Smoothness of the velocity field and three-dimensional surface detection from optic flow

Five experiments were conducted to determine the importance of smoothness of the velocity field in detecting 3-D surfaces from optic flow. Subjects were presented with optic flow displays simulating either points positioned on a corrugated 3-D surface or points randomly positioned within a 3-D volume. The subject’s task was to indicate whether or not the display appeared to be a 3-D surface. Smoothness of the velocity field was examined by systematically varying the speed of individual velocities in the flow field according to a Gaussian distribution withM = 0 andSD = σ. Variations in frequency, amplitude, density, and surface complexity were also examined. Detection of the corrugated surfaces systematically declined with an increase in σ. An increase in frequency of the corrugation for simple (single-frequency corrugation) surfaces resulted in a decrease in surface detection accuracy. Accuracy increased with an increase in density and amplitude for both simple and complex (multiplefrequency corrugation) surfaces. An analysis of the deformation of the displays predicted performance on the basis of human observers, providing further support for the importance of deformation for 3-D surface detection.     

Aging and the discrimination of 3-D shape from motion and binocular disparity

Two experiments evaluated the ability of younger and older adults to visually discriminate 3-D shape as a function of surface coherence. The coherence was manipulated by embedding the 3-D surfaces in volumetric noise (e.g., for a 55?% coherent surface, 55?% of the stimulus points fell on a 3-D surface, while 45?% of the points occupied random locations within the same volume of space). The 3-D surfaces were defined by static binocular disparity, dynamic binocular disparity, and motion. The results of both experiments demonstrated significant effects of age: Older adults required more coherence (tolerated volumetric noise less) for reliable shape discrimination than did younger adults. Motion-defined and static-binocular-disparity-defined surfaces resulted in similar coherence thresholds. However, performance for dynamic-binocular-disparity-defined surfaces was superior (i.e., the observers?? surface coherence thresholds were lowest for these stimuli). The results of both experiments showed that younger and older adults possess considerable tolerance to the disrupting effects of volumetric noise; the observers could reliably discriminate 3-D surface shape even when 45?% of the stimulus points (or more) constituted noise.     

Cooperativity, priming, and 3-D surface detection from optic flow

Three experiments were conducted to determine whether the mechanisms responsible for the detection of three-dimensional (3-D) surfaces from optic flow operate in a cooperative manner. The first experiment was conducted to determine whether a hysteresis effect occurs for 3-D surface detection from optic flow. The results of the first experiment demonstrated a hysteresis effect with lower thresholds occurring for decreasing texture density than for increasing texture density. The second experiment used a priming methodology to determine whether this form of cooperativity was based on preactivation of shear detectors or preactivation of 2-D motion detectors. The results suggest that only shear detectors were primed. The third experiment utilized a similar methodology to determine whether a surface representation would produce a priming effect. We found no evidence that the priming effect found in the second experiment was the result of preactivation of a generic representation of the test stimuli. The results of the experiments, considered together, suggest priming of the mechanisms responsible for recovering shear.     

Active heading control in simulated flight based on vertically extended contours

In two experiments, we manipulated the properties of 3-D objects and terrain texture in order to investigate their effects on active heading control during simulated flight. Simulated crosswinds were used to introduce a rotational component into the retinal flow field that presumably provided the visual cues used for heading control An active control task was used so that the results could be generalized to real-world applications such as flight simulation. In Experiment 1, we examined the effects of three types of terrain, each of which was presented with and without 3-D objects (trees), and found that the presence of 3-D objects was more important than terrain texture for precise heading control In Experiment 2, we investigated the effects of varying the height and density of 3-D objects and found that increasing 3-D object density improved heading control, but that 3-D object height had only a small effect. On the basis of these results, we conclude that the vertical contours improved active heading control by enhancing the motion parallax information contained in the retinal flow.     

The observer-relative velocity field as the basis for effective motion parallax

Earlier studies of motion parallax found unambiguous relative depth perception when random dot patterns were systematically translated in accordance with either motion of the observer's head or motion of the display scope. The need for such relative motion between an observer and a flow field was examined by placing a flow field in a limited area (window) in a large scope and translating the window relative to the observer. Accuracy in judging surface orientation and quantitative depth estimates were determined by the velocity field relative to the observer and were not measurably affected by whether this field was produced with a stationary or a moving window. Accuracy was consistently higher for smaller ratios of maximum to minimum projected velocities, reaching 100% in one experiment with a 1.12:1 ratio. We conclude that fully effective motion parallax does not require relative motion between the observer's head and the contours of a flow field.     

Factors influencing perceived angular velocity

The assumption that humans are able to perceive and process-angular kinematics is critical to many structure-from-motion and optical flow models. The current studies investigate this sensitivity, and examine several factors likely to influence angular velocity perception. In particular, three factors are considered: (1) the extent to which perceived angular velocity is determined by edge transitions of surface elements, (2) the extent to which angular velocity estimates are influenced by instantaneous linear velocities of surface elements, and (3) whether element-velocity effects are related to three-dimensional (3-D) tangential velocities or to two-dimensional (2-D) image velocities. Edge-transition rate biased angular velocity estimates only when edges were highly salient. Element velocities influenced perceived angular velocity; this bias was related to 2-D image velocity rather than 3-D tangential velocity. Despite these biases, however, judgments were most strongly determined by the true angular velocity. Sensitivity to this higher order motion parameter was surprisingly good, for rotations both in depth (y-axis) and parallel to the line of sight (z-axis).     

Visual short-term memory benefit for objects on different 3-D surfaces

Visual short-term memory (VSTM) plays an important role in visual cognition. Although objects are located on different 3-dimensional (3-D) surfaces in the real world, how VSTM capacity may be influenced by the presence of multiple 3-D surfaces has never been examined. By manipulating binocular disparities of visual displays, the authors found that more colored objects could be held in VSTM when they were placed on 2 rather than on 1 planar 3-D surfaces. This between-surface benefit in VSTM was present only when binding of objects' colors to their 3-D locations was required (i.e., when observers needed to remember which color appeared where). When binding was not required, no between-surface benefit in VSTM was observed. This benefit in VSTM could not be attributed to the number of spatial locations attended within a given surface. It was not due to a general perceptual grouping effect either, because grouping by motion and grouping by different regions of the same surface did not yield the same benefit. This increment in capacity indicates that VSTM benefits from the placement of objects in a 3-D scene.     

Factors influencing perceived angular velocity.

The assumption that humans are able to perceive and process angular kinematics is critical to many structure-from-motion and optical flow models. The current studies investigate this sensitivity, and examine several factors likely to influence angular velocity perception. In particular, three factors are considered: (1) the extent to which perceived angular velocity is determined by edge transitions of surface elements, (2) the extent to which angular velocity estimates are influenced by instantaneous linear velocities of surface elements, and (3) whether element-velocity effects are related to three-dimensional (3-D) tangential velocities or to two-dimensional (2-D) image velocities. Edge-transition rate biased angular velocity estimates only when edges were highly salient. Element velocities influenced perceived angular velocity; this bias was related to 2-D image velocity rather than 3-D tangential velocity. Despite these biases, however, judgments were most strongly determined by the true angular velocity. Sensitivity to this higher order motion parameter was surprisingly good, for rotations both in depth (y-axis) and parallel to the line of sight (z-axis).     

Shape from shading from images rendered with various surface types and light fields

Shape constancy is referred to as the tendency for the perceived shape of an object to remain unchanged even under changed viewing and illumination conditions. We investigated, in two experiments, whether shape constancy would hold for images of 3-D solid objects defined by shading only, whose renderings differed in terms of surface material type (bi-directional reflectance distribution functions), light field, light direction, shape, and specularity. Observers were presented with the image of a sphere or an ellipsoid and required to set perceived orientation and cross-section profile on designated points of the image. Results showed that shape judgments varied with all the aforementioned variables except specularity. Shape estimates were more precise with specular than asperity scattering surfaces, collimated than hemispherical diffuse lighting conditions, lower than higher elevations, spherical than ellipsoidal shapes, but not different between surfaces having differing specularity. These results suggest that shape judgments are made largely on the basis of the overall intensity distribution of shading, and that the portions of intensity distribution that are due to nonstructural variables such as surface material type or light field are not excluded in the process of shape estimation, as if being due to structural components. It is concluded that little constancy is expected in the perception of shape from shading.     

Visual and nonvisual information disambiguate surfaces specified by motion parallax.

Motion parallax has been shown to be an effective and unambiguous source of information about the structure of three-dimensional (3-D) surfaces, both when an observer makes lateral movements with respect to a stationary surface and when the surface translates with respect to a stationary observer (Rogers & Graham, 1979). When the same pattern of relative motions among parts of the simulated surface is presented to a stationary observer on an unmoving monitor, the perceived corrugations are unstable with respect to the direction of the peaks and troughs. The lack of ambiguity in the original demonstrations could be due to the presence of (1) non-visual information (proprioceptive and vestibular signals) produced when the observer moves or tracks a moving surface, and/or (2) additional optic flow information available in the whole array. To distinguish between these two possibilities, we measured perceived ambiguity in simulated 3-D surfaces in situations where either nonvisual information or one of four kinds of visual information was present. Both visual and nonvisual information were effective in disambiguating the direction of depth within the simulated surface. Real perspective shape transformations affecting the elements of the display were most effective in disambiguating the display.     

Visual and nonvisual information disambiguate surfaces specified by motion parallax

Motion parallax has been shown to be an effective and unamhiguous:source of information about the structure of three-dimensional (3-D) surfaces, both when an observer makes lateral movementswith respect to a stationary surface and when the surface translates with respect to a stationary observer (Rogers & Graham, 1979). When the same pattern of relative motions among parts of the simulated surface is presented to a stationary observer on an unmoving monitor, the perceived corrugations are unstable with respect to the direction of the peaks and troughs. The lack of ambiguity in the original demonstrations could be due to the presence of (1) non-visual information (proprioceptive and vestibular signals) produced when the observer moves or tracks a moving surface, andlor (2) additional optic flow information available in the whole array. To distinguish between these two possibilities, we measured perceived ambiguity in simulated 3-D surfaces in situations where either nonvisual information or one of four kinds of visual information was present. Both visual and nonvisual information were effective in disambiguating the direction of depth within the simulated surface. Real perspective shape transformations affecting the elements of the display were most effective in disambiguating the display.     

The ground dominance effect in the perception of relative distance in 3-D scenes is mainly due to characteristics of the ground surface

Previously, we (Bian, Braunstein, and& Andersen, 2005) reported a dominance effect of the ground plane over other environmental surfaces in determining the perceived relative distance of objects in 3-D scenes. In the present study, we conducted three experiments to investigate whether this ground dominance is due to inherent differences between ground and ceiling surfaces, or to the locations of these surfaces in the visual field. In Experiment 1, two vertical posts were positioned between a ground surface and a ceiling surface, and optical contact was manipulated so that the two surfaces provided contradictory information about the relative distances of the posts from the participant. The two surfaces were either both above, both below, or one above and one below fixation. In Experiment 2, only one surface was presented, either above, below, or at fixation. In Experiment 3, the posts were replaced by two red dots, and the eccentricity of the optical contact on the two surfaces was equated in each of five locations in the visual field. In all three experiments, participants judged which of the two objects appeared to be closer. Overall, we found a higher proportion of judgments consistent with a ground surface than with a ceiling surface in all locations, indicating that the ground dominance effect is mainly due to characteristics of the ground surface, with location in the visual field having only a minor effect.     

2-D contour perception resulting from kinetic occlusion

Kinetic occlusion, the progressive deletion or accretion of texture elements as one surface covers or uncovers another, has been shown to be an important source of information for determining depth order. In the present study, the importance of this information for 2-D contour perception was examined. In Experiment 1, subjects were asked to discriminate four different target shapes defined solely by kinetic occlusion. Discrimination increased with an increase in texture density and velocity, with density as the major factor. In Experiment 2, the targets were defined by static untextured regions as well as by kinetic occlusion. Overall, accuracy was similar to that found in Experiment 1, indicating that the presence of static information had little impact on accuracy. In Experiment 3, subjects were unable to discriminate among the four targets when presented with static versions of the displays used in Experiment 2. The results from these experiments indicate that kinetic occlusion can be used for discrimination of different 2-D shapes and that density has a more important role in determining accuracy than velocity.     

Aging and the perception of depth and 3-D shape from motion parallax

The ability of younger and older observers to perceive 3-D shape and depth from motion parallax was investigated. In Experiment 1, the observers discriminated among differently curved 3-dimensional (3-D) surfaces in the presence of noise. In Experiment 2, the surfaces' shape was held constant and the amount of front-to-back depth was varied; the observers estimated the amount of depth they perceived. The effects of age were strongly task dependent. The younger observers' performance in Experiment 1 was almost 60% higher than that of the older observers. In contrast, no age effect was obtained in Experiment 2. Older observers can effectively perceive variations in depth from patterns of motion parallax, but their ability to discriminate 3-D shape is significantly compromised.     

Necessity of spatial pooling for the perception of heading in nonrigid environments

This study examined whether the perception of heading is determined by spatially pooling velocity information. Observers were presented displays simulating observer motion through a volume of 3-D objects. To test the importance of spatial pooling, the authors systematically varied the nonrigidity of the flow field using two types of object motion: adding a unique rotation or translation to each object. Calculations of the signal-to-noise (observer velocity-to-object motion) ratio indicated no decrements in performance when the ratio was .39 for object rotation and .45 for object translation. Performance also increased with the number of objects in the scene. These results suggest that heading is determined by mechanisms that use spatial pooling over large regions.     

Three gradients and the perception of flat and curved surfaces

Researchers of visual perception have long been interested in the perceived slant of a surface and in the gradients that purportedly specify it. Slant is the angle between the line of sight and the tangent to the planar surface at any point, also called the surface normal. Gradients are the sources of information that grade, or change, with visual angle as one looks from one's feet upward to the horizon. The present article explores three gradients--perspective, compression, and density--and the phenomenal impression of flat and curved surfaces. The perspective gradient is measured at right angles to the axis of tilt at any point in the optic array; that is, when looking down a hallway at the tiles of a floor receding in the distance, perspective is measured by the x-axis width of each tile projected on the image plane orthogonal to the line of sight. The compression gradient is the ratio of y/x axis measures on the projected plane. The density gradient is measured by the number of tiles per unit solid visual angle. For flat surfaces and many others, perspective and compression gradients decrease with distance, and the density gradient increases. We discuss the manner in which these gradients change for various types of surfaces. Each gradient is founded on a different assumption about textures on the surfaces around us. In Experiment 1, viewers assessed the three-dimensional character of projections of flat and curved surfaces receding in the distance. They made pairwise judgments of preference and of dissimilarity among eight stimuli in each of four sets. The presence of each gradient was manipulated orthogonally such that each stimulus had zero, one, two, or three gradients appropriate for either a flat surface or a curved surface. Judgments were made were made for surfaces with both regularly shaped and irregularly shaped textures scattered on them. All viewer assessment were then scaled in one dimension. Multiple correlation and regression on the scale values revealed that greater than 98% of the variance in scale values was accounted for by the gradients. For the flat surfaces a mean of 65% of the variance was accounted for by the perspective gradient, 28% by the density gradient, and 6% by the compression gradient. For curved surfaces, on the other hand, a mean of 96% of the variance was accounted for by the compression gradient, and less than 2% by either the perspective gradient or the density gradient.(ABSTRACT TRUNCATED AT 400 WORDS)     

The discriminability of smooth stereoscopic surfaces.

In this study the sensitivity of human vision to the smoothness of stereoscopic surface structure was investigated. In experiments 1 and 2 random-dot stereograms were used to evaluate the discrimination of smooth versus 'noisy' sinusoidal surfaces differing in the percentages of points on a single smooth surface. Fully coherent smooth surfaces were found to be much more discriminable than other less smooth randomly perturbed surfaces. In experiment 3 the discrimination between discontinuous triangle-wave surfaces and similarly shaped smoothly curved surfaces obtained from the addition of the fundamental and the third harmonic of the corresponding triangle-wave surface was evaluated. The triangle-wave surfaces were found to be more accurately discriminated from the smoothly curved surfaces than would be predicted from the detectability of the difference in their Fourier power spectra. This superior discriminability was attributed to differences between the curvature and/or discontinuity of the two surfaces. In experiment 3 the effects of incoherent 'noise' points on the discrimination between the two surface types were also evaluated. These randomly positioned noise points had a relatively small effect on the discrimination between the two surfaces. In general, the results of these experiments indicate that smooth surfaces are salient for stereopsis and that isolated local violations of smoothness are highly discriminable.     

The absence of depth constancy in contour stereograms

Stereoscopic surfaces constructed from Kanizsa-type illusory contours or explicit luminance contours were tested for three-dimensional (3-D) shape constancy. The curvature of the contours and the apparent viewing distance between the surface and the observer were manipulated. Observers judged which of two surfaces appeared more curved. Experiment 1 allowed eye movements and revealed a bias in 3-D shape judgment with changes in apparent viewing distance, such that surfaces presented far from the observer appeared less curved than surfaces presented close to the observer. The lack of depth constancy was approximately the same for illusory-contour surfaces and for explicit-contour surfaces. Experiment 2 showed that depth constancy for explicit-contour surfaces improved slightly when fixation was required and eye movements were restricted. These experiments suggest that curvature in depth is misperceived, and that illusory-contour surfaces are particularly sensitive to this distortion.     

Brain areas sensitive to coherent visual motion

Detection of coherent motion versus noise is widely used as a measure of global visual-motion processing. To localise the human brain mechanisms involved in this performance, functional magnetic resonance imaging (fMRI) was used to compare brain activation during viewing of coherently moving random dots with that during viewing spatially and temporally comparable dynamic noise. Rates of reversal of coherent motion and coherent-motion velocities (5 versus 20 deg s-1) were also compared. Differences in local activation between conditions were analysed by statistical parametric mapping. Greater activation by coherent motion compared to noise was found in V5 and putative V3A, but not in V1. In addition there were foci of activation on the occipital ventral surface, the intraparietal sulcus, and superior temporal sulcus. Thus, coherent-motion information has distinctive effects in a number of extrastriate visual brain areas. The rate of motion reversal showed only weak effects in motion-sensitive areas. V1 was better activated by noise than by coherent motion, possibly reflecting activation of neurons with a wider range of motion selectivities. This activation was at a more anterior location in the comparison of noise with the faster velocity, suggesting that 20 deg s-1 is beyond the velocity range of the V1 representation of central visual field. These results support the use of motion-coherence tests for extrastriate as opposed to V1 function. However, sensitivity to motion coherence is not confined to V5, and may extend beyond the classically defined dorsal stream.     

Interpolation across surface discontinuities in structure from motion Asad Satdpour

Interpolation across orientation discontinuities in simulated three-dimensional (3-D). surfaces was studied in three experiments with the use of structure-from-motion (SFM). displays. The displays depicted dots on two slanted planes with a region devoid of dots (a gap). between them. If extended through the gap at constant slope, the planes would meet at a dihedral edge. Subjects were required to place an SFM probe dot, located within the gap, on the perceived surface. Probe dot placements indicated that subjects perceived a smooth surface connecting the planes rather than a surface with a discontinuity. Probe dot placements varied with slope of the planes, density of the dots, and gap size, but not with orientation (horizontal or vertical). of the dihedral edge or of the axis of rotation. Smoothing was consistent with models of 2-D interpolation proposed by Ullman (1976). and Kellman and Shipley (1991). and with a model of 3-D interpolation proposed by Grimson (1981).