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R van Ee

Publications and source records attributed to R van Ee.

At least 19 recordsLinked to original sources
PubMed2006

Direct extraction of curvature-based metric shape from stereo by view-modulated receptive fields.

Any computation of metric surface structure from horizontal disparities depends on the viewing geometry, and analysing this dependence allows us to narrow down the choice of viable schemes. For example, all depth-based or slant-based schemes (i.e. nearly all existing models) are found to be unrealistically sensitive to natural errors in vergence. Curvature-based schemes avoid these problems and require only moderate, more robust view-dependent corrections to yield local object shape, without any depth coding. This fits the fact that humans are strikingly insensitive to global depth but accurate in discriminating surface curvature. The latter also excludes coding only affine structure. In view of new adaptation results, our goal becomes to directly extract retinotopic fields of metric surface curvatures (i.e. avoiding intermediate disparity curvature). To find a robust neural realisation, we combine new exact analysis with basic neural and psychophysical constraints. Systematic, step-by-step 'design' leads to neural operators which employ a novel family of 'dynamic' receptive fields (RFs), tuned to specific (bi-)local disparity structure. The required RF family is dictated by the non-Euclidean geometry that we identify as inherent in cyclopean vision. The dynamic RF-subfield patterns are controlled via gain modulation by binocular vergence and version, and parameterised by a cell-specific tuning to slant. Our full characterisation of the neural operators invites a range of new neurophysiological tests. Regarding shape perception, the model inverts widely accepted interpretations: It predicts the various types of errors that have often been mistaken for evidence against metric shape extraction.

Animals↗
PubMed2006

Attentional control over either of the two competing percepts of ambiguous stimuli revealed by a two-parameter analysis: means do not make the difference.

We studied distributions of perceptual rivalry reversals, as defined by the two fitted parameters of the Gamma distribution. We did so for a variety of bi-stable stimuli and voluntary control exertion tasks. Subjects' distributions differed from one another for a particular stimulus and control task in a systematic way that reflects a constraint on the describing parameters. We found a variety of two-parameter effects, the most important one being that distributions of subjects differ from one another in the same systematic way across different stimuli and control tasks (i.e., a fast switcher remains fast across all conditions in a parameter-specified way). The cardinal component of subject-dependent variation was not the conventionally used mean reversal rate, but a component that was oriented-for all stimuli and tasks-roughly perpendicular to the mean rate. For the Necker cube, we performed additional experiments employing specific variations in control exertion, suggesting that subjects have to a considerable extent independent control over the reversal rate of either of the two competing percepts.

Attention↗
PubMed2005

Colour helps to solve the binocular matching problem.

The spatial differences between the two retinal images, called binocular disparities, can be used to recover the three-dimensional (3D) aspects of a scene. The computation of disparity depends upon the correct identification of corresponding features in the two images. Understanding what image features are used by the brain to solve this binocular matching problem is an important issue in research on stereoscopic vision. The role of colour in binocular vision is controversial and it has been argued that colour is ineffective in achieving binocular vision. In the current experiment subjects were required to indicate the amount of perceived depth. The stimulus consisted of an array of fronto-parallel bars uniformly distributed in a constant sized volume. We studied the perceived depth in those 3D stimuli by manipulating both colour (monochrome, trichrome) and luminance (congruent, incongruent). Our results demonstrate that the amount of perceived depth was influenced by colour, indicating that the visual system uses colour to achieve binocular matching. Physiological data have revealed cortical cells in macaque V2 that are tuned both to binocular disparity and to colour. We suggest that one of the functional roles of these cells may be to help solve the binocular matching problem.

Color↗
PubMed2005

Voluntary control and the dynamics of perceptual bi-stability.

Voluntary control and conscious perception seem to be related: when we are confronted with ambiguous images we are in some cases and to some extent able to voluntarily select a percept. However, to date voluntary control has not been used in neurophysiological studies on the correlates of conscious perception, presumably because the dynamic of perceptual reversals was not suitable. We exposed the visual system to four ambiguous stimuli that instigate bi-stable perception: slant rivalry, orthogonal grating rivalry, house-face rivalry, and Necker cube rivalry. In the preceding companion paper [van Ee, R. (2005). Dynamics of perceptual bi-stability for stereoscopic slant rivalry and a comparison with grating, house-face, and Necker cube rivalry. Vision Research] we focussed on the temporal dynamics of the perceptual reversals. Here we examined the role of voluntary control in the dynamics of perceptual reversals. We asked subjects to attempt to hold percepts and to speed-up the perceptual reversals. The investigations across the four stimuli revealed qualitative similarities concerning the influence of voluntary control on the temporal dynamics of perceptual reversals. We also found differences. In comparison to the other rivalry stimuli, slant rivalry exhibits: (1) relatively long percept durations; (2) a relatively clear role of voluntary control in modifying the percept durations. We advocate that these aspects, alongside with its metrical (quantitative) aspects, potentially make slant rivalry an interesting tool in studying the neural underpinnings of visual awareness.

Humans↗
PubMed2001

Motion direction, speed and orientation in binocular matching.

The spatial differences between the images seen by the two eyes, called binocular disparities, can be used to recover the volumetric (three-dimensional) aspects of a scene. The computation of disparity depends upon the correct identification of corresponding features in the two images. Understanding what image features are used by the brain to solve this matching problem is one of the main issues in stereoscopic vision. Many cortical neurons in visual areas V1 (ref. 2), MT (refs 3, 4) and MST (refs 5, 6) that are tuned to binocular disparity are also tuned to orientation, motion direction and speed. Although psychophysical work has shown that motion direction can facilitate binocular matching, the psychophysical literature on the role of orientation is mixed, and it has been argued that speed differences are ineffective in aiding correspondence. Here we use a different psychophysical paradigm to show that the visual system uses similarities in orientation, motion direction and speed to achieve binocular correspondence. These results indicate that cells that multiplex orientation, motion direction, speed and binocular disparity may help to solve the binocular matching problem.

Depth Perception↗
PubMed2001

Perceptual learning without feedback and the stability of stereoscopic slant estimation.

Subjects were examined for practice effects in a stereoscopic slant-estimation task involving surfaces that comprised a large portion of the visual field. In most subjects slant estimation was significantly affected by practice, but only when an isolated surface (an absolute disparity gradient) was present in the visual field. When a second, unslanted, surface was visible (providing a second disparity gradient and thereby also a relative disparity gradient) none of the subjects exhibited practice effects. Apparently, stereoscopic slant estimation is more robust or stable over time in the presence of a second surface than in its absence. In order to relate the practice effects, which occurred without feedback, to perceptual learning, results are interpreted within a cue-interaction framework. In this paradigm the contribution of a cue depends on its reliability. It is suggested that normally absolute disparity gradients contribute relatively little to perceived slant and that subjects learn to increase this contribution by utilizing proprioceptive information. It is argued that--given the limited computational power of the brain--a relatively small contribution of absolute disparity gradients in perceived slant enhances the stability of stereoscopic slant perception.

Cues↗
PubMed2001

Occlusion junctions do not improve stereoacuity.

Occlusion geometry gives rise to interocular shifts in the positions of binocularly viewed contour junctions. Since these shifts do not give rise to normal binocular disparities, they have been called 'pseudodisparities'. Previous work has shown that the unmatched contour segments of a partially occluded contour at occlusion junctions can be used to recover the geometry of the occluding surface through the construction of 'illusory' contours. Here, experiments were performed to determine whether such junction shifts could enhance stereoscopic depth detection when the relative disparity between the contours was below threshold. Our results showed that stereoscopic depth detection does not improve when pseudodisparity is present. We conclude that the visual system is less sensitive to pseudodisparity than to conventional disparity information. We suggest that the primary role of pseudodisparity is to overcome conditions of camouflage.

Cues↗
PubMed2000

Unconstrained stereoscopic matching of lines.

The computation of horizontal binocular disparities used in stereoscopic depth perception depends upon the identification of corresponding features in the two retinal images. In principle, binocular matching is a two-dimensional problem that considers matches in all possible meridians. Normally, constraints such as end points or crossing points limit the direction and magnitude of matches. If matching is unconstrained, such as is the case with long lines, it is completely ambiguous. Under these conditions the default match will be determined by the operating range, or upper disparity limit, of matchable vertical and horizontal disparities. We computed the operating range of vertical matches for stereoscopic depth as a function of line orientation. Our results suggest that the two-dimensional operating range is anisotropic for vertical and horizontal disparity and that unconstrained matches are not based upon either epipolar geometry or nearest neighbor constraints, but rather the mean of disparity estimates within the operating range for binocular matches. This operating range can be extended vertically when matches are constrained by image primitives.

Depth Perception↗
PubMed2000

Is there an interaction between perceived direction and perceived aspect ratio in stereoscopic vision?

In monocular vision, the horizontal/vertical aspect ratio (shape) of a frontoparallel rectangle can be based on the comparison of the perceived directions of the rectangle's edges. In binocular vision of a typical three-dimensional scene (when occlusions are present), this is not the case: Frontoparallel rectangles would be perceived in a distorted fashion if an observer were to base perceived aspect ratio on the perceived directions of the rectangle's edges. We psychophysically investigated stereoscopically perceived aspect ratios of frontoparallel occluding and occluded rectangles for various distances and fixation depths. We found that observers did not perceive the distortions that would be predicted on the basis of the above-mentioned comparison of the perceived visual directions of the edges of the rectangle. Our results strongly suggest that the mechanism that determines perceived aspect ratio is dissociated from the mechanism that determines perceived direction. The consequences of the findings for the Kanizsa, Poggendorff, and horizontal/vertical illusions are discussed.

Depth Perception↗
PubMed1999

The influence of large scanning eye movements on stereoscopic slant estimation of large surfaces.

The results of several experiments demonstrate that the estimated magnitude of perceived slant of large stereoscopic surfaces increases with the duration of the presentation. In these experiments, subjects were free to make eye movements. A possible explanation for the increase is that the visual system needs to scan the stimulus with eye movements (which take time) before it can make a reliable estimate of slant. We investigated the influence of large scanning eye movements on stereoscopic slant estimation of large surfaces. Six subjects estimated the magnitude of slant about the vertical or horizontal axis induced by large-field stereograms of which one half-image was transformed by horizontal scale, horizontal shear, vertical scale, vertical shear, divergence or rotation relative to the other half-image. The experiment was blocked in three sessions. Each session was devoted to one of the following fixation strategies: central fixation, peripheral (20 deg) fixation and active scanning of the stimulus. The presentation duration in each of the sessions was 0.5, 2 or 8 s. Estimations were done with and without a visual reference. The magnitudes of estimated slant and the perceptual biases were not significantly influenced by the three fixation strategies. Thus, our results provide no support for the hypothesis that the time used for the execution of large scanning eye movements explains the build-up of estimated slant with the duration of the stimulus presentation.

Anisotropy↗
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↗
PubMed1999

Perceived visual direction near an occluder.

When an opaque object occludes a more distant object, the two eyes often see different parts of the distant object. Hering's laws of visual direction make an interesting prediction for this situation: the part seen by both eyes should be seen in a different direction than the part seen by one eye. We examined whether this prediction holds by asking observers to align a vertical monocular line segment with a nearby vertical binocular segment. We found it necessary to correct the alignment data for vergence errors, which were measured in a control experiment, and for monocular spatial distortions, which were also measured in a control experiment. Settings were reasonably consistent with Hering's laws when the monocular and binocular targets were separated by 30 arcmin or more. Observers aligned the targets as if they were viewing them from one eye only when they were separated by 2 arcmin; this behavior is consistent with an observation reported by Erkelens and colleagues. The same behavior was observed when the segments were horizontal and when no visible occluder was present. Perceived visual direction when the two eyes see different parts of a distant target is assigned in a fashion that minimizes, but does not eliminate, distortions of the shape of the occluded object.

Form Perception↗
PubMed1999

An analysis of binocular slant contrast.

When a small frontoparallel surface (a test strip) is surrounded by a larger slanted surface (an inducer), the test strip is perceived as slanted in the direction opposite to the inducer. This has been called the depth-contrast effect, but we call it the slant-contrast effect. In nearly all demonstrations of this effect, the inducer's slant is specified by stereoscopic signals; and other signals, such as the texture gradient, specify that it is frontoparallel. We present a theory of slant estimation that determines surface slant via linear combination of various slant estimators; the weight of each estimator is proportional to its reliability. The theory explains slant contrast because the absolute slant of the inducer and the relative slant between test strip and inducer are both estimated with greater reliability than the absolute slant of the test strip. The theory predicts that slant contrast will be eliminated if the signals specifying the inducer's slant are consistent with one another. It also predicts reversed slant contrast if the inducer's slant is specified by nonstereoscopic signals rather than by stereo signals. These predictions were tested and confirmed in three experiments. The first showed that slant contrast is greatly reduced when the stereo-specified and nonstereo-specified slants of the inducer are made consistent with one another. The second showed that slant contrast is eliminated altogether when the stimulus consists of real planes rather than images on a display screen. The third showed that slant contrast is reversed when the nonstereo-specified slant of the inducer varies and the stereo-specified slant is zero. We conclude that slant contrast is a byproduct of the visual system's reconciliation of conflicting information while it attempts to determine surface slant.

Contrast Sensitivity↗
PubMed1998

Temporal aspects of stereoscopic slant estimation: an evaluation and extension of Howard and Kaneko's theory.

We investigated temporal aspects of stereoscopically perceived slant produced by the following transformations: horizontal scale, horizontal shear, vertical scale, vertical shear, divergence and rotation, between the half-images of a stereogram. Six subjects viewed large field stimuli (70 degrees diameter) both in the presence and in the absence of a visual reference. The presentation duration was: 0.1, 0.4, 1.6, 6.4 or 25.6 s. Without reference we found the following: rotation and divergence evoked considerable perceived slant in a number of subjects. This finding violates the recently published results of Howard and Kaneko. Slant evoked by vertical scale and shear was similar to slant evoked by horizontal scale and shear but was generally less. With reference we found the following: vertical scale and vertical shear did not evoke slant. Slant due to rotation and divergence was similar to slant due to horizontal scale and shear but was generally less. According to the theory of Howard and Kaneko, perceived slant depends on the difference between horizontal and vertical scale and shear disparities. We made their theory more explicit by translating their proposals into linear mathematical expressions that contain weighting factors that allow for both slant evoked by rotation or divergence, subject-dependent underestimation of slant and other related phenomena reported in the literature. Our data for all stimulus durations and for all subjects is explained by this 'unequal-weighting' extension of Howard and Kaneko's theory.

Adult↗
PubMed1998

A computational model of depth perception based on headcentric disparity.

It is now well established that depth is coded by local horizontal disparity and global vertical disparity. We present a computational model which explains how depth is extracted from these two types of disparities. The model uses the two (one for each eye) headcentric directions of binocular targets, derived from retinal signals and oculomotor signals. Headcentric disparity is defined as the difference between headcentric directions of corresponding features in the left and right eye's images. Using Helmholtz's coordinate systems we decompose headcentric disparity into azimuthal and elevational disparity. Elevational disparities of real objects are zero if the signals which contribute to headcentric disparity do not contain any errors. Azimuthal headcentric disparity is a 1D quantity from which an exact equation relating distance and disparity can be derived. The equation is valid for all headcentric directions and for all binocular fixation positions. Such an equation does not exist if disparity is expressed in retinal coordinates. Possible types of errors in oculomotor signals (six) produce global elevational disparity fields which are characterised by different gradients in the azimuthal and elevational directions. Computations show that the elevational disparity fields uniquely characterise both the type and size of the errors in oculomotor signals. Our model uses a measure of the global elevational disparity field together with local azimuthal disparity to accurately derive headcentric distance throughout the visual field. The model explains existing data on whole-field disparity transformations as well as hitherto unexplained aspects of stereoscopic depth perception.

Computational Biology↗
PubMed1997

Capture of visual direction: an unexpected phenomenon in binocular vision.

Binocular perception of visual direction is based on laws which were formulated more than 100 years ago. These laws govern the directions in which human beings perceive objects visible to both eyes (binocular objects) and objects visible to only one eye (monocular objects). We report here that the laws do not hold for monocular objects adjacent to binocular objects. The perceived directions of these monocular objects are captured by those of nearby binocular objects. Capture of binocular visual direction is an unexpected phenomenon because it refutes the generally accepted notion that a particular retinal location gives rise to a particular subjective visual direction. The practical consequence is that the subjective techniques for measuring eye position which are widely used in fundamental research and clinical practice are unreliable if they are used in densely structured stimuli. We suggest that capture results from a mechanism of lateral interaction between adjacent visual directions. This mechanism ensures that, despite eye movements, objects have the same spatial order in monocular and binocular vision. This conservation of spatial order also explains why retinal blind spots are not manifest in binocular vision.

Convergence, Ocular↗