Competition in bistable vision is attribute-specific

A new 'CFS tracking' paradigm reveals uniform suppression depth regardless of target complexity or salience

When the eyes view separate and incompatible images, the brain suppresses one image and promotes the other into visual awareness. Periods of interocular suppression can be prolonged during continuous flash suppression (CFS) - when one eye views a static 'target' while the other views a complex dynamic stimulus. Measuring the time needed for a suppressed image to break CFS (bCFS) has been widely used to investigate unconscious processing, and the results have generated controversy regarding the scope of visual processing without awareness. Here, we address this controversy with a new 'CFS tracking' paradigm (tCFS) in which the suppressed monocular target steadily increases in contrast until breaking into awareness (as in bCFS) after which it decreases until it again disappears (reCFS), with this cycle continuing for many reversals. Unlike bCFS, tCFS provides a measure of suppression depth by quantifying the difference between breakthrough and suppression thresholds. tCFS confirms that (i) breakthrough thresholds indeed differ across target types (e.g. faces vs gratings, as bCFS has shown) - but (ii) suppression depth does not vary across target types. Once the breakthrough contrast is reached for a given stimulus, all stimuli require a strikingly uniform reduction in contrast to reach the corresponding suppression threshold. This uniform suppression depth points to a single mechanism of CFS suppression, one that likely occurs early in visual processing because suppression depth was not modulated by target salience or complexity. More fundamentally, it shows that variations in bCFS thresholds alone are insufficient for inferring whether the barrier to achieving awareness exerted by interocular suppression is weaker for some categories of visual stimuli compared to others.

Bistable perception: neural bases and usefulness in psychological research

Bistable images have the possibility of being perceived in two different ways. Due to their physical characteristics, these visual stimuli allow two different perceptions, associated with top-down and bottom-up modulating processes. Based on an extensive literature review, the present article aims to gather the conceptual models and the foundations of perceptual bistability. This theoretical article compiles not only notions that are intertwined with the understanding of this perceptual phenomenon, but also the diverse classification and uses of bistable images in psychological research, along with a detailed explanation of the neural correlates that are involved in perceptual reversibility. We conclude that the use of bistable images as a paradigmatic resource in psychological research might be extensive. In addition, due to their characteristics, visual bistable stimuli have the potential to be implemented as a resource in experimental tasks that seek to understand diverse concerns linked essentially to attention, sensory, perceptual and memory processes.

Can Rotational Grouping Be Determined by the Initial Conditions?

Objects rotating in depth with an ambiguous rotation direction frequently appear to rotate together. Corotation is especially strong when the objects are interpretable as having a shared axis. We manipulated the initial conditions of the experiment by having pairs of objects initially appear to be unambiguous, and then make either a sudden or gradual transition to ambiguous spin. We find that in neither case do coaxial counter-rotating objects persist in being perceived as counter-rotating. This implies that the perceptual constraint that favors coaxial corotation overrides the initial perceptual state of the objects.

Fluctuations of visual awareness: combining motion-induced blindness with binocular rivalry

Binocular rivalry (BR) and motion-induced blindness (MIB) are two phenomena of visual awareness where perception alternates between multiple states despite constant retinal input. Both phenomena have been extensively studied, but the underlying processing remains unclear. It has been suggested that BR and MIB involve the same neural mechanism, but how the two phenomena compete for visual awareness in the same stimulus has not been systematically investigated. Here we introduce BR in a dichoptic stimulus display that can also elicit MIB and examine fluctuations of visual awareness over the course of each trial. Exploiting this paradigm we manipulated stimulus characteristics that are known to influence MIB and BR. In two experiments we found that effects on multistable percepts were incompatible with the idea of a common oscillator. The results suggest instead that local and global stimulus attributes can affect the dynamics of each percept differently. We conclude that the two phenomena of visual awareness share basic temporal characteristics but are most likely influenced by processing at different stages within the visual system.

Dynamics of temporally interleaved percept-choice sequences: interaction via adaptation in shared neural populations

At the onset of visually ambiguous or conflicting stimuli, our visual system quickly ‘chooses’ one of the possible percepts. Interrupted presentation of the same stimuli has revealed that each percept-choice depends strongly on the history of previous choices and the duration of the interruptions. Recent psychophysics and modeling has discovered increasingly rich dynamical structure in such percept-choice sequences, and explained or predicted these patterns in terms of simple neural mechanisms: fast cross-inhibition and slow shunting adaptation that also causes a near-threshold facilitatory effect. However, we still lack a clear understanding of the dynamical interactions between two distinct, temporally interleaved, percept-choice sequences—a type of experiment that probes which feature-level neural network connectivity and dynamics allow the visual system to resolve the vast ambiguity of everyday vision. Here, we fill this gap. We first show that a simple column-structured neural network captures the known phenomenology, and then identify and analyze the crucial underlying mechanism via two stages of model-reduction: A 6-population reduction shows how temporally well-separated sequences become coupled via adaptation in neurons that are shared between the populations driven by either of the two sequences. The essential dynamics can then be reduced further, to a set of iterated adaptation-maps. This enables detailed analysis, resulting in the prediction of phase-diagrams of possible sequence-pair patterns and their response to perturbations. These predictions invite a variety of future experiments.

Multistable binocular feature-integrated percepts are frozen by intermittent presentation

When two different stimuli are presented continuously to each eye, the percept alternates over time between the left-eye stimulus and right-eye stimulus. The perceptual alternation can be slowed or even stopped, however, if the same stimuli are presented intermittently (D. A. Leopold, M. Wilke, A. Maier, & N. K. Logothetis, 2002; J. Orbach, D. Ehrlich, & H. A. Heath, 1963). A basic question is the nature of the persisting neural representation, which mediates the stabilized percept. Is it a representation for the dominant eye, for the stimulus in one eye or for a feature-integrated percept incorporating features presented separately to each eye? We define a feature-integrated percept as one constructed by the visual system but which never is presented as a stimulus. This was tested using a feature-integrated percept resulting from rivalrous, equiluminant chromatic patterns (S. W. Hong & S. K. Shevell, 2009). Measurements showed that the feature-integrated percept was stabilized by intermittent viewing: when the percept at the end of the initial viewing period was feature-integrated, this same integrated percept was seen on subsequent intermittent presentations. The results showed that the stabilized percept from these intermittent rivalrous patterns was due to a persisting neural representation at or after binocular feature integration, not to a persisting dominant eye or neural representation of a retinal stimulus.

Intermittent ambiguous stimuli: implicit memory causes periodic perceptual alternations

When viewing a stimulus that has multiple plausible real-world interpretations, perception alternates between these interpretations every few seconds. Alternations can be halted by intermittently removing the stimulus from view. The same interpretation dominates over many successive presentations, and perception stabilizes. Here we study perception during long sessions of such intermittent presentation. We demonstrate that, rather than causing truly stable perception, intermittent presentation gives rise to a perceptual alternation cycle with its own characteristics and dependencies, different from those during continuous presentation. Alternations during intermittent viewing typically occur once every few minutes--much less frequently than the seconds-scale alternations during continuous viewing. Strikingly, alternations during intermittent viewing occur at fairly regular intervals, making for a surprisingly periodic alternation cycle. The duration of this cycle becomes longer as the blank duration between presentations is increased, reaching dozens of minutes in some cases. We interpret our findings in terms of a mathematical model that describes a neural network with competition between alternative interpretations. Network sensitivities depend on prior dominance, thus providing a memory for past perception. Slow changes in sensitivity produce both perceptual stabilization and the regular but infrequent alternations, meaning that the same memory traces are responsible for both. This model provides a good description of psychophysical findings, and offers several indications regarding their neural basis.

General validity of Levelt's propositions reveals common computational mechanisms for visual rivalry

The mechanisms underlying conscious visual perception are often studied with either binocular rivalry or perceptual rivalry stimuli. Despite existing research into both types of rivalry, it remains unclear to what extent their underlying mechanisms involve common computational rules. Computational models of binocular rivalry mechanisms are generally tested against Levelt's four propositions, describing the psychophysical relation between stimulus strength and alternation dynamics in binocular rivalry. Here we use a bistable rotating structure-from-motion sphere, a generally studied form of perceptual rivalry, to demonstrate that Levelt's propositions also apply to the alternation dynamics of perceptual rivalry. Importantly, these findings suggest that bistability in structure-from-motion results from active cross-inhibition between neural populations with computational principles similar to those present in binocular rivalry. Thus, although the neural input to the computational mechanism of rivalry may stem from different cortical neurons and different cognitive levels the computational principles just prior to the production of visual awareness appear to be common to the two types of rivalry.

Sensory memory for ambiguous vision

In recent years the overlap between visual perception and memory has shed light on our understanding of both. When ambiguous images that normally cause perception to waver unpredictably are presented briefly with intervening blank periods, perception tends to freeze, locking into one interpretation. This indicates that there is a form of memory storage across the blank interval. This memory trace codes low-level characteristics of the stored stimulus. Although a trace is evident after a single perceptual instance, the trace builds over many separate stimulus presentations, indicating a flexible, variable-length time-course. This memory shares important characteristics with priming by non-ambiguous stimuli. Computational models now provide a framework to interpret many empirical observations.

Caloric vestibular stimulation reveals discrete neural mechanisms for coherence rivalry and eye rivalry: A meta-rivalry model

Binocular rivalry is an extraordinary visual phenomenon that has engaged investigators for centuries. Since its first report, there has been vigorous debate over how the brain achieves the perceptual alternations that occur when conflicting images are presented simultaneously, one to each eye. Opposing high-level/stimulus-representation models and low-level/eye-based models have been proposed to explain the phenomenon, recently merging into an amalgam view. Here, we provide evidence that during viewing of Díaz-Caneja stimuli, coherence rivalry -- in which aspects of each eye's presented image are perceptually regrouped into rivalling coherent images -- and eye rivalry operate via discrete neural mechanisms. We demonstrate that high-level brain activation by unilateral caloric vestibular stimulation shifts the predominance of perceived coherent images (coherence rivalry) but not half-field images (eye rivalry). This finding suggests that coherence rivalry (like conventional rivalry according to our previous studies) is mediated by interhemispheric switching at a high level, while eye rivalry is mediated by intrahemispheric mechanisms, most likely at a low level. Based on the present data, we further propose that Díaz-Caneja stimuli induce 'meta-rivalry' whereby the discrete high- and low-level competitive processes themselves rival for visual consciousness.

Discontinuous presentation of ambiguous figures: How interstimulus-interval durations affect reversal dynamics and ERPs

If we observe an ambiguous figure, our percept is unstable and alternates between the possible interpretations. Periodically interrupting the presentation sizably modulates the spontaneous reversal rate. We here studied event-related potential (ERP) correlates of the neural processes underlying these strong modulations. An ambiguous Necker stimulus was presented discontinuously with four randomly varying interstimulus intervals (ISI; 14, 43, 130, 390 ms) while participants indicated perceptual reversals. EEG was selectively averaged with respect to the participants' percept and ISI. ERP traces varied markedly between ISIs. A simple model explained a major part of this variation and showed that the ISI-dependent ERP modulation occurs after disambiguation has already taken place. We suggest that perceptual stability (or reversal) depends on a system state, slowly changing from one reversal to the next. ISI can shift this state on a scale between stability and instability.