mapping↗