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PubMed2006

Perception of illumination direction in images of 3-D convex objects: influence of surface materials and light fields.

We investigated the perception of illumination direction in images of 3-D convex objects under variations of light field and surface material properties. In a first experiment, we used an illumination-matching procedure in order to measure observers' ability to estimate the direction of illumination in images of 3-D polyhedra rendered under different light fields and illumination directions. Match deviations were larger in frontal direction than in rear directions, mainly counterclockwise in azimuth component, and diverged, in elevation component, from the image plane. In a second experiment, we examined whether the direction estimate was affected by the surface material type (BRDF), the light field, and the illumination direction. Angular deviations varied with material surface type and were largest in the test elevation direction 0 degrees. Elevation component deviations also differed with surface type and were larger in hemispherical diffuse lighting than in collimated lighting. These results suggest that the direction estimation is better with images of evenly distributed intensity gradients than with those of drastically varying gradients, and that the visual system may not take intensity variations due to the surface material or the light field into account in estimating the direction of illumination.

Cues↗
PubMed2006

Perception of surface slant from oriented textures.

When a surface covered with a regular texture is viewed in perspective, the projected texture provides a number of cues to 3D surface orientation. For oriented textures, one cue is perspective convergence: symmetry lines that are parallel along the surface project to lines that vary systematically in orientation. We investigated the contribution of perspective convergence to perception of 3D slant and tested whether slant from convergence depends on oriented spectral components. Subjects judged the sign of slant about a vertical axis of rotation. Textures were composed of filled circles in three spatial arrangements: a hex grid with symmetry lines at 0 and +/-60 deg relative to the tilt direction (aligned condition), a hex grid with symmetry lines at 90 and +/-30 deg (perpendicular condition), and random arrangements with similar average spacing (isotropic condition). The two hex grid textures differed in the amount of spectral energy present in the tilt direction (horizontal) but were otherwise closely matched. Slant discrimination thresholds for monocular stimuli were higher for isotropic textures than for either of the two hex grid textures and were higher for the perpendicular texture than for the aligned texture. In a second experiment, we measured the weight given to texture relative to binocular slant information for cue conflict stimuli (+/-5 deg). Weights were found to agree with individual subjects' monocular thresholds, in accordance with optimal estimation theory. We conclude that the visual system uses perspective convergence to perceive slant and that effective use of convergence requires the presence of spectral components aligned with the tilt direction.

Cues↗
PubMed2003

Spatial perception in normal and strabismic subjects: role of stereopsis and monocular clues.

PURPOSE: To evaluate the roles of binocular vision, monocular clues, and experience in spatial perception in a prospective, nonrandomized, case-control study. METHODS: A test was created consisting of three wood blocks arranged along the frontal plane inside Panum area. To produce some degrees of horizontal disparity, blocks were moved along a sagittal plane. Thirteen normal subjects (control group) and 13 nonamblyopic strabismic subjects (study group) were asked to identify, both under binocular and monocular vision, the position of the blocks in a series of 12 randomized presentations (phase 1). In phase II of the trial, a letter E in three different sizes, acting as a monocular clue, marked the three blocks. In both phases, the number of correct answers to the test during binocular and monocular vision was recorded. RESULTS: Binocular normal responses appeared greater than monocular normal responses and strabismic binocular responses in both phases (p < 0.001). Binocular strabismic responses and monocular answers of both groups appeared similar. CONCLUSIONS: In our experimental model, spatial orientation of strabismic subjects in binocular vision is substantially identical to that of normal subjects in monocular vision. Monocular clues were used in the same manner in both groups of subjects, and experience seemed to play no role in spatial localization of visual objects.

Adolescent↗
PubMed1995

[Effect of traumatic iris defects on spatial perception].

Perforating injury to the eye often results in iris defects in addition to loss of the lens. To determine whether iris defects can impair stereopsis, we tested the stereoscopic vision of 23 patients treated at the University Eye Clinic in Kiel for perforating eye injuries involving the iris and, in most cases, the lens. The patients were divided into three groups according to the severity of the iris defect and the status of the intraocular lens. Individuals with strabismus or visual acuity < 0.5 (20/40) were excluded. Stereopsis was measured using conventional stereopsis tests (Titmus contour stereopsis, Lang global stereopsis) and by means of the Pulfrich pendulum phenomenon. Patients with traumatic aphakia without an iris defect (n = 33) served as controls. Optical rehabilitation was achieved in the controls lenses (n = 16) or intraocular lens implant (n = 17). In group I (intact lens, iris defect < 135 degrees), two of five patients had attained global stereopsis. In group II (implanted or contact lens, iris defect < 135 degrees), only one of the nine patients exhibited global stereopsis. In group III (no lens, iris defect > 135 degrees), none of the nine patients achieved global stereopsis. In the control group, by contrast, more than half of the patients with an implant (10 of 17) and 3 of 16 patients with a contact lens attained global stereopsis. The results indicate that severe iris defects are especially likely to impair stereoscopic vision, more so than traumatic loss of lens alone without an accompanying iris defect.

Adolescent↗
PubMed1990

The perception of 3-dimensional affine structure from minimal apparent motion sequences.

The research described in the present article was designed to identify the minimal conditions for the visual perception of 3-dimensional structure from motion by comparing the theoretical limitations of ideal observers with the perceptual performance of actual human subjects on a variety of psychophysical tasks. The research began with a mathematical analysis, which showed that 2-frame apparent motion sequences are theoretically sufficient to distinguish between rigid and nonrigid motion and to identify structural properties of an object that remain invariant under affine transformations, but that 3 or more distinct frames are theoretically necessary to adequately specify properties of euclidean structure such as the relative 3-dimensional lengths or angles between nonparallel line segments. A series of four experiments was then performed to verify the psychological validity of this analysis. The results demonstrated that the determination of structure from motion in actual human observers may be restricted to the use of first order temporal relations, which are available within 2-frame apparent motion sequences. That is to say, the accuracy of observers' judgments did not improve in any of these experiments as the number of distinct frames in an apparent motion sequence was increased from 2 to 8, and performance on tasks involving affine structure was of an order of magnitude greater than performance on similar tasks involving euclidean structure.

Attention↗
PubMed1999

Horizontal and vertical disparity, eye position, and stereoscopic slant perception.

The slant of a stereoscopically defined surface cannot be determined solely from horizontal disparities or from derived quantities such as horizontal size ratio (HSR). There are four other signals that, in combination with horizontal disparity, could in principle allow an unambiguous estimate of slant: the vergence and version of the eyes, the vertical size ratio (VSR), and the horizontal gradient of VSR. Another useful signal is provided by perspective slant cues. The determination of perceived slant can be modeled as a weighted combination of three estimates based on those signals: a perspective estimate, a stereoscopic estimate based on HSR and VSR, and a stereoscopic estimate based on HSR and sensed eye position. In a series of experiments, we examined human observers' use of the two stereoscopic means of estimation. Perspective cues were rendered uninformative. We found that VSR and sensed eye position are both used to interpret the measured horizontal disparities. When the two are placed in conflict, the visual system usually gives more weight to VSR. However, when VSR is made difficult to measure by using short stimuli or stimuli composed of vertical lines, the visual system relies on sensed eye position. A model in which the observer's slant estimate is a weighted average of the slant estimate based on HSR and VSR and the one based on HSR and eye position accounted well for the data. The weights varied across viewing conditions because the informativeness of the signals they employ vary from one situation to another.

Cues↗
PubMed1996

Regularity vs genericity in the perception of collinearity.

The perception of collinearity is investigated, with the focus on the minimal case of three dots. As suggested previously, from the standpoint of probabilistic inference, the observer must classify each dot triplet as having arisen either from a one-dimensional curvilinear process or from a two-dimensional patch. The normative distributions of triplets arising from these two classes are unavailable to the observer, and are in fact somewhat counterintuitive. Hence in order to classify triplets, the observer invents distributions for each of the two opposed types, 'regular' (collinear) triplets and 'generic' (ie not regular) triplets. The collinear prototype is centered at 0 degree (ie perfectly straight), whereas the generic prototype, contrary to the normative statistics, is centered at 120 degrees away from straight-apparently because this is the point most distant in triplet space from straight and thus creates the maximum possible contrast between the two prototypes. By default, these two processes are assumed to be equiprobable in the environment. An experiment designed to investigate how subjects' judgments are affected by conspicuous environmental deviations from this assumption is reported. The results suggest that observers react by elevating or depressing the expected probability of the generic prototype relative to the regular one, leaving the prototype structure otherwise intact.

Adult↗
PubMed1994

The perception of spatial structure with oblique viewing: an explanation for Byzantine perspective?

Earlier work has confirmed that (i) observers can judge divergent receding lines, placed directly in front of them, to be parallel, and (ii) converging lines which are displaced laterally, so that they are viewed obliquely, can also be judged to be parallel. The former observation is in accord with traditional views of perspective while the latter, which is in accord with the depictions of objects found in Byzantine painting, is not in accord with perspective but is predicted by the relative magnitude of the visual angles subtended by the near and far ends of the pair of lines. To investigate whether these effects occurred when the stimulus was clearly three-dimensional, experiments were conducted with a novel apparatus, consisting of a framework of computer-controlled motor-driven luminous rods. This could be remotely adjusted so that all visible sides appeared to be parallel, ie to resemble a cube. Results showed that observers set the sides of this trapezohedron framework as diverging when it was viewed immediately in front of them, a result which is concordant with linear perspective, ie they see the normal projection of a cube as having converging edges. When the framework was displaced from the median plane so that it was viewed obliquely, the sides were set as converging and the magnitude of this effect was significantly related to angle of view, ie observers see the normal projection of a cube as having diverging sides.(ABSTRACT TRUNCATED AT 250 WORDS)

Art↗
PubMed2000

Perception, representation and recognition: a holistic view of recognition.

It is clear that humans have mental representations of their spatial environments and that these representations are useful, if not essential, in a wide variety of cognitive tasks such as identification of landmarks and objects, guiding actions and navigation and in directing spatial awareness and attention. Determining the properties of mental representation has long been a contentious issue (see Pinker, 1984). One method of probing the nature of human representation is by studying the extent to which representation can surpass or go beyond the visual (or sensory) experience from which it derives. From a strictly empiricist standpoint what is not sensed cannot be represented; except as a combination of things that have been experienced. But perceptual experience is always limited by our view of the world and the properties of our visual system. It is therefore not surprising when human representation is found to be highly dependent on the initial viewpoint of the observer and on any shortcomings thereof. However, representation is not a static entity; it evolves with experience. The debate as to whether human representation of objects is view-dependent or view-invariant that has dominated research journals recently may simply be a discussion concerning how much information is available in the retinal image during experimental tests and whether this information is sufficient for the task at hand. Here we review an approach to the study of the development of human spatial representation under realistic problem solving scenarios. This is facilitated by the use of realistic virtual environments, exploratory learning and redundancy in visual detail.

Computer Simulation↗
PubMed2001

Large errors in the perception of vertically are generated by luminance borders (integrated across space) not by subjective borders.

The rod-and-frame illusion shows large errors in the judgment of visual vertical in the dark if the frame is large and there are no other visible cues (Witkin and Asch, 1948 Journal of Experimental Psychology 38 762-782). Three experiments were performed to investigate other characteristics of the frame critical for generating these large errors. In the first experiment, the illusion produced by an 11 degrees tilted frame made by luminance borders (standard condition) was considerably larger than that produced by a subjective-contour frame. In the second experiment, with a 33 degrees frame tilt, the illusion was in the direction of frame tilt with a luminance-border frame but in the opposite direction in the subjective-contour condition. In the third experiment, to contrast the role of local and global orientation, the sides of the frame were made of short separate luminous segments. The segments could be oriented in the same direction as the frame sides, in the opposite direction, or could be vertical. The orientation of the global frame dominated the illusion while local orientation produced much smaller effects. Overall, to generate a large rod-and-frame illusion in the dark, the tilted frame must have luminance, not subjective, contours. Luminance borders do not need to be continuous: a frame made of sparse segments is also effective. The mechanism responsible for the large orientation illusion is driven by integrators of orientation across large areas, not by figural operators extracting shape orientation in the absence of oriented contours.

Adult↗
PubMed1996

Does the human visual system implement an ideal observer theory of slant from texture?

Texture information about surface shape can be decomposed into three constituents: compression, density and scaling. Blake, Bülthoff and Sheinberg's (1993, Vision Research, 33, 1723-1737) Ideal Observer theory of slant from texture predicts that the relative contribution of compression and density to the percept of planar surface slant should vary with field of view (FOV). The contribution of compression and density should both increase as FOV increases but statistical analysis shows that the reliability, and hence the expected contribution, of density increases at a greater rate than compression with increasing FOV. Specific predictions are that at FOV < 20 deg compression should be more effective than density, at FOV approximately 20 deg, compression and density should be equally effective, and at FOV > 20 deg, compression should be less effective than density. These predictions were tested by pitting these components of texture against one another. The method used was similar to that described in Frisby and Buckley [1992 In Orban, G. & Nagel H.-H. (Eds) Artificial and biological visual systems. Berlin: Springer]: observers judged binocularly the slant of a large table on to which was projected a texture created using computer graphics to contain various texture component cues to slant. The size of the projected texture patch determined the FOV which was by this means set to either 10, 20 or 30 deg. It was found that compression was the dominant cue irrespective of FOV, even if it was perturbed by noise. Possible reasons for this divergence of human observers from predictions of the Ideal Observer theory of slant from texture are discussed.

Adolescent↗
PubMed2002

Achromatic transparency and the role of local contours.

In this paper we investigate the role of contours and junctions in the perception of single-plane achromatic transparency. In order to measure the accuracy with which observers encode transparency, a six-luminance stimulus was employed in which the figural properties could be easily manipulated. Accuracy was measured by requiring subjects to select (either by the method of adjustment or by using a forced-choice procedure) the luminance that best completed a simulated transparent filter. The X junctions in the stimulus were destroyed or perturbed in three experiments. Simple occlusion of the junction (experiment 1), and perturbation of the orientation of the contours of the filter as they pass through the junction (experiment 3) resulted in small but significant reductions in performance. On the other hand, a sudden change in orientation of the background (material) contours (experiment 2) resulted in a small but significant enhancement of overall performance compared with the control stimulus. In the forced-choice task, reversals in the polarity of contours (as defined by the brightness order of flanking regions) around the junction were shown to effect large changes in subjects' accuracy in processing transparency. The overall results show that X and Psi junctions are indeed salient properties of transparent stimuli. The findings suggest that jagged contours with sudden changes in direction are more likely to be attributed to reflectance (material) changes than to changes due to a transparent filter (or to illumination).

Depth Perception↗
PubMed1989

[Visual fatigue evaluation of field sequential stereoscopic 3D display using near point distance].

The purpose of this paper was to evaluate quantitatively visual fatigue encountered in viewing stereoscopic 3D television. By examining the difference in the visual function involved in watching 2D and stereoscopic 3D displays, we could extract only the effect on visual perception in stereoscopic 3D television. Since the visual fatigue with the 3D display may be caused by the discrepancy between convergence and accommodation, the near point distances were measured by an Accommodo Polyrecorder, where Landolt's ring was pursued with binocularity. Five subjects, all employees of NEC Corporation, participated in the experiment. All subjects had normal or corrected-to-normal vision. Two of the subjects watched a 3D video movie. After a week, they watched another video movie in 2D. The rest of them watched the movies in reverse order. One video movie viewing requires about 120 minutes. The near point distances were measured before and after watching for 30, 60, 90 and 120 minutes. The 3D television system used here is a 3D video disc system with field sequence using glasses with liquid crystal shutters. In the case of 2D, each subject wore the glasses whose shutters were kept open. In the experiment, instruction to watch the video movies in a relaxed posture was given. There was a significant effect in regard to 3D/2D treatments on the change rates of the near point distance measured after watching every 30 minutes to one before watching. In addition, the change rates in 3D watching after 60, 90 and 120 minutes were significantly increased, compared with the corresponding rates in 2D.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMedDate not supplied

Rapid identification of ocular dominance columns in macaques using cytochrome oxidase, Zif268, and dark-field microscopy.

Strabismus induces an abnormal pattern of alternating light and dark columns of cytochrome oxidase (CO) activity in macaque striate cortex. This pattern may arise because visual perception is suppressed in one eye to avoid diplopia. To test whether CO activity is reduced in the ocular dominance columns of the suppressed eye, we performed monocular enucleation to co-label the ocular dominance columns with Zif268 immunohistochemistry in seven exotropic adult Macaca fascicularis. This approach was unsuccessful, for two reasons. First, Zif268 yielded inconsistent labelling, that was usually greater in the enucleated eye's ocular dominance columns, but was sometimes greater in the intact eye's columns. Therefore, Zif268 was not a reliable method for identifying the ocular dominance columns serving each eye. Second, in three control animals we found that a brief survival period following monocular enucleation (needed for Zif268 levels to change) was long enough to alter CO staining. For example, a survival time of only 3 h was sufficient to induce CO columns, indicating that the activity of this enzyme fluctuates more rapidly than realized previously. Independent of these findings, we have also discovered that acute monocular enucleation produces a vivid pattern of ocular dominance columns visible in unstained or CO-stained sections under dark-field illumination. The ocular dominance columns of the acutely enucleated eye appear dark. This was verified by labelling the ocular dominance columns with [3H]proline. Dark-field illumination of the cortex after acute monocular enucleation offers a new, easy method for identifying the ocular dominance columns in macaques.

Animals↗
PubMed2002

Perceptual assignment of opacity to translucent surfaces: the role of image blur.

In constructing the percept of transparency, the visual system must decompose the light intensity at each image location into two components one for the partially transmissivc surface, the other for the underlying surface seen through it. Theories of perceptual transparency have typically assumed that this decomposition is defined quantitatively in terms of the inverse of some physical model (typically, Metelli's 'episcotister model'). In previous work, we demonstrated that the visual system uses Michelson contrast as a critical image variable in assigning transmittance to transparent surfaces not luminance differences as predicted by Metelli's model [F Metelli, 1974 Scientific American 230(4) 90 98]. In this paper, we study the contribution of another variable in determining perceived transmittance, namely, the image blur introduced by the light-scattering properties of translucent surfaces and materials. Experiment 1 demonstrates that increasing the degree of blur in the region of transparency leads to a lowering in perceived transmittance, even if Michelson contrast remains constant in this region. Experiment 2 tests how this addition of blur affects apparent contrast in the absence of perceived transparency. The results demonstrate that, although introducing blur leads to a lowering in apparent contrast, the magnitude of this decrease is relatively small, and not sufficient to explain the decrease in perceived transmittance observed in experiment 1. The visual system thus takes the presence of blur in the region of transparency as an additional image cue in assigning transmittance to partially transmissive surfaces.

Contrast Sensitivity↗
PubMed1989

Integration of stereo and texture cues in the formation of discontinuities during three-dimensional surface interpolation.

A series of stereograms are presented which demonstrate that texture boundaries can strongly influence the perception of discontinuities between neighbouring three-dimensional (3-D) surfaces portrayed by means of stereo cues. In these demonstration figures, no stereo information is available in the immediate vicinity of the boundary between the two 3-D stereo surfaces because all texture in that region is removed in one eye's view. On the other hand, various forms of texture boundary information are provided in the resulting monocular region. This stimulus paradigm is used to explore the question: what influence does texture boundary information have on the nature of the perceived 3-D surface that is interpolated between two stimulus regions which carry stereo cues? It is shown that if a clear-cut texture boundary is present in the monocular region then this is used by the human visual system to fix the perceived location of 3-D crease and step surface discontinuities between the stereo regions. Collett (1985) explored this issue with a similar methodology and reported weak and unreliable assistance from monocular texture boundaries in helping shape 3-D stereo surface discontinuities. The strong and robust phenomena demonstrated here seem to rely on two main differences between the present stimuli and those of Collett. In the present stimuli, figurally continuous textures containing strong texture boundaries are used, together with a technique for minimising the complications, including binocular rivalry, that arise from the borders of the stimulus regions present in only one half of each stereogram.

Cues↗
PubMed1997

Frame-of-reference and hierarchical-organisation effects in the rod-and-frame illusion.

Two hypotheses proposed as alternatives by Rock--frame of reference and hierarchical organisation of perception--were tested in a series of experiments with the use of the rod-and-frame illusion. This illusion produces errors in the apparent vertical due to the presence of a tilted frame surrounding the test rod. The apparent vertical is shifted in the direction of the frame tilt. When an upright square was added inside the tilted frame, rod-setting errors varied according to the visual characteristics of the display. In the case of a large display presented in the dark (experiment 1), there continued to be large errors in the direction of the outer-square tilt. This finding supports the frame-of-reference hypothesis, which proposes that the orientation of all objects in the visual field is dominated by the most peripheral reference. In the case of a small display presented in a lit environment (experiments 2 and 3), the direction of errors was the opposite. This latter finding was taken to indicate that the rod was set with reference to the perceived tilt of the inner upright square. Thus, according to a hierarchical-organisation hypothesis, the orientation of an object in the visual field is influenced by objects in the immediate surroundings not by outermost reference. Overall, the results confirm the presence of two qualitatively different classes of orientational phenomena: one is concerned with the definition of egocentric coordinates and one with an object-centred visual representation.

Adult↗
PubMed2003

Experience-dependent visual cue recalibration based on discrepancies between visual and haptic percepts.

We studied the hypothesis that observers can recalibrate their visual percepts when visual and haptic (touch) cues are discordant and the haptic information is judged to be reliable. Using a novel visuo-haptic virtual reality environment, we conducted a set of experiments in which subjects interacted with scenes consisting of two fronto-parallel surfaces. Subjects judged the distance between the two surfaces based on two perceptual cues: a visual stereo cue obtained when viewing the scene binocularly and a haptic cue obtained when subjects grasped the two surfaces between their thumb and index fingers. Visual and haptic cues regarding the scene were manipulated independently so that they could either be consistent or inconsistent. Experiment 1 explored the effect of visuo-haptic inconsistencies on depth-from-stereo estimates. Our findings suggest that when stereo and haptic cues are inconsistent, subjects recalibrate their interpretations of the visual stereo cue so that depth-from-stereo percepts are in greater agreement with depth-from-haptic percepts. In Experiment 2 the visuo-haptic discrepancy took a different form when the two surfaces were near the subject than when they were far from the subject. The results indicate that subjects recalibrated their interpretations of the stereo cue in a context-sensitive manner that depended on viewing distance, thereby making them more consistent with depth-from-haptic estimates at all viewing distances. Together these findings suggest that observers' visual and haptic percepts are tightly coupled in the sense that haptic percepts provide a standard to which visual percepts can be recalibrated when the visual percepts are deemed to be erroneous.

Adolescent↗