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Segmentation of a visual scene in "figure" and "ground" is essential for perception of the three-dimensional layout of a scene. In cases of bi-stable perception, two distinct figure-ground interpretations alternate over time. We were interested in the temporal dynamics of these alternations, in particular when the same image is presented repeatedly, with short blank periods in-between. Surprisingly, we found that the intermittent presentation of Rubin's classical "face-or-vase" figure, which is frequently taken as a standard case of bi-stable figure-ground perception, often evoked perceptual switches during the short presentations and stabilization was not prominent. Interestingly, bi-stable perception of Kanizsa's anomalous transparency figure did strongly stabilize across blanks. We also found stabilization for the Necker cube, which we used for comparison. The degree of stabilization (and the lack of it) varied across stimuli and across individuals. Our results indicate, against common expectation, that the stabilization phenomenon cannot be generally evoked by intermittent presentation. We argue that top-down feedback factors such as familiarity, semantics, expectation, and perceptual bias contribute to the complex processes underlying the temporal dynamics of bi-stable figure-ground perception., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

A previous study has shown that diverting attention from binocular rivalry to a visual distractor task results in a slowing of rivalry alternation rate between simple orthogonal orientations. Here, we investigate whether the slowing of visual perceptual alternations will occur when attention is diverted to an auditory distractor task, and we extend the investigation by testing this for two kinds of binocular rivalry stimuli and for the Necker cube. Our results show that doing the auditory attention task does indeed slow visual perceptual alternations, that the slowing effect is a graded function of attentional load, and that the attentional slowing effect is less pronounced for grating rivalry than for house/face rivalry and for the Necker cube. These results are explained in terms of supramodal attentional resources modulating a high-level interpretative process in perceptual ambiguity, together with a role for feedback to early visual processes in the case of binocular rivalry., (Copyright 2010 Elsevier Ltd. All rights reserved.)

Visual rivalry is thought to be a distributed process that simultaneously takes place at multiple levels in the visual processing hierarchy. Also, the different types of rivalry, such as binocular and monocular rivalry, are thought to engage shared underlying mechanisms. We hypothesized that the amount of perceptual suppression during rivalry as measured by the total duration of fully exclusive perceptual dominance is determined by a distance in a neurally represented feature space. This hypothesis can be contrasted with the possibility that the brain constructs an internal model of the outside world using full-fledged object representations, and that perceptual suppression is due to an appraisal of the likelihood of the particular stimulus configuration at a high, object-based level. We applied color and stereo-depth differences between monocular rivalry stimulus gratings, and manipulated color and eye-of-origin information in binocular rivalry using the flicker & switch presentation paradigm. Our data show that exclusivity in visual rivalry increases with increased difference in feature space without regard for real-world constraints, and that eye-of-origin information may be regarded as a segregating feature that functions in a manner similar to color and stereo-depth information. Moreover, distances defined in multiple feature dimensions additively and independently increase the amount of perceptual exclusivity and coherence in both monocular and binocular rivalry. We conclude that exclusivity in visual rivalry is determined by a distance in feature space that is subtended by multiple stimulus features.

After viewing directional motion, one is likely to perceive a subsequently presented directionally ambiguous motion as being in the same direction as the prior motion. The perceptual bias towards the most recent percept gradually develops as the interval between the prior stimulus and a subsequent test becomes longer. This form of positive bias, or priming, is created in an automatic fashion. It remain unclear how such perceptual bias could be eliminated by a stimulus manipulation. Here we examine whether presentation of a stimulus, which was neutral as to the competing perceptual interpretations, during the interval between prior and test stimuli, disrupts the development of the priming effect. In experiments with ambiguous motion, we used stationary gratings as the neutral stimuli, and in an experiment with binocular rivalry between orthogonal gratings, we used a plaid pattern consisting of the two rival gratings. In both cases, presenting the neutral stimuli reduced the perceptual bias. These findings show that the visual system dynamically calibrates its internal bias using a recent percept and that this internal bias can be nullified by presenting neutral stimuli.

Recent work investigated the influence of exogenous attention on initial percept dominance at the onset of binocular rivalry. It was reported that cueing attention to one of two binocularly presented transparent stimuli immediately prior to rivalrous viewing provided the cued stimulus with a competitive advantage in subsequent binocular rivalry. This effect was independent of the eye containing the cued stimulus during the rivalry phase. In this recent work, the attention cue was always presented to both eyes. This leaves unclear the extent to which cueing affects binocular and/or monocular stimulus representations. To disambiguate this issue, we compared the cueing strength when the cue was presented ipsi-, contra- or bi-laterally with respect to the eye containing the cued stimulus during subsequent binocular rivalry. Besides replicating previous findings, we found that stimulus cueing readily transfers across eyes, suggesting that binocular mechanisms mediate exogenous attention effects on dominance selection at the onset of binocular rivalry.

When the two eyes are presented with sufficiently different stimuli, the stimuli will engage in binocular rivalry. During binocular rivalry, a subject's perceptual state alternates between awareness of the stimulus presented to the right eye and that presented to the left eye. There are instances in which competition is not eye-based, but instead takes place between stimulus features, as is the case in flicker and switch rivalry (F&S). Here we investigate another such instance, interocular grouping, using a Diaz-Caneja type stimulus in conjunction with synchronous stimulus flicker. Our results indicate that stimulus flicker increases the total duration of interocularly bound percepts, and that this effect occurs for a range of temporal flicker frequencies. Furthermore, the use of contrast-inversion flicker causes a decrease of total dominance duration of the interocularly bound percepts. We argue that different flickering regimes can be used to differentially stimulate lower and higher levels of visual processing involved in binocular rivalry. We propose that the amount of interocularly combined pattern-completed percept can be regarded as a measure of the level at which binocular rivalry is resolved.

We investigated the influence of changing physical parameters and task on bistable perception of an ambiguously rotating sphere (SFM). Increasing dot-density and velocity decreased the duration of perceptual phases during both passive viewing and voluntary control exertion. Our main finding is that voluntary control of perception depends on the physical parameters constituting the stimulus. This dependency places important constraints on the mechanisms mediating voluntary control as these mechanisms cannot operate independently of stimulus characteristics. In addition, local asymmetries in dot-densities can trigger alternations towards the most salient direction, which is not necessarily associated with largest number of dots: competition between perceptual interpretations during SFM appears to occur between surface-based representations rather than between individual elements. Finally, we show that voluntary control remains effective, even when attentive tracking of individual stimulus elements is no longer possible.

Although it is known that high-level spatial attention affects adaptation for a variety of stimulus features (including binocular disparity), the influence of voluntary attentional control-and the associated awareness-on adaptation has remained unexplored. We developed an ambiguous surface slant adaptation stimulus with conflicting monocular and binocular slant signals that instigated two mutually exclusive surface percepts with opposite slants. Using intermittent stimulus removal, subjects were able to voluntarily select one of the two rivaling slant percepts for extended adaptation periods, enabling us to dissociate slant adaptation due to awareness from stimulus-induced slant adaptation. We found that slant aftereffects (SAE) for monocular and binocular test patterns had opposite signs when measured simultaneously. There was no significant influence of voluntarily controlled perceptual state during adaptation on SAEs of monocular or binocular signals. In addition, the magnitude of the binocular SAE did not correlate with the magnitude of perceived slant. Using adaptation to one slant cue, and testing with the other cue, we demonstrated that multiple slant signals adapt independently. We conclude that slant adaptation occurs before the level of slant awareness. Our findings place the site of stereoscopic slant adaptation after disparity and eye posture are interpreted for slant [as demonstrated by Berends et al. (Berends, E. M., Liu, B., & Schor, C. M. (2005). Stereo-slant adaptation is high level and does not involve disparity coding. Journal of Vision 5 (1), 71-80), using that disparity scales with distance], but before other slant signals are integrated for the resulting awareness of the presented slant stimulus.

We have investigated the role of saccades and fixation positions in two perceptual rivalry paradigms (slant rivalry and Necker cube) and in two binocular rivalry paradigms (grating and house-face rivalry), and we compared results obtained from two different voluntary control conditions (natural viewing and hold percept). We found that for binocular rivalry, rather than for perceptual rivalry, there is a marked positive temporal correlation between saccades and perceptual flips at about the moment of the flip. Across different voluntary control conditions the pattern of temporal correlation did not change (although the amount of correlation did frequently, but not always, change), indicating that subjects do not use different temporal eye movement schemes to exert voluntary control. Analysis of the fixation positions at about the moment of the flips indicates that the fixation position by itself does not determine the percept but that subjects prefer to fixate at different positions when asked to hold either of the different percepts.

We exposed the visual system to an ambiguous 3D slant rivalry stimulus consisting of a grid for which monocular (perspective) and binocular (disparity) cues independently specified a slant about a horizontal axis. When those cues specified similar slants, observers perceived a single slant. When the difference between the specified slants was large, observers alternatively perceived a perspective- or a disparity-dominated slant. Eye movement measurements revealed that there was no positive correlation between a perceptual flip and both saccades (microsaccades as well as larger saccades) and blinks that occurred prior to a perceptual flip. We also found that changes in horizontal vergence were not responsible for perceptual flips. Thus, eye movements were not essential to flip from one percept to the other. After the moment of a perceptual flip the occurrence probabilities of both saccades and blinks were reduced. The reduced probability of saccades mainly occurred for larger voluntary saccades, rather than for involuntary microsaccades. We suggest that the reduced probability of voluntary saccades reflects a reset of saccade planning.

A way to study conscious perception is to expose the visual system to an ambiguous stimulus that instigates bi-stable perception. This provides the opportunity to study neural underpinnings related to the percepts rather than to the stimulus. We have recently developed a slant-rivalry paradigm that has beneficial metrical (quantitative) aspects and that exhibits temporal aspects of perceptual reversals that seemed to be under considerable voluntary control of the observer. Here we examined a range of different aspects of the temporal dynamics of the perceptual reversals of slant rivalry and we compared these with the dynamics of orthogonal grating rivalry, house-face rivalry, and Necker cube rivalry. We found that slant rivalry exhibits a qualitatively similar pattern of dynamics. The drift of the perceptual reversal rate, both across successive experimental repetitions, and across successive 35-s portions of data were similar. The sequential dependence of the durations of perceptual phases, too, revealed very similar patterns. The main quantitative difference, which could make slant rivalry a useful stimulus for future neurophysiological studies, is that the percept durations are relatively long compared to the other rivalry stimuli. In the paper that accompanies this paper [van Ee, R., van Dam, L. C. J., Brouwer, G. J. (2005). Voluntary control and the dynamics of perceptual bi-stability. Vision Research,] we focused on the role of voluntary control in the dynamics of perceptual reversals.

In order to perceive depth from binocular disparities the visual system has to identify matching features of the two retinal images. Normally, the assigned disparity is unambiguously determined by monocularly visible matching constraints. The assigned disparity is ambiguous when matching is unconstrained, such as when we view an isolated long oblique disparate line. Recently we found that in order to perceive a depth probe at the same depth as the oblique line, the probe needs to have the same horizontal disparity as the line (i.e. matching occurs along horizontal "search-zones" [Vis. Res. 40 (2000) 151]). Here we examined whether the depth probe disparity in unconstrained matching of long lines is influenced by cyclovergence, by cyclorotation between stereogram half-images, or by combinations of the two. We measured retinal rotation (>6 deg in cyclovergence conditions). We found that in those conditions in which the retinal images were the same (a condition with, say, both zero cyclovergence and zero cyclorotation between the half-images, creates the same retinal images as a condition with both 6 deg cyclovergence and 6 deg cyclorotation) assigned depth was the same too, i.e. independent of cyclovergence. Thus, the assigned depth of the test-line seems to be determined solely by the retinal test-line orientation, implying that the binocular matching algorithm does not seem to incorporate the eyes' cyclovergence when matching is unconstrained.

Two different colours, one presented to one eye and the other presented to the other eye, often create the impression of a third colour. This percept is known as binocular colour mixture. Here we use coloured stereograms to study binocular colour appearance. Vivid pastel colours are induced in monocular, achromatic patches, if these are placed in stereograms whose left and right images differ in colour. The build-up of the colours is slow and takes tens of seconds or even minutes in certain individuals. The induced colours remain visible during monocular viewing of the patch and decay gradually. The same colours are induced irrespective of whether the patches are placed in fusible or rivalrous stereograms. We show that these colour effects cannot be induced by monocular colour mechanisms, either alone or in combination with binocular colour mixing. We suggest that the colours are induced by a binocular feedback mechanism, which reduces colour differences between the colour appearances of two monocular images. Induced colours are not observed if the achromatic patches are binocular. However, induced colours are apparent if one switches to monocular viewing after prolonged binocular viewing of the binocular patches. This aftereffect suggests that binocular colour induction acts on the monocular representations of binocular images. We suggest that during binocular viewing the fast process of binocular colour mixing masks the changes in colour appearance produced by the much slower process of binocular colour induction.

During binocular fixation, the eyes usually point in different directions, and yet, each object is judged to lie in a single direction. It is commonly believed that a particular location in the head serves as the origin for such directional judgments. This location is known as the cyclopean eye. We argue here that observers can judge visually perceived directions from angular information alone, and do not require positional information supplied by a cyclopean eye. We show that experimental findings reported as evidence for the cyclopean concept can also be explained solely by angular information without the need for a cyclopean eye. Recent findings concerning binocular shape perception and the cyclopean illusion demonstrate that binocular perception is incompatible with vision from a single vantage point. The concept of the cyclopean eye is sometimes inappropriate and always irrelevant as far as vision is concerned.

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.

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.

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.

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.

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.

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.

Investigations of binocular visual direction have concentrated mainly on stationary objects. Eye positions were generally not measured and binocular fixation was assumed to be perfect. During the viewing of stationary objects, vergence errors are not negligible but small. During the viewing of moving objects, however, errors in binocular fixation are much larger. Existing rules for binocular visual direction were examined under the latter, more demanding viewing conditions. Eye movements were measured objectively by the scleral coil technique. Subjects viewed a large stereogram in which the half-images oscillated in counterphase. The stereogram contained two square random-dot patterns placed side by side with a gap in between. A vertical line, visible only to one eye, oscillated in the gap. Subjects were asked to adjust the amplitude of line motion until the line was perceived to be stationary. In so doing, they set amplitudes equal to the amplitudes of half-image motion if the gap between the patterns was narrow. They set amplitudes significantly smaller in wider gaps. Subjects made considerable fixational errors in following the oscillations of the line and the random-dot patterns. The results of the settings and of the retinal errors together refute existing rules for binocular visual direction of monocular objects. Perceived directions of monocular objects cannot be specified by geometrical rules that include only the positions of the objects and of the two eyes. The results suggest that perceived directions of monocular objects are captured by the binocular visual directions of adjacent binocular objects. Capture of binocular visual direction was found to be effective for gaps as wide as 8 deg between the binocular objects. The phenomenon of binocular capture has negative consequences for the general use of nonius lines as indicators of eye position.

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.

We systematically analyse the binocular disparity field under various eye, head and stimulus positions and orientations. From the literature we know that certain classes of disparity which involve the entire disparity field (such as those caused by horizontal lateral shift, differential rotation, horizontal scale and horizontal shear between the entire half-images of a stereogram) lead to relatively poor depth perception in the case of limited observation periods. These classes of disparity are found to be similar to the classes of disparities which are brought about by eye and head movements. Our analysis supports the suggestion that binocular depth perception is based primarily (for the first few hundred milliseconds) on classes of disparity that do not change as a result of ego-movement.

We investigated Werner's binocular depth-contrast effect. Subjects viewed stereograms consisting of a test pattern and an inducing pattern. The half-images of the inducing pattern were either horizontally scaled or sheared relative to each other. Subjects judged the (induced) perceived slant of the test pattern. We were interested in what influence the spatial configuration of the test pattern and the inducing pattern had on the depth-contrast effect. We conclude that the depth-contrast effect is a global effect. In other words, it is not restricted to the location of the inducing pattern. The effect decreases with distance, however, in an anisotropic way. The depth-contrast effect was present most prominently when the test pattern was positioned in the direction along the slant (rotation) axis of the inducing pattern. We suggest that Werner's depth-contrast effect can be explained by the (previously reported) findings that: (1) stereopsis is relatively insensitive to whole-field horizontal scale and shear; and (2) stereopsis is very sensitive to horizontal scale and shear of two stimuli relative to each other.

A generally accepted notion in binocular vision is that we see the world as if viewed by a single eye, the cyclopean eye. A consequence of seeing the world from a single point in space is that the outlines of occluding and occluded surfaces have the same shape. We designed stereograms in which subjects aligned binocularly visible lines to each other. The lines were lying in different depth planes. In the vicinity of occluded areas, binocular alignment was achieved by alignment of the lines in the eye that viewed the monocularly visible details. Stereograms in which shapes of surfaces lying in different depth planes were compared to each other show that occluding and occluded surfaces do not have the same shape: a square surface occludes rectangular surfaces in other depth planes of which the horizontal widths are smaller than the vertical widths. This difference is perceived shape is not possible if the centre of binocular direction has a fixed position in the head.

We investigate temporal aspects of binocular slant perception in the presence and absence of a visual reference. Subjects judge slant induced by large-field stereograms of which one half-image is either horizontally scaled or sheared relative to the other half-image. Each stimulus is presented for different observation periods ranging from 0.1 to 19.2 sec. We quantitatively corroborate earlier findings that perceived slant develops significantly faster and to higher levels with visual reference than without it. In daily life, when we are active, there will not be much time for slant to develop. We find that if observation periods are brief (a few seconds or less) slant is poorly perceived if there is no visual reference. We conclude that the visual system is relatively insensitive to large-field horizontal scale and shear.