Visible binocular beats from invisible monocular stimuli during binocular rivalry

When two qualitatively different stimuli are presented at the same time, one to each eye, the stimuli can either integrate or compete with each other. When they compete, one of the two stimuli is alternately suppressed, a phenomenon called binocular rivalry [1,2]. When they integrate, observers see some form of the combined stimuli. Many different properties (for example, shape or color) of the two stimuli can induce binocular rivalry. Not all differences result in rivalry, however. Visual 'beats', for example, are the result of integration of high-frequency flicker between the two eyes [3,4], and are thus a binocular fusion phenomenon. It remains in dispute whether binocular fusion and rivalry can co-exist with one another [5-7]. Here, we report that rivalry and beats, two apparently opposing phenomena, can be perceived at the same time within the same spatial location. We hypothesized that the interocular difference in visual attributes that are predominantly processed in the Parvocellular pathway will lead to rivalry, and differences in visual attributes that are predominantly processed in the Magnocellular pathway tend to integrate. Further predictions based on this hypothesis were tested and confirmed.     

Suppressed patterns alter vision during binocular rivalry

Binocular rivalry occurs when incongruent patterns are presented to corresponding regions of the retinas, leading to fluctuations of awareness between the patterns . One attribute of a stimulus may rival whereas another may combine between the eyes , but it is typically assumed that the dominant features are perceived veridically. Here, we show this is not necessarily the case and that a suppressed visual feature can alter dominant perception. The cortical representations of oriented gratings can interact even when one of them is perceptually suppressed, such that the perceived orientation of the dominant grating is systematically biased depending on the orientation of the suppressed grating. A suppressed inducing pattern has the same qualitative effect as a visible one, but suppression reduces effective contrast by a factor of around six. A simple neural model quantifies and helps explain these illusions. These results demonstrate that binocular rivalry suppression operates in a graded fashion across multiple sites in the visual hierarchy rather than truncating processing at a single site and that suppressed visual information can alter dominant vision in real-time.     

Subcortical discrimination of unperceived objects during binocular rivalry

Rapid identification of behaviorally relevant objects is important for survival. In humans, the neural computations for visually discriminating complex objects involve inferior temporal cortex (IT). However, less detailed but faster form processing may also occur in a phylogenetically older subcortical visual system that terminates in the amygdala. We used binocular rivalry to present stimuli without conscious awareness, thereby eliminating the IT object representation and isolating subcortical visual input to the amygdala. Functional magnetic resonance imaging revealed significant brain activation in the left amygdala but not in object-selective IT in response to unperceived fearful faces compared to unperceived nonface objects. These findings indicate that, for certain behaviorally relevant stimuli, a high-level cortical representation in IT is not required for object discrimination in the amygdala.     

Visual motion,binocular correspondence and binocular rivalry

The human pupillary control system has been the subject of interest to biologists and engineers as an example of a sensorimotor reflex which can be embedded in a control system paradigm. We present a nonlinear feedback model whose compact structure allows us to hypothesize possible physiological mechanisms which generate the proper behavior of the pupil system. The important pupil responses, including pupil size effect, asymmetry, and response to high-frequency stimuli, are defined. This model was simulated on a digital computer and comparisons to the paradigm experimental responses were performed, demonstrating a fit to each of the observed conditions. Improvements on previous models are discussed.     

The role of temporally coarse form processing during binocular rivalry

Presenting the eyes with spatially mismatched images causes a phenomenon known as binocular rivalry-a fluctuation of awareness whereby each eye's image alternately determines perception. Binocular rivalry is used to study interocular conflict resolution and the formation of conscious awareness from retinal images. Although the spatial determinants of rivalry have been well-characterized, the temporal determinants are still largely unstudied. We confirm a previous observation that conflicting images do not need to be presented continuously or simultaneously to elicit binocular rivalry. This process has a temporal limit of about 350 ms, which is an order of magnitude larger than the visual system's temporal resolution. We characterize this temporal limit of binocular rivalry by showing that it is independent of low-level information such as interocular timing differences, contrast-reversals, stimulus energy, and eye-of-origin information. This suggests the temporal factors maintaining rivalry relate more to higher-level form information, than to low-level visual information. Systematically comparing the role of form and motion-the processing of which may be assigned to ventral and dorsal visual pathways, respectively-reveals that this temporal limit is determined by form conflict rather than motion conflict. Together, our findings demonstrate that binocular conflict resolution depends on temporally coarse form-based processing, possibly originating in the ventral visual pathway.     

Reduced models for binocular rivalry

Binocular rivalry occurs when two very different images are presented to the two eyes, but a subject perceives only one image at a given time. A number of computational models for binocular rivalry have been proposed; most can be categorised as either “rate” models, containing a small number of variables, or as more biophysically-realistic “spiking neuron” models. However, a principled derivation of a reduced model from a spiking model is lacking. We present two such derivations, one heuristic and a second using recently-developed data-mining techniques to extract a small number of “macroscopic” variables from the results of a spiking neuron model simulation. We also consider bifurcations that can occur as parameters are varied, and the role of noise in such systems. Our methods are applicable to a number of other models of interest.     

The dynamic model of binocular rivalry

Binocular rivalry is an interesting phenomenon observed in the human vision. It occurs when the right and left eyes are given different stimuli (pictures). This paper describes a mathematical model which explains the mechanism of binocular rivalry. Our basic assumption is that binocular rivalry is elicited by the mutual inhibition between the right and left visual neuron systems. The mutual inhibition between two neurons is first discussed in detail, where a special emphasis is put on a fatigue effect of neurons, and then its results are applied to a simulation model of binocular rivalry.     

Quantum formalism to describe binocular rivalry

On the basis of the general character and operation of the process of perception, a formalism is sought to mathematically describe the subjective or abstract/mental process of perception. It is shown that the formalism of orthodox quantum theory of measurement, where the observer plays a key role, is a broader mathematical foundation which can be adopted to describe the dynamics of the subjective experience. The mathematical formalism describes the psychophysical dynamics of the subjective or cognitive experience as communicated to us by the subject. Subsequently, the formalism is used to describe simple perception processes and, in particular, to describe the probability distribution of dominance duration obtained from the testimony of subjects experiencing binocular rivalry. Using this theory and parameters based on known values of neuronal oscillation frequencies and firing rates, the calculated probability distribution of dominance duration of rival states in binocular rivalry under various conditions is found to be in good agreement with available experimental data. This theory naturally explains an observed marked increase in dominance duration in binocular rivalry upon periodic interruption of stimulus and yields testable predictions for the distribution of perceptual alteration in time.     

Neural field model of binocular rivalry waves

We present a neural field model of binocular rivalry waves in visual cortex. For each eye we consider a one-dimensional network of neurons that respond maximally to a particular feature of the corresponding image such as the orientation of a grating stimulus. Recurrent connections within each one-dimensional network are assumed to be excitatory, whereas connections between the two networks are inhibitory (cross-inhibition). Slow adaptation is incorporated into the model by taking the network connections to exhibit synaptic depression. We derive an analytical expression for the speed of a binocular rivalry wave as a function of various neurophysiological parameters, and show how properties of the wave are consistent with the wave-like propagation of perceptual dominance observed in recent psychophysical experiments. In addition to providing an analytical framework for studying binocular rivalry waves, we show how neural field methods provide insights into the mechanisms underlying the generation of the waves. In particular, we highlight the important role of slow adaptation in providing a “symmetry breaking mechanism” that allows waves to propagate.     

The interaction between binocular rivalry and negative afterimages

Afterimage formation, historically attributed to retinal mechanisms, may also involve postretinal process. Consistent with this notion are results from experiments, reported here, investigating the interaction between binocular rivalry and negative afterimages (AIs). In Experiment 1, one eye was exposed to a grating never consciously experienced by the observer because this grating remained suppressed in rivalry throughout induction (the exclusively dominant stimulus was designed to preclude formation of an AI). As expected, the suppressed grating generated a vivid AI whose orientation could be accurately identified; not surprisingly, the strength of this AI varied with induction contrast. Experiment 2 revealed, however, that the strength of this AI produced during suppression was significantly weaker than the AI produced by that same stimulus when it was visible throughout the entire induction period, implying that some component of AI induction is susceptible to interocular suppression. In Experiment 3, AIs of dichoptic, orthogonally oriented gratings were induced in a way ensuring that one of the two gratings was exclusively dominant during the induction period. Dissimilar monocular AIs engaged in rivalry, as expected, but, surprisingly, the AI induced by the suppressed grating initially dominated. We offer two alternative accounts of this counterintuitive finding, both based on differential neural adaptation.     

Independent binocular rivalry processes for motion and form

During binocular rivalry, conflicting monocular images undergo alternating suppression. This study explores rivalry suppression by probing visual sensitivity during rivalry with various probe stimuli. When two faces engage in rivalry, sensitivity to face probes is reduced 4-fold during suppression. Rivaling global motions also rivaled very deeply when probed with a global motion. However, in a surprising finding, sensitivity to face probes is completely unimpaired during global motion rivalry, and motion sensitivity is unimpaired during face rivalry. This suggests that rivalry suppression is localized to the neurons representing the image conflict, which means that probes of a different kind suffer no suppression. Sensibly, this would leave visual processes not involved in rivalry free to function normally.     

Bistable percepts in the brain: FMRI contrasts monocular pattern rivalry and binocular rivalry

The neural correlates of binocular rivalry have been actively debated in recent years, and are of considerable interest as they may shed light on mechanisms of conscious awareness. In a related phenomenon, monocular rivalry, a composite image is shown to both eyes. The subject experiences perceptual alternations in which the two stimulus components alternate in clarity or salience. The experience is similar to perceptual alternations in binocular rivalry, although the reduction in visibility of the suppressed component is greater for binocular rivalry, especially at higher stimulus contrasts. We used fMRI at 3T to image activity in visual cortex while subjects perceived either monocular or binocular rivalry, or a matched non-rivalrous control condition. The stimulus patterns were left/right oblique gratings with the luminance contrast set at 9%, 18% or 36%. Compared to a blank screen, both binocular and monocular rivalry showed a U-shaped function of activation as a function of stimulus contrast, i.e. higher activity for most areas at 9% and 36%. The sites of cortical activation for monocular rivalry included occipital pole (V1, V2, V3), ventral temporal, and superior parietal cortex. The additional areas for binocular rivalry included lateral occipital regions, as well as inferior parietal cortex close to the temporoparietal junction (TPJ). In particular, higher-tier areas MT+ and V3A were more active for binocular than monocular rivalry for all contrasts. In comparison, activation in V2 and V3 was reduced for binocular compared to monocular rivalry at the higher contrasts that evoked stronger binocular perceptual suppression, indicating that the effects of suppression are not limited to interocular suppression in V1.     

Perceptual grouping of biological motion promotes binocular rivalry

Investigation of perceptual rivalry between conflicting stimuli presented one to each eye can further understanding of the neural underpinnings of conscious visual perception. During rivalry, visual awareness fluctuates between perceptions of the two stimuli. Here, we demonstrate that high-level perceptual grouping can promote rivalry between stimulus pairs that would otherwise be perceived as nonrivalrous. Perceptual grouping was generated with point-light walker stimuli that simulate human motion, visible only as lights placed on the joints. Although such walking figures are unrecognizable when stationary, recognition judgments as complex as gender and identity can accurately be made from animated displays, demonstrating the efficiency with which our visual system can group dynamic local signals into a globally coherent walking figure. We find that point-light walker stimuli presented one to each eye and in different colors and configurations results in strong rivalry. However, rivalry is minimal when the two walkers are split between the eyes or both presented to one eye. This pattern of results suggests that processing animated walker figures promotes rivalry between signals from the two eyes rather than between higher-level representations of the walkers. This leads us to hypothesize that awareness during binocular rivalry involves the integrated activity of high-level perceptual mechanisms in conjunction with lower-level ocular suppression modulated via cortical feedback.     

Stimulus motion propels traveling waves in binocular rivalry

State transitions in the nervous system often take shape as traveling waves, whereby one neural state is replaced by another across space in a wave-like manner. In visual perception, transitions between the two mutually exclusive percepts that alternate when the two eyes view conflicting stimuli (binocular rivalry) may also take shape as traveling waves. The properties of these waves point to a neural substrate of binocular rivalry alternations that have the hallmark signs of lower cortical areas. In a series of experiments, we show a potent interaction between traveling waves in binocular rivalry and stimulus motion. The course of the traveling wave is biased in the motion direction of the suppressed stimulus that gains dominance by means of the wave-like transition. Thus, stimulus motion may propel the traveling wave across the stimulus to the extent that the stimulus motion dictates the traveling wave's direction completely. Using a computational model, we show that a speed-dependent asymmetry in lateral inhibitory connections between retinotopically organized and motion-sensitive neurons can explain our results. We argue that such a change in suppressive connections may play a vital role in the resolution of dynamic occlusion situations.     

Cooperation between different spatial frequencies in binocular rivalry

In binocular rivalry, stimuli made up of any limited spatial frequency (sf-) range are perceived for a shorter time than patterns consisting of the whole sf-spectrum. This finding indicates a non-linear summation of primarily independent sf-channels in the human visual system.     

Willpower and conscious percept: volitional switching in binocular rivalry

When dissimilar images are presented to the left and right eyes, awareness switches spontaneously between the two images, such that one of the images is suppressed from awareness while the other is perceptually dominant. For over 170 years, it has been accepted that even though the periods of dominance are subject to attentional processes, we have no inherent control over perceptual switching. Here, we revisit this issue in response to evidence that top-down attention can target perceptually suppressed 'vision for action' representations in the dorsal stream. We investigated volitional control over rivalry between apparent motion (AM), drifting (DM) and stationary (ST) grating pairs. Observers demonstrated a remarkable ability to generate intentional switches in the AM and D conditions, but not in the ST condition. Corresponding switches in the pursuit direction of optokinetic nystagmus verified this finding objectively. We showed it is unlikely that intentional perceptual switches were triggered by saccadic eye movements, because their frequency was reduced substantially in the volitional condition and did not change around the time of perceptual switches. Hence, we propose that synergy between dorsal and ventral stream representations provides the missing link in establishing volitional control over rivalrous conscious percepts.     

Disrupting parietal function prolongs dominance durations in binocular rivalry

Human brain imaging studies of bistable perceptual phenomena revealed that frontal and parietal areas are activated during perceptual switches between the two conflicting percepts. However, these studies do not provide information about causality, i.e., whether activity reports a consequence or a cause of the perceptual change. Here we used functional magnetic resonance imaging to individually localize four parietal regions involved in perceptual switches during binocular rivalry in 15 subjects and subsequently disturbed their neural processing and that of a control site using 2 Hz repetitive transcranial magnetic stimulation (TMS) during binocular rivalry. We found that TMS over one of the sites, the right intraparietal sulcus (IPS), prolonged the periods of stable percepts. Additionally, the more lateralized the blood oxygen level-dependent signal was in IPS, the more lateralized the TMS effects were. Lateralization varied considerably across subjects, with a right-hemispheric bias. Control replay experiments rule out nonspecific effects of TMS on task performance, reaction times, or eye blinks. Our results thus demonstrate a causal, destabilizing, and individually lateralized effect of normal IPS function on perceptual continuity in rivalry. This is in accord with a role of IPS in perceptual selection, relating its role in rivalrous perception to that in attention.