On the relation between dichoptic masking and binocular rivalry

When our two eyes view incompatible images, the brain invokes suppressive processes to inhibit one image, and favor the other. Two phenomena are typically observed: dichoptic masking (reduced sensitivity to one image) for brief presentations, and binocular rivalry (alternation between the two images), over longer exposures. However, it is not clear if these two phenomena arise from a common suppressive process. We investigated this by measuring both threshold elevation in simultaneous dichoptic masking and mean percept durations in rivalry, whilst varying relative stimulus orientation. Masking and rivalry showed significant correlations, such that strong masking was associated with long dominance durations. A second experiment suggested that individual differences across both measures are also correlated. These findings are consistent with varying the magnitude of interocular suppression in computational models of both rivalry and masking, and imply the existence of a common suppressive process. Since dichoptic masking has been localised to the monocular neurons of V1, this is a plausible first stage of binocular rivalry.     

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.     

Audiovisual interactions in binocular rivalry

When the two eyes are presented with dissimilar images, human observers report alternating percepts-a phenomenon coined binocular rivalry. These perceptual fluctuations reflect competition between the two visual inputs both at monocular and binocular processing stages. Here we investigated the influence of auditory stimulation on the temporal dynamics of binocular rivalry. In three psychophysics experiments, we investigated whether sounds that provide directionally congruent, incongruent, or non-motion information modulate the dominance periods of rivaling visual motion percepts. Visual stimuli were dichoptically presented random-dot kinematograms (RDKs) at different levels of motion coherence. The results show that directional motion sounds rather than auditory input per se influenced the temporal dynamics of binocular rivalry. In all experiments, motion sounds prolonged the dominance periods of the directionally congruent visual motion percept. In contrast, motion sounds abbreviated the suppression periods of the directionally congruent visual motion percepts only when they competed with directionally incongruent percepts. Therefore, analogous to visual contextual effects, auditory motion interacted primarily with consciously perceived visual input rather than visual input suppressed from awareness. Our findings suggest that auditory modulation of perceptual dominance times might be established in a top-down fashion by means of feedback mechanisms.     

Failure of rivalry at low contrast: evidence of a suprathreshold binocular summation process.

Presentation of different images to the two eyes normally results in a time-varying alternation between the two images (binocular rivalry). However, we find that when orthogonal gratings are viewed dichoptically at low contrast, a stable summation between the two images is perceived in the form of a dichoptic plaid. The range of perception of the dichoptic plaid depends on spatial frequency, contrast and luminance of the gratings. This phenomenon differs from the "false fusion", a fleeting summation of different images perceived only under very brief presentation of the stimuli. The observations suggest that there exists a neural process that performs a summation of dissimilar images, and that is distinct from the competitive process of suppression and binocular rivalry.     

Functional hierarchies of nonconscious visual processing

A number of psychophysical techniques can be used to eliminate the registration of stimuli in visual awareness and to study the dynamics of conscious and nonconscious information processing in the visual system. However, little is known about how these techniques relate to each other. We chose to compare binocular rivalry, induced by orthogonal gratings presented separately to the two eyes, and metacontrast suppression, produced when a target stimulus is followed by a spatially surrounding mask stimulus, to investigate relative levels and correlates of nonconscious processing. Combined with prior results, our findings indicate that binocular rivalry expresses its suppressive effects prior to the level at which the mechanism of metacontrast does. Implications for theories of masking and interpretations of the loss or perceptual effects when stimulus visibility is suppressed by different psychophysical methods are discussed.     

Modulation of spatiotemporal dynamics of binocular rivalry by collinear facilitation and pattern-dependent adaptation

The role of collinear facilitation was investigated to test predictions of a model for traveling waves of dominance during binocular rivalry (H. Wilson, R. Blake, & S. Lee, 2001). In Experiment 1, we characterized traveling wave dynamics using a recently developed technique called periodic perturbation (M.-S. Kang, D. Heeger, & R. Blake, 2009). Results reveal that the propagation speed of waves for a collinear stimulus increased regardless of whether that stimulus was suppressed (replicating earlier work) or dominant; this latter finding is contrary to the model's prediction. In Experiment 2, we measured perceptual dominance durations within a localized region in the center of two rival stimuli that varied in degree of collinearity. Results reveal that increased collinearity did not change average dominance durations regardless of the rivalry phase of the stimulus, again contrary to the model's prediction. Incorporating pattern-dependent modulation of adaptation rate into the model accounted for results from both experiments. Using model simulations, we show how interactions between collinear facilitation and pattern-dependent adaptation may influence the dynamics of binocular rivalry. We also discuss alternative interpretations of our findings, including the possible role of surround suppression.     

Perceptual dominance during binocular rivalry is prolonged by a dynamic surround

We examined whether dynamic stimulation that surrounds a rival target influences perceptual alternations during binocular rivalry. We presented a rival target surrounded by dynamic random-dot patterns to both eyes, and measured dominance durations for each eye’s rival target. We found that rival target dominance durations were longer when surrounds were dynamic than when they were static or absent. Additionally, prolonged dominance durations were more apparent when the dynamic surround was alternately presented between the two eyes than when it was presented simultaneously to both eyes. These results indicate that dynamic stimulation that surrounds a rival target plays a role in maintaining the current perceptual state, and causes less perceptual alternations during binocular rivalry. Our findings suggest that dynamic signals on the retina may suppress rivalry, and thus provide useful information for stabilizing perceptions in daily life.     

Distributions of alternation rates in various forms of bistable perception

Studying the temporal dynamics of bistable perception can be useful for understanding neural mechanisms underlying the phenomenon. We take a closer look at those temporal dynamics, using data from four different ambiguous stimuli. We focus our analyses on two recurrent themes in bistable perception literature. First, we address the question whether percept durations follow a gamma distribution, as is commonly assumed. We conclude that this assumption is not justified by the gamma distribution's approximate resemblance to distributions of percept durations. We instead present two straightforward distributions of reciprocal percept durations (i.e., rates) that both easily surpass the classic gamma distribution in terms of resemblance to empirical data. Second, we compare the distributions arising from binocular rivalry with those from other forms of bistable perception. Parallels in temporal dynamics between those classes of stimuli are often mentioned as an indication of a similar neural basis, but have never been studied in detail. Our results demonstrate that the distributions arising from binocular rivalry and other forms of bistable perception are indeed similar up to a high level of detail.     

A binocular rivalry study of motion perception in the human brain

The relationship between brain activity and conscious visual experience is central to our understanding of the neural mechanisms underlying perception. Binocular rivalry, where monocular stimuli compete for perceptual dominance, has been previously used to dissociate the constant stimulus from the varying percept. We report here fMRI results from humans experiencing binocular rivalry under a dichoptic stimulation paradigm that consisted of two drifting random dot patterns with different motion coherence. Each pattern had also a different color, which both enhanced rivalry and was used for reporting which of the two patterns was visible at each time. As the perception of the subjects alternated between coherent motion and motion noise, we examined the effect that these alternations had on the strength of the MR signal throughout the brain. Our results demonstrate that motion perception is able to modulate the activity of several of the visual areas which are known to be involved in motion processing. More specifically, in addition to area V5 which showed the strongest modulation, a higher activity during the perception of motion than during the perception of noise was also clearly observed in areas V3A and LOC, and less so in area V3. In previous studies, these areas had been selectively activated by motion stimuli but whether their activity reflects motion perception or not remained unclear; here we show that they are involved in motion perception as well. The present findings therefore suggest a lack of a clear distinction between 'processing' versus 'perceptual' areas in the brain, but rather that the areas involved in the processing of a specific visual attribute are also part of the neuronal network that is collectively responsible for its perceptual representation.     

Discriminating binocular fusion from false fusion

In a series of psychophysical experiments, observers discriminated between briefly flashed stimuli (cosine gratings, cosine plaids) that were either identical to the two eyes (dioptic) or differed between the two eyes (dichoptic). Although dioptic and dichoptic binocular stimuli were perceptually similar, they were distinguishable well above chance at exposure durations too brief for the onset of binocular rivalry. Random variations in display contrast did not alter this pattern of results. These results show that the neural events that signal false fusion of dissimilar monocular stimuli are not equivalent to those that underlie binocular fusion of matched monocular views.     

Exogenous attention and endogenous attention influence initial dominance in binocular rivalry

We investigated the influence of exogenous and endogenous attention on initial selection in binocular rivalry. Experiment 1 used superimposed +/-45 degrees gratings viewed dioptically for 3s, followed by a brief contrast increment in one of the gratings to direct exogenous attention to that grating. After a brief blank period, dichoptic stimuli were presented for various durations (100-700 ms). Exogenous attention strongly influenced which stimulus was initially dominant in binocular rivalry, replicating an earlier report (Mitchell, Stoner, & Reynolds. (2004). Object-based attention determines dominance in binocular rivalry. Nature, 429, 410-413). In Experiment 2, endogenous attention was manipulated by having participants track one of two oblique gratings both of which independently and continuously changed their orientations and spatial frequencies during a 5s period. The initially dominant grating was most often the one whose orientation matched the grating correctly tracked using endogenous attention. In Experiment 3, we measured the strength of both exogenous and endogenous attention by varying the contrast of one of two rival gratings when attention was previously directed to that grating. The contrast of the attended grating had to be reduced by an amount in the neighborhood of 0.3 log-units, to counteract attention's boost to initial dominance. Evidently both exogenous and endogenous attention can influence initial dominance of binocular rivalry, effectively boosting the stimulus strength of the attended rival stimulus.     

Alternation rate in perceptual bistability is maximal at and symmetric around equi-dominance

When an ambiguous stimulus is viewed for a prolonged time, perception alternates between the different possible interpretations of the stimulus. The alternations seem haphazard, but closer inspection of their dynamics reveals systematic properties in many bistable phenomena. Parametric manipulations result in gradual changes in the fraction of time a given interpretation dominates perception, often over the entire possible range of zero to one. The mean dominance durations of the competing interpretations can also vary over wide ranges (from less than a second to dozens of seconds or more), but finding systematic relations in how they vary has proven difficult. Following the pioneering work of W. J. M. Levelt (1968) in binocular rivalry, previous studies have sought to formulate a relation in terms of the effect of physical parameters of the stimulus, such as image contrast in binocular rivalry. However, the link between external parameters and "stimulus strength" is not as obvious for other bistable phenomena. Here we show that systematic relations readily emerge when the mean dominance durations are examined instead as a function of "percept strength," as measured by the fraction of dominance time, and provide theoretical rationale for this observation. For three different bistable phenomena, plotting the mean dominance durations of the two percepts against the fraction of dominance time resulted in complementary curves with near-perfect symmetry around equi-dominance (the point where each percept dominates half the time). As a consequence, the alternation rate reaches a maximum at equi-dominance. We next show that the observed behavior arises naturally in simple double-well energy models and in neural competition models with cross-inhibition and input normalization. Finally, we discuss the possibility that bistable perceptual switches reflect a perceptual "exploratory" strategy, akin to foraging behavior, which leads naturally to maximal alternation rate at equi-dominance if perceptual switches come with a cost.     

Independent binocular integration for form and colour

Although different features of an object are processed in anatomically distinct regions of the cerebral cortex, they often appear bound together in perception. Here, using binocular rivalry, we reveal that the awareness of form can occur independently from the awareness of colour. First, we report that, if both eyes briefly view a grating stimulus prior to the presentation of the same grating in one eye and an orthogonal grating in the other, subjects tend to report perceptual dominance of the non-primed grating. The primer was most effective when it was similar in orientation, spatial frequency and spatial phase to one of the rival images. Next, we showed that the process underlying the binocular integration of chromatic information was selectively influenced by the colour of a previously presented stimulus. We then combined these paradigms by using a primer that had the same colour as one rival stimulus, but the same form as the other stimulus. In this situation, we found that rival stimuli differing in form and colour can sometimes achieve states of dominance in which the chromatic information from one eye's image combines with the form of the other eye's image temporarily creating a binocular impression that corresponds with neither monocular component. Finally, we demonstrated that during continuous viewing of rival stimuli differing in form and colour, chromatic integration could occur independently of form rivalry. Paradoxically, however, we found that changes to the form of the stimulus had more of an influence on chromatic integration than on form rivalry. Together these phenomena show that the neural processes involved in integrating information from the two eyes can operate selectively on different stimulus features.     

Stimulus flicker alters interocular grouping during 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.     

Development of rivalry and dichoptic masking in human infants

PURPOSE: To examine the development of rivalry, dichoptic masking, and binocular interactions in infants more than 5 months of age using the visual evoked potential (VEP). METHODS: VEPs were recorded in 35 infants between 5 and 15 months of age and 23 adults between 13 and 59 years of age. Counterphasing, sinusoidal, 1 cycle/deg gratings were presented dichoptically. Responses from each eye were isolated by "tagging" each half-image with a different temporal frequency (5 or 7.5 Hz). Observers were presented with fixed 80% contrast gratings in each eye in experiment 1. Rivalry was detected on the basis of a negative correlation between the simultaneously measured response amplitudes at the second harmonics of the two eye-tagging frequencies. In a second analysis of the same data, response amplitudes recorded under dichoptic viewing conditions were compared to those obtained in a monocular control condition (dichoptic masking). In experiment 2, a 40% fixed-contrast grating was presented to one eye, whereas the other eye viewed a grating that was swept in contrast from 1% to 67%. Dichoptic masking was measured as the reduction in the fixed-grating response caused by the variable contrast grating. RESULTS: Experiment 1: although adults showed evidence of VEP amplitude alternations between the eyes for cross-oriented half-images (physiological rivalry), infants did not. This immature response to rivalrous stimuli occurred despite the presence of responses at nonlinear combination frequencies recorded with gratings of the same orientation in each eye, a definitive indication of binocular interaction. In addition, both iso- and cross-oriented half-images produced less dichoptic masking in infants than in adults in this experiment. Experiment 2: dichoptic masking in the infants was equivalent to that seen in adults with parallel gratings in the two eyes; however, masking with cross-oriented configurations was approximately five times weaker in the infants relative to the adults. CONCLUSIONS: The authors have identified a set of stimulus conditions under which infants between 5 and 15 months of age fail to demonstrate physiological rivalry despite the presence of binocular interactions. The observed lack of binocular rivalry may be the result of a specific immaturity in dichoptic, cross-orientation suppression.     

Configuration saliency revealed in short duration binocular rivalry.

Supra-threshold spatial integration was studied by testing the saliency of multi-Gabor element configurations in short duration binocular rivalry (dichoptic masking) conditions. Dichoptic presentations allow for a competition between spatially overlapping supra-threshold stimuli that involve non-overlapping monocular receptive fields in the first stage of visual filtering. Different spatial configurations of Gabor patches (sigma = lambda = 0.12 degree) were presented to one eye (target) together with a bandpass noise presented to the other eye (mask). After a short rivalry period (120 ms) in which a dominance of one eye was established, a probe (a randomly positioned small rectangle of reduced contrast in the target) was presented for additional detection period (80 ms). Probe detection performance was measured (two-alternative-forced choice paradigm (2AFC) by finding the mask contrast leading to 79% correct response. Results show that configuration saliency is consistently expressed as dominance in short-duration binocular rivalry, with similar results obtained for longer durations (200 ms and continuous presentations). We find that textures of high-contrast randomly oriented patches are more dominant than uniform textures where the effect decreases and eventually reverses with decreasing of contrast. For supra-threshold contours, however, we find that smooth collinear contours are more dominant than 'jagged' ones, regardless of phase and contrast. These findings suggest principles underlying early lateral integration mechanisms based on contrast dependent inhibitory and excitatory connections. This mechanism could be based on iso-orientation surround (2D) inhibition and collinear (1D) facilitation, with inhibition being more effective at high contrasts.     

Binocular motion rivalry in macaque monkeys: eye dominance and tracking eye movements

When the two eyes are exposed to markedly different patterns, perception becomes unstable, falling into oscillations, so that the image of one eye is seen first and then that from the other. With large stimuli the alternation is piecemeal, whilst when small stimuli are used the whole pattern alternates in unison. The purpose of this study was to determine whether a reliable, objective indicator of the perceptual state during binocular rivalry could be developed in the nonhuman primate. Monkeys (Macaca mulatta) were trained to discriminate direction of motion when presented with vertically drifting gratings moving in opposite directions in the two eyes. A high correlation was found between the direction of the slow phase of the optokinetic nystagmus (OKN) elicited by the drifting gratings during rivalry and the direction of motion reported by the monkey even though the gain of the OKN was reduced during rivalry, and the latency was longer. Behavioral eye dominance during rivalry varied significantly over time, between individuals and as a function of interocular contrast differences. Since the direction of tracking eye movements can be used to reliably monitor perceptual state during binocular motion rivalry, the opportunity exists in nonhuman primates to study the neurophysiological mechanisms underlying motion perception during the perceptually ambiguous condition of binocular rivalry.     

Monocular rivalry exhibits three hallmarks of binocular rivalry: Evidence for common processes

Binocular rivalry occurs when different images are presented one to each eye: the images are visible only alternately. Monocular rivalry occurs when different images are presented both to the same eye: the clarity of the images fluctuates alternately. Could both sorts of rivalry reflect the operation of a general visual mechanism for dealing with perceptual ambiguity? We report four experiments showing similarities between the two phenomena. First, we show that monocular rivalry can occur with complex images, as with binocular rivalry, and that the two phenomena are affected similarly by the size (Experiment 1) and colour (Experiment 2) of the images. Second, we show that the distribution of dominance periods during monocular rivalry has a gamma shape and is stochastic (Experiment 3). Third, we show that during periods of monocular-rivalry suppression, the threshold to detect a probe (a contrast pulse to the suppressed stimulus) is raised compared with during periods of dominance (Experiment 4). The threshold elevation is much weaker than during binocular rivalry, consistent with monocular rivalry’s weak appearance. We discuss other similarities between monocular and binocular rivalry, and also some differences, concluding that part of the processing underlying both phenomena is a general visual mechanism for dealing with perceptual ambiguity.     

Some psychophysical tasks measure ocular dominance plasticity more reliably than others

In the recent decade, studies have shown that short-term monocular deprivation strengthens the deprived eye''s contribution to binocular vision. However, the magnitude of the change in eye dominance after monocular deprivation (i.e., the patching effect) has been found to be different between different methods and within the same method. There are three possible explanations for the discrepancy. First, the mechanisms underlying the patching effect that are probed by different measurement tasks might exist at different neural sites. Second, the test–retest variability of the same test can produce inconsistent results. Third, the magnitude of the patching effect itself within the same observer can vary across separate days or experimental sessions. To explore these possibilities, we assessed the test–retest reliability of the three most commonly used tasks (binocular rivalry, binocular combination, and dichoptic masking) and the repeatability of the shift in eye dominance after short-term monocular deprivation for each of the task. Two variations for binocular phase combination were used, at one and many contrasts of the stimuli. Also, two variations for dichoptic masking were employed; the orientation of the mask grating was either horizontal or vertical. Thus, five different tasks were evaluated. We hoped to resolve some of the inconsistencies reported in the literature concerning this form of visual plasticity. In this study, we also aimed to recommend a measurement method that would allow us to better understand its physiological basis and the underpinning of visual disorders.     

Retinal image shifts, but not eye movements per se, cause alternations in awareness during binocular rivalry

Particularly promising studies on visual awareness exploit a generally used perceptual bistability phenomenon, "binocular rivalry"--in which the two eyes' images alternately dominate--because it can dissociate the visual input from the perceptual output. To successfully study awareness, it is crucial to know the extent to which eye movements alter the input. Although there is convincing evidence that perceptual alternations can occur without eye movements, the literature on their exact role is mixed. Moreover, recent work has demonstrated that eye movements, first, correlate positively with perceptual alternations in binocular rivalry, and second, often accompany covert attention shifts (that were previously thought to be purely mental). Here, we asked whether eye movements cause perceptual alternations, and if so, whether it is either the execution of the eye movement or the resulting retinal image change that causes the alternation. Subjects viewed repetitive line patterns, enabling a distinction of saccades that did produce foveal image changes from those that did not. Subjects reported binocular rivalry alternations. We found that, although a saccade is not essential to initiate percept changes, the foveal image change resulting from a (micro)saccade is a deciding factor for percept dominance. We conclude that the foveal image must change to have a saccade cause a change in awareness. This sheds new light on the interaction between spatial attention shifts and perceptual alternations.