Individual differences in the temporal dynamics of binocular rivalry and stimulus rivalry

Tracking rivalry with neural rhythms: multivariate SSVEPs reveal perception during binocular rivalry

The contents of awareness can substantially change without any modification to the external world. Such effects are exemplified in binocular rivalry, where a different stimulus is presented to each eye causing instability in perception. This phenomenon has made binocular rivalry a quintessential method for studying consciousness and the necessary neural correlates for awareness. However, to conduct research on binocular rivalry usually requires self-reports of changes in percept, which can produce confounds and exclude states and contexts where self-reports are undesirable or unreliable. Here, we use a novel multivariate spatial filter dubbed 'Rhythmic Entrainment Source Separation' to extract steady state visual evoked potentials from electroencephalography data. We show that this method can be used to quantify the perceptual switch-rate of participants during binocular rivalry and therefore may be valuable in experimental contexts where self-reports are methodologically problematic or impossible, particularly as an adjunct. Our analyses also reveal that 'no-report' conditions may affect the deployment of attention and thereby neural correlates, another important consideration for consciousness research.

Cortical theta phase synchronization involved in mismatch-driven perceptual alternation in binocular rivalry

When two conflicting images are presented to each eye, a phenomenon called binocular rivalry occurs in which we initially perceive one image, and then our perception switches to the other over time. An enhancement of θ-band phase coherence in visual mismatch oscillatory response (vMOR) is reported to be involved in the facilitation of perceptual alternation when the deviant stimulus is presented unconsciously. In this study, we investigated the modulation effect of θ-band transcranial alternating current stimulation (tACS) on perceptual alternation in binocular rivalry, with a focus on its relationship with the θ-band vMOR. The results showed that tACS had no significant effect on the mean proportion of perceptual alternation. Analyzing the differential effects of the modulation, however, we found a positive correlation between the increase in inter-trial phase coherence of the vMOR and the promotion of perceptual alternation under the unconscious deviant condition. Additionally, our findings indicate that the θ-band phase synchrony between frontal and occipital electrode sides, as measured by the phase lag index, is implicated in perceptual alternation, with an increase (decrease) in connection density observed in participants whose perceptual alternation was increased (decreased) by tACS. These results support the hypothesis that deviant visual stimuli evoke θ-band phase synchrony between the frontal and occipital cortices, thereby enhancing perceptual alternation in binocular rivalry.

The role of pattern coherence in interocular grouping during binocular rivalry: Insights from individual differences

Interocular grouping during binocular rivalry occurs when two images presented to each eye combine into a coherent pattern. The experience of interocular grouping is thought to be influenced by both eye-of-origin, which involves excitatory lateral connections among monocular neurons, and pattern coherence, which results from top-down intervention from higher visual areas. However, it remains unclear which factor plays a more significant role in the interocularly-grouped percepts during binocular rivalry. The current study employed an individual difference approach to investigate whether grouping dynamics are mainly determined by eye-of-origin or pattern coherence. We found that participants who perceived interocularly-driven coherent percepts for a longer duration also tended to experience longer periods of monocularly-driven coherent percepts. In contrast, participants who experienced non-coherent piecemeal percepts for an extended duration in conventional rivalry also had longer duration of non-coherent percepts in the interocular coherence setting. This individual differences in experiencing interocular grouping suggest that pattern coherence exerts a stronger influence on grouping dynamics during binocular rivalry compared to eye-of-origin factors.

Neural Dynamics during Binocular Rivalry: Indications from Human Lateral Geniculate Nucleus

When two sufficiently different stimuli are presented to each eye, perception alternates between them. This binocular rivalry is conceived as a competition for representation in the single stream of visual consciousness. The magnocellular (M) and parvocellular (P) pathways, originating in the retina, encode disparate information, but their potentially different contributions to binocular rivalry have not been determined. Here, we used functional magnetic resonance imaging to measure the human lateral geniculate nucleus (LGN), where the M and P neurons are segregated into layers receiving input from a single eye. We had three participants (one male, two females) and used achromatic stimuli to avoid contributions from color opponent neurons that may have confounded previous studies. We observed activity in the eye-specific regions of LGN correlated with perception, with similar magnitudes during rivalry or physical stimuli alternations, also similar in the M and P regions. These results suggest that LGN activity reflects our perceptions during binocular rivalry and is not simply an artifact of color opponency. Further, perception appears to be a global phenomenon in the LGN, not just limited to a single information channel.

Individual differences point to two separate processes involved in the resolution of binocular rivalry

Although binocular rivalry is different from other perceptually bistable phenomena in requiring interocular conflict, it also shares numerous features with those phenomena. This raises the question of whether, and to what extent, the neural bases of binocular rivalry and other bistable phenomena overlap. Here we examine this question using an individual-differences approach. In a first experiment, observers reported perception during four binocular rivalry tasks that differed in the features and retinal locations of the stimuli used. Perceptual dominance durations were highly correlated when compared between stimuli that differed in location only. Correlations were substantially weaker, however, when comparing stimuli comprised of different features. Thus, individual differences in binocular-rivalry perception partly reflect a feature-specific factor that is not shared among all variants of binocular rivalry. Our second experiment again included several binocular rivalry variants, but also a different form of bistability: moving plaid rivalry. Correlations in dominance durations between binocular rivalry variants that differed in feature content were again modest. Moreover, and surprisingly, correlations between binocular rivalry and moving plaid rivalry were of similar magnitude. This indicates a second, more general, factor underlying individual differences in binocular rivalry perception: one that is shared across binocular rivalry and moving plaid rivalry. We propose that the first, feature-specific factor corresponds to feature-tuned mechanisms involved in the treatment of interocular conflict, whereas the second, general factor corresponds to mechanisms involved in representing surfaces. These latter mechanisms would operate at a binocular level and be central to both binocular rivalry and other forms of bistability.

Predictive context biases binocular rivalry in children and adults with no positive relation to two measures of social cognition

Integration of prior experience and contextual information can help to resolve perceptually ambiguous situations and might support the ability to understand other peoples' thoughts and intentions, called Theory of Mind. We studied whether the readiness to incorporate contextual information for resolving binocular rivalry is positively associated with Theory-of-Mind-related social cognitive abilities. In children (12 to 13 years) and adults (18 to 25 years), a predictive temporal context reliably modulated the onset of binocular rivalry to a similar degree. In contrast, adult participants scored better on measures of Theory of Mind compared to children. We observed considerable interindividual differences regarding the influence of a predictive context on binocular rivalry, which were associated with differences in sensory eye dominance. The absence of a positive association between predictive effects on perception and Theory of Mind performance suggests that predictive effects on binocular rivalry and higher-level Theory-of-Mind-related abilities stem from different neurocognitive mechanisms. We conclude that the influence of predictive contextual information on basic visual processes is fully developed at an earlier age, whereas social cognitive skills continue to evolve from adolescence to adulthood.

What happens in the brain of meditators when perception changes but not the stimulus?

During the observation of an ambiguous figure our perception alternates between mutually exclusive interpretations, although the stimulus itself remains unchanged. The rate of these endogenous reversals has been discussed as reflecting basic aspects of endogenous brain dynamics. Recent evidence indicates that extensive meditation practice evokes long-term functional and anatomic changes in the brain, also affecting the endogenous brain dynamics. As one of several consequences the rate of perceptual reversals during ambiguous figure perception decreases. In the present study we compared EEG-correlates of endogenous reversals of ambiguous figures between meditators and non-meditating controls in order to better understand timing and brain locations of this altered endogenous brain dynamics. A well-established EEG paradigm was used to measure the neural processes underlying endogenous perceptual reversals of ambiguous figures with high temporal precision. We compared reversal-related ERPs between experienced meditators and non-meditating controls. For both groups we found highly similar chains of reversal-related ERPs, starting early in visual areas, therewith replicating previous findings from the literature. Meditators, however, showed an additional frontal ERP signature already 160 ms after stimulus onset (Frontal Negativity). We interpret the additional, meditation-specific ERP results as evidence that extensive meditation practice provides control of frontal brain areas over early sensory processing steps. This may allow meditators to overcome phylogenetically evolved perceptual and attentional processing automatisms.

Individual variation in inter-ocular suppression and sensory eye dominance

The competitive and inhibitory interactions between the two eyes' images are a pervasive aspect of binocular vision. Over the last decade, our understanding of the neural processes underpinning binocular rivalry (BR) and continuous flash suppression (CFS) has increased substantially, but we still have little understanding of the relationship between these two effects and their variation in the general population. Studies that pool data across individuals and eyes risk masking substantial variations in binocular vision that exist in the general population. To investigate this issue we compared the depth of inter-ocular suppression evoked by BR with that elicited by CFS, in a group (N = 25) of visually normal individuals. A noise pattern (either static for BR or dynamic for CFS) was presented to one eye and its suppressive influence on a probe grating presented simultaneously to the other eye was measured. We found substantial individual differences in the magnitude of suppression (a 10-fold variation in probe detection threshold) evoked by each task, but performance on BR was a significant predictor of performance on the CFS task. However many individuals showed marked asymmetries between the two eyes' ability to detect a suppressed target, that were not necessarily the same for the two tasks. There was a tendency for the magnitude of the asymmetry to increase as the refresh rate of the dynamic noise increased. The results suggest a common underlying mechanism is likely to be responsible, at least in part, for driving inter-ocular suppression under BR and CFS. The marked asymmetries in inter-ocular suppression at higher noise refresh rates, may be indicative of a difference in temporal processing between the eyes.

Tagged MEG measures binocular rivalry in a cortical network that predicts alternation rate

Binocular rivalry (BR) is a dynamic visual illusion that provides insight into the cortical mechanisms of visual awareness, stimulus selection, and object identification. When dissimilar binocular images cannot be fused, perception switches every few seconds between the left and right eye images. The speed at which individuals switch between alternatives is a stable, partially heritable trait. In order to isolate the monocular and binocular processes that determine the speed of rivalry, we presented stimuli tagged with a different flicker frequency in each eye and applied stimulus-phase locked MEG source imaging. We hypothesized that the strength of the evoked fundamental or intermodulation frequencies would vary when comparing Fast and Slow Switchers. Ten subjects reported perceptual alternations, with mean dominance durations between 1.2–4.0 sec. During BR, event-related monocular input in V1, and broadly in higher-tier ventral temporal cortex, waxed and waned with the periods of left or right eye dominance/suppression. In addition, we show that Slow Switchers produce greater evoked intermodulation frequency responses in a cortical network composed of V1, lateral occipital, posterior STS, retrosplenial & superior parietal cortices. Importantly, these dominance durations were not predictable from the brain responses to either of the fundamental tagging frequencies in isolation, nor from any responses to a pattern rivalry control condition, or a non-rivalrous control. The novel cortical network isolated, which overlaps with the default-mode network, may contain neurons that compute the level of endogenous monocular difference, and monitor accumulation of this conflict over extended periods of time. These findings are the first to relate the speed of rivalry across observers to the ‘efficient coding’ theory of computing binocular differences that may apply to binocular vision generally.

Frequency of alpha oscillation predicts individual differences in perceptual stability during binocular rivalry

When ambiguous visual stimuli have multiple interpretations, human perception can alternate between them, producing perceptual multistability. There is a large variation between individuals in how long stable percepts endure, on average, between switches, but the underlying neural basis of this individual difference in perceptual dynamics remains obscure. Here, we show that in one widely studied multistable paradigm-binocular rivalry-perceptual stability in individuals is predicted by the frequency of their neural oscillations within the alpha range (7-13 Hz). Our results suggest revising models of rivalry to incorporate effects of neural oscillations on perceptual alternations, and raise the possibility that a common factor may influence dynamics in many neural processes.

Two paradigms of bistable plaid motion reveal independent mutual inhibition processes

Perception is sometimes bistable, switching between two possible interpretations. Levelt developed several propositions to explain bistable perception in binocular rivalry, based on a model of competing neural populations connected through reciprocal inhibition. Here we test Levelt's laws with bistable plaid motion. Plaids are typically tristable, either a coherent pattern, transparent with one component in front, or transparent with the opposite depth order. In Experiment 1, we use a large angle between component directions to prevent plaid coherence, limiting the ambiguity to alternations of grating depth order. Similar to increasing contrast in binocular rivalry, increasing component speed led to higher switch rates (analogous to Levelt's fourth proposition). In Experiment 2, we used occlusion cues to prevent one depth order and limit bistability to one transparent depth order alternating with coherence. Increasing grating speed shortened coherent motion periods but left transparent periods largely unchanged (analogous to Levelt's second proposition). Switch dynamics showed no correlation between the experiments. These data suggest that plaid component speed acts like contrast in binocular rivalry to vary switch dynamics through a mutual inhibition model. The lack of correlation between both experiments suggests reciprocal inhibition mediates bistability between a variety of neural populations across the visual system.

Revisiting individual differences in the time course of binocular rivalry

Simultaneously showing an observer two incompatible displays, one to each eye, causes binocular rivalry, during which the observer regularly switches between perceiving one eye's display and perceiving the other. Observers differ in the rate of this perceptual cycle, and these individual differences have been reported to correlate with differences in the perceptual switch rate for other bistable perception phenomena. Identifying which psychological or neural factors explain this variability can help clarify the mechanisms underlying binocular rivalry and of bistable perception generally. Motivated by the prominent theory that perceptual switches during binocular rivalry are brought about by neural adaptation, we investigated whether perceptual switch rates are correlated with the strength of neural adaptation, indexed by visual aftereffects. We found no compelling evidence for such correlations. Moreover, we did not corroborate previous findings that switch rates are correlated between binocular rivalry and other forms of bistable perception. This latter nonreplication prompted us to perform a meta-analysis of existing research into correlations among forms of bistable perception, which revealed that evidence for such correlations is much weaker than is generally believed. By showing no common factor linking individual differences in binocular rivalry and in our other paradigms, these results fit well with other work that has shown such common factors to be rare among visual phenomena generally.

Stimulus rivalry and binocular rivalry share a common neural substrate

When two incompatible images are shown separately to each eye, a perceptual process known as binocular rivalry occurs by which the two images compete for awareness. The site of competition for binocular rivalry has been a topic of debate, and recent theories are that it may occur either at low levels of the visual system where the inputs from the two eyes are combined or at high levels of the visual system where the two images are processed. One of the major pieces of evidence for a high-level image account of rivalry is a phenomenon known as stimulus rivalry, in which two competing stimuli are swapped between the eyes at 3 Hz. However, there is little available neurophysiological evidence for a neural substrate for this high-level competition. Here, we used frequency tagging of two competing stimuli in binocular rivalry and stimulus rivalry in humans to evaluate whether the steady-state visually evoked potentials (SSVEPs) show similar signatures of neural competition for both conditions. We found that flickering the stimuli generates spectral power at the tagged frequencies in both types of rivalry in the early visual cortex. We then quantified dynamic signatures of competition by tracking amplitude changes in the frequency tags, which showed that both types of rivalry colocalized in occipital regions of the cortex. Thus, contrary to our hypothesis that stimulus rivalry was being mediated by high-level competition between the images, we find that neural competition measured by the SSVEP instead suggests that the sites of competition for stimulus rivalry and binocular rivalry may similarly include the occipital pole and middle temporal gyrus (hMT+/V5) of the visual system, consistent with a low-level, binocular interpretation.

The Independent and Shared Mechanisms of Intrinsic Brain Dynamics: Insights From Bistable Perception

In bistable perception, constant input leads to alternating perception. The dynamics of the changing perception reflects the intrinsic dynamic properties of the “unconscious inferential” process in the brain. Under the same condition, individuals differ in how fast they experience the perceptual alternation. In this study, testing many forms of bistable perception in a large number of observers, we investigated the key question of whether there is a general and common mechanism or multiple and independent mechanisms that control the dynamics of the inferential brain. Bistable phenomena tested include binocular rivalry, vase-face, Necker cube, moving plaid, motion induced blindness, biological motion, spinning dancer, rotating cylinder, Lissajous-figure, rolling wheel, and translating diamond. Switching dynamics for each bistable percept was measured in 100 observers. Results show that the switching rates of subsets of bistable percept are highly correlated. The clustering of dynamic properties of some bistable phenomena but not an overall general control of switching dynamics implies that the brain’s inferential processes are both shared and independent – faster in constructing 3D structure from motion does not mean faster in integrating components into an objects.

Does direction of walking impact binocular rivalry between competing patterns of optic flow?

When dissimilar monocular images are viewed simultaneously by the two eyes, stable binocular vision gives way to unstable vision characterized by alternations in dominance between the two images in a phenomenon called binocular rivalry. These alternations in perception reveal the existence of inhibitory interactions between neural representations associated with conflicting visual inputs. Binocular rivalry has been studied since the days of Wheatstone, but one recent strategy is to investigate its susceptibility to influences caused by one's own motor activity. This paper focused on the activity of walking, which produces an expected, characteristic direction of optic flow dependent upon the direction of one's walking. In a set of experiments, we employed virtual reality technology to present dichoptic stimuli to observers who walked forward, backward, or were sitting. Optic flow was presented to a given eye, and was sometimes congruent with the direction of walking, sometimes incongruent, and sometimes random, except when the participant was sitting. Our results indicate that, while walking had a reliable influence on rivalry dynamics, the predominance of congruent or incongruent motion did not.

The effect of stimulus strength on binocular rivalry rate in healthy individuals: Implications for genetic, clinical and individual differences studies

Binocular rivalry (BR) occurs when conflicting images concurrently presented to corresponding retinal locations of each eye stochastically alternate in perception. Anomalies of BR rate have been examined in a range of clinical psychiatric conditions. In particular, slow BR rate has been proposed as an endophenotype for bipolar disorder (BD) to improve power in large-scale genome-wide association studies. Examining the validity of BR rate as a BD endophenotype however requires large-scale datasets (n=1000s to 10,000s), a standardized testing protocol, and optimization of stimulus parameters to maximize separation between BD and healthy groups. Such requirements are indeed relevant to all clinical psychiatric BR studies. Here we address the issue of stimulus optimization by examining the effect of stimulus parameter variation on BR rate and mixed-percept duration (MPD) in healthy individuals. We aimed to identify the stimulus parameters that induced the fastest BR rates with the least MPD. Employing a repeated-measures within-subjects design, 40 healthy adults completed four BR tasks using orthogonally drifting grating stimuli that varied in drift speed and aperture size. Pairwise comparisons were performed to determine modulation of BR rate and MPD by these stimulus parameters, and individual variation of such modulation was also assessed. From amongst the stimulus parameters examined, we found that 8cycles/s drift speed in a 1.5° aperture induced the fastest BR rate without increasing MPD, but that BR rate with this stimulus configuration was not substantially different to BR rate with stimulus parameters we have used in previous studies (i.e., 4cycles/s drift speed in a 1.5° aperture). In addition to contributing to stimulus optimization issues, the findings have implications for Levelt's Proposition IV of binocular rivalry dynamics and individual differences in such dynamics.

Attention model of binocular rivalry

When the corresponding retinal locations in the two eyes are presented with incompatible images, a stable percept gives way to perceptual alternations in which the two images compete for perceptual dominance. As perceptual experience evolves dynamically under constant external inputs, binocular rivalry has been used for studying intrinsic cortical computations and for understanding how the brain regulates competing inputs. Converging behavioral and EEG results have shown that binocular rivalry and attention are intertwined: binocular rivalry ceases when attention is diverted away from the rivalry stimuli. In addition, the competing image in one eye suppresses the target in the other eye through a pattern of gain changes similar to those induced by attention. These results require a revision of the current computational theories of binocular rivalry, in which the role of attention is ignored. Here, we provide a computational model of binocular rivalry. In the model, competition between two images in rivalry is driven by both attentional modulation and mutual inhibition, which have distinct selectivity (feature vs. eye of origin) and dynamics (relatively slow vs. relatively fast). The proposed model explains a wide range of phenomena reported in rivalry, including the three hallmarks: (i) binocular rivalry requires attention; (ii) various perceptual states emerge when the two images are swapped between the eyes multiple times per second; (iii) the dominance duration as a function of input strength follows Levelt's propositions. With a bifurcation analysis, we identified the parameter space in which the model's behavior was consistent with experimental results.

Chromatic interocular-switch rivalry

Interocular-switch rivalry (also known as stimulus rivalry) is a kind of binocular rivalry in which two rivalrous images are swapped between the eyes several times a second. The result is stable periods of one image and then the other, with stable intervals that span many eye swaps (Logothetis, Leopold, & Sheinberg, 1996). Previous work used this close kin of binocular rivalry with rivalrous forms. Experiments here test whether chromatic interocular-switch rivalry, in which the swapped stimuli differ in only chromaticity, results in slow alternation between two colors. Swapping equiluminant rivalrous chromaticities at 3.75 Hz resulted in slow perceptual color alternation, with one or the other color often continuously visible for two seconds or longer (during which there were 15+ eye swaps). A well-known theory for sustained percepts from interocular-switch rivalry with form is inhibitory competition between binocular neurons driven by monocular neurons with matched orientation tuning in each eye; such binocular neurons would produce a stable response when a given orientation is swapped between the eyes. A similar model can account for the percepts here from chromatic interocular-switch rivalry and is underpinned by the neurophysiological finding that color-preferring binocular neurons are driven by monocular neurons from each eye with well-matched chromatic selectivity (Peirce, Solomon, Forte, & Lennie, 2008). In contrast to chromatic interocular-switch rivalry, luminance interocular-switch rivalry with swapped stimuli that differ in only luminance did not result in slowly alternating percepts of different brightnesses.

Filling-in rivalry: Perceptual alternations in the absence of retinal image conflict

During perceptual rivalry, an observer's perceptual experience alternates over time despite constant sensory stimulation. Perceptual alternations are thought to be driven by conflicting or ambiguous retinal image features at a particular spatial location and modulated by global context from surrounding locations. However, rivalry can also occur between two illusory stimuli-such as two filled-in stimuli within the retinal blind spot. In this "filling-in rivalry," what observers perceive in the blind spot changes in the absence of local stimulation. It remains unclear if filling-in rivalry shares common mechanisms with other types of rivalry. We measured the dynamics of rivalry between filled-in percepts in the blind spot, finding a high degree of exclusivity (perceptual dominance of one filled-in percept, rather than a perception of transparency), alternation rates that were highly consistent for individual observers, and dynamics that closely resembled other forms of perceptual rivalry. The results suggest that mechanisms common to a wide range of rivalry situations need not rely on conflicting retinal image signals.

Serial correlations in Continuous Flash Suppression

Research on visual rivalry has demonstrated that consecutive dominance durations are serially dependent, implying that the underlying competition mechanism is not driven by some random process but includes a memory component. Here we asked whether serial dependence is also observed in continuous flash suppression (CFS). We addressed this question by analyzing a large dataset of time series of suppression durations obtained in a series of so-called “breaking CFS” experiments in which the duration of the period is measured until a suppressed target breaks through the CFS mask. Across experimental manipulations, stimuli, and observers, we found that (i) the distribution of breakthrough rates was fit less well by a gamma distribution than in conventional visual rivalry paradigms, (ii) the suppression duration on a previous trial influenced the suppression duration on a later trial up to as long as a lag of eight trials, and (iii) the mechanism underlying these serial correlations was predominantly monocular. We conclude that the underlying competition mechanism of CFS also includes a memory component that is primarily, but not necessarily exclusively, monocular in nature. We suggest that the temporal dependency structure of suppression durations in CFS is akin to those observed in binocular rivalry, which might imply that both phenomena tap into similar rather than distinct mechanisms.