Bistable Stimuli Research Articles

Perceptual Incongruence Influences Bistability and Cortical Activation

We employed a parametric psychophysical design in combination with functional imaging to examine the influence of metric changes in perceptual incongruence on perceptual alternation rates and cortical responses. Subjects viewed a bistable stimulus defined by incongruent depth cues; bistability resulted from incongruence between binocular disparity and monocular perspective cues that specify different slants (slant rivalry). Psychophysical results revealed that perceptual alternation rates were positively correlated with the degree of perceived incongruence. Functional imaging revealed systematic increases in activity that paralleled the psychophysical results within anterior intraparietal sulcus, prior to the onset of perceptual alternations. We suggest that this cortical activity predicts the frequency of subsequent alternations, implying a putative causal role for these areas in initiating bistable perception. In contrast, areas implicated in form and depth processing (LOC and V3A) were sensitive to the degree of slant, but failed to show increases in activity when these cues were in conflict.

Adaptive Estimation of Three-Dimensional Structure in the Human Brain

Perceiving the three-dimensional (3D) properties of the environment relies on the brain bringing together ambiguous cues (e.g., binocular disparity, shading, texture) with information gained from short- and long-term experience. Perceptual aftereffects, in which the perception of an ambiguous 3D stimulus is biased away from the shape of a previously viewed stimulus, provide a sensitive means of probing this process, yet little is known about their neural basis. Here, we investigate 3D aftereffects using psychophysical and functional MRI (fMRI) adaptation paradigms to gain insight into the cortical circuits that mediate the perceptual interpretation of ambiguous depth signals. Using two classic bistable stimuli (Mach card, kinetic depth effect), we test aftereffects produced by 3D shapes defined by binocular (disparity) or monocular (texture, shading) depth cues. We show that the processing of ambiguous 3D stimuli in dorsal visual cortical areas (V3B/KO, V7) and posterior parietal regions is modulated by adaptation in line with perceptual aftereffects. Similar behavioral and fMRI adaptation effects for the two types of bistable stimuli suggest common neural substrates for depth aftereffects independent of the inducing depth cues (disparity, texture, shading). In line with current thinking about the role of adaptation in sensory optimization, our findings provide evidence that estimation of 3D shape in dorsal cortical areas takes account of the adaptive context to resolve depth ambiguity and interpret 3D structure.

Relaxation-Based Feature Selection for Single-Trial Decoding of Bistable Perception

Bistable perception refers to the phenomenon of spontaneously alternating percepts while viewing the same stimulus continuously. Bistable stimuli allow dissociation between stimuli and perception, and thus, provide a unique opportunity for understanding the neural basis of visual perception. In this paper, we focus on a relaxation (RELAX) based algorithm to select features from the multitaper spectral estimates of the multichannel intracortical local field potential (LFP), simultaneously collected from the middle temporal visual cortex of a macaque monkey, for decoding its bistable structure-from-motion (SFM) perception. We demonstrate that RELAX surpasses the conventional sequential forward selection (SFS) by offering the flexibility of modifying selected features. We propose a redundancy reduction preprocessing technique to significantly reduce the computational load for both SFS and RELAX. We exploit the support vector machines classifier based on the selected features for single-trial decoding the reported perception. Our results demonstrate the excellent performance of the RELAX feature selection algorithm. Furthermore, we find that the features in the gamma frequency band (30-100 Hz) of LFP are most relevant to bistable SFM perception. This finding is novel in awake monkey studies and suggests that gamma oscillations carry the most discriminative information for bistable perception of SFM stimuli.

The Mixture of Bernoulli Experts: a theory to quantify reliance on cues in dichotomous perceptual decisions.

The appearances of perceptually bistable stimuli can by definition be reported with confidence, so these stimuli may be useful to investigate how visual cues are learned and combined to construct visual appearance. However, interpreting experimental data (percent of trials seen one way or the other) requires a theoretically motivated measure of cue effectiveness. Here we describe a simple Bayesian theory for dichotomous perceptual decisions: the Mixture of Bernoulli Experts or MBE. In this theory, a cue's subjective reliability is the product of a weight and an estimate of the cue's ecological validity. The theory (1) justifies the use of probit analysis to measure the system's reliance on a cue and (2) enables hypothesis testing. To illustrate, we used apparent 3D rotation direction in perceptually ambiguous Necker cube movies to test whether the visual system relied on a newly recruited cue (position of the stimulus within the visual field) to the same extent when a long-trusted cue (binocular disparity) was present or not present in the display. For six trainees, reliance on the newly recruited cue was similar whether or not the long-trusted cue was present, suggesting that the visual system assumed the new cue to be conditionally independent.

Widespread fMRI activity differences between perceptual states in visual rivalry are correlated with differences in observer biases

When observing bistable stimuli, the percept can change in the absence of changes in the stimulus itself. When intermittently presenting a bistable stimulus, the number of perceptual alternations can increase or decrease, depending on the duration of the period that the stimulus is removed from screen between stimulus presentations (off-period). Longer off-periods lead to stabilization of the percept, while short off-periods produce perceptual alternations. Here we compare fMRI brain activation across percept repetitions and alternations when observing an intermittently presented ambiguously rotating structure from motion sphere. In the first experimental session, subjects were requested to voluntarily control the percept into either a repeating or an alternating perceptual regime at a single off-period. In a consecutive session, subjects observed the sphere uninstructed, and reported alternations and repetitions. The behavioral data showed that there were marked individual biases for observing the sphere as either repeating or alternating. The fMRI data showed activation differences between alternating and repeating perceptual regimes in an extensive network that included parietal cortex, dorsal premotor area, dorsolateral prefrontal cortex, supplementary motor area, insula, and cerebellum. However, these activation differences could all be explained by intersubject differences in the bias for one of the two perceptual regimes. The stronger the bias was for a particular perceptual regime, the less activation and vice versa. We conclude that widespread activation differences between perceptual regimes can be accounted for by differences in the perceptual bias for one of the two regimes.

The role of the corpus callosum in the perception of reversible figures in children

To test the role of interhemispheric competition through the corpus callosum in the perceptual alternation of reversible figures, we compared children with callosal pathology and typically developing children on a bistable stimulus task. The children with corpus callosum pathology reported significantly less changes of percepts per minute than the age-matched typically developing children. In addition, older typically developing children reported significantly more changes of percepts than the younger ones. These results support the hypothesis that the rate of reversal between two interpretations of a bistable stimulus may be partly mediated by the corpus callosum.

Multisensory Integration: A Functional Role for Inter-Area Synchronization?

It has been suggested that synchronization of neural activity in distinct areas of the brain is important in the 'perceptual binding' of the various features relating to a specific object. A recent study has provided new evidence that inter-area synchronization is important in multisensory integration.

Perceptual coupling of multiple point-light figures

When confronted with multiple bistable stimuli at the same time, the visual system tends to generate a common interpretation for all stimuli. We exploit this perceptual-coupling phenomenon to investigate the perception of depth in bistable point-light figures. Observers indicate the global depth orientation of simultaneously presented point-light figures while the similarity between the stimuli is manipulated. In a first experiment, a higher occurrence of coupled percepts is found for identical figures, but coupling breaks down when either the movement pattern or both the viewpoint and the phase-relation are changed. A second experiment confirms these results, but also demonstrates that different point-light actions can be subject to perceptual coupling as long as they share the same viewpoint and exhibit equivalent degrees of perceptual ambiguity. The data are consistent with an explanation in terms of differential contributions of stored, view-dependent object and action representations and of an interaction between stages processing local stimulus features. The consequences of these results are discussed in the framework of an explicit model for the perception of depth in biological motion.

Early interactions between neuronal adaptation and voluntary control determine perceptual choices in bistable vision

At the onset of bistable stimuli, the brain needs to choose which of the competing perceptual interpretations will first reach awareness. Stimulus manipulations and cognitive control both influence this choice process, but the underlying mechanisms and interactions remain poorly understood. Using intermittent presentation of bistable visual stimuli, we demonstrate that short interruptions cause perceptual reversals upon the next presentation, whereas longer interstimulus intervals stabilize the percept. Top-down voluntary control biases this process but does not override the timing dependencies. Extending a recently introduced low-level neural model, we demonstrate that percept-choice dynamics in bistable vision can be fully understood with interactions in early neural processing stages. Our model includes adaptive neural processing preceding a rivalry resolution stage with cross-inhibition, adaptation, and an interaction of the adaptation levels with a neural baseline. Most importantly, our findings suggest that top-down attentional control over bistable stimuli interacts with low-level mechanisms at early levels of sensory processing before perceptual conflicts are resolved and perceptual choices about bistable stimuli are made.

Perceptual grouping and inverse fMRI activity patterns in human visual cortex

We used functional magnetic resonance imaging (fMRI) to measure activity in human visual cortex, including a higher object processing area, the lateral occipital complex (LOC), and primary visual cortex (V1), in response to a perceptually bistable stimulus whose elements were perceived as either grouped into a shape or randomly arranged. We found activity increases in the LOC and simultaneous reductions of activity in V1 when the elements were perceived as a coherent shape. Consistent with a number of inferential models of visual processing, our results suggest that feedback from higher visual areas to lower visual areas serves to reduce activity during perceptual grouping. The implications of these findings with respect to these models are discussed.

Single-Trial Decoding of Bistable Perception Based on Sparse Nonnegative Tensor Decomposition

The study of the neuronal correlates of the spontaneous alternation in perception elicited by bistable visual stimuli is promising for understanding the mechanism of neural information processing and the neural basis of visual perception and perceptual decision-making. In this paper, we develop a sparse nonnegative tensor factorization-(NTF)-based method to extract features from the local field potential (LFP), collected from the middle temporal (MT) visual cortex in a macaque monkey, for decoding its bistable structure-from-motion (SFM) perception. We apply the feature extraction approach to the multichannel time-frequency representation of the intracortical LFP data. The advantages of the sparse NTF-based feature extraction approach lies in its capability to yield components common across the space, time, and frequency domains yet discriminative across different conditions without prior knowledge of the discriminating frequency bands and temporal windows for a specific subject. We employ the support vector machines (SVMs) classifier based on the features of the NTF components for single-trial decoding the reported perception. Our results suggest that although other bands also have certain discriminability, the gamma band feature carries the most discriminative information for bistable perception, and that imposing the sparseness constraints on the nonnegative tensor factorization improves extraction of this feature.

Optimal spatio-temporal pooling of neural responses in area MT

Bistable visual stimuli such as Rubin's vase/face or the Necker cube refer to the phenomena of spontaneously alternating percepts while viewing the same visual image. The uncoupling between the stimulus and percept offers a means for understanding neural basis of visual perception. Here we asked whether pooling the responses of a large population of MT neurons over time and space could improve the predictability of perceptual decisions during ambiguous visual stimulation. Two well trained rhesus monkeys indicated the perceived direction of rotation of bistable structure-from-motion (SFM) stimuli by pushing one of two levers. During this task, multi-channel intracortical recordings including single-unit activity (SUA), multi-unit activity (MUA), and local field potentials (LFP) were collected from area MT. We sorted the neural data according to the monkeys' behavioral choices and employed statistical algorithms to classify brain states (i.e., the subjective interpretation of a bistable stimulus). Classification was performed with linear discriminant analysis with leave-one-out cross-validation. We found that SUA, MUA and LFP all had a rather modest capability of predicting the monkeys' perceptual report when considered in isolation. We developed dynamic models for spatio-temporal integration of distributed neural signals. We found that the discriminative information of neuronal population activity accumulates over time and the combination of simultaneously collected data greatly improved the prediction accuracy of each of the signals. The accuracy and statistical power of determining the monkeys' perception increased with the number of channels as well as with the types of neural signals used for analysis. Our results demonstrate that simultaneous collection of multiple neural responses in area MT can improve the determination of perceptual states.

Maximum alternation rate in bi-stable perception occurs at equidominance: experiments and modeling

We present ambiguous stimuli, such as a pair of superimposed gratings exhibiting bi-stable depth ordering and binocular rivalry stimulus, to human subjects and measure the amount of time each of the two possible percepts dominates the other. Parameters of the stimuli are manipulated in order to strengthen gradually one of the percepts compared to the other, that is, to increase the fraction of time that this percept is dominant. For some choice of the parameters, the two percepts become equidominant: they dominate for the same fraction of time, or, equivalently, their mean dominance durations are the same. When the parameter which controls the strength of the percepts is varied from the equidominance value, its effect on the mean dominance duration for each percept is not the same. For the percept which becomes stronger, the mean dominance duration increases greatly in comparison to its value at equidominance, while for the percept which becomes weaker, the mean dominance duration is only slightly reduced. This result implies that as a function of the parameter that controls percept strength, the alternation rate between the two interpretations reaches a maximum at the equidominance point. We show that these features naturally arise in a bi-stable energy-based attractor model [1] where parameter manipulations produce symmetrical deformations of the energy landscape. Based on this formalism, we construct a two-population rate-based model with divisively normalized inputs, which produces behavior qualitatively similar to the one observed experimentally. A general class of neuronal competition models which describe rivalry during ambiguous stimulus presentation [2,3] exhibits the described behavior as well, as long as divisively normalized inputs are introduced. We calculate the entropy of a binary system in which probabilities are equal to the fraction of dominance of each state and show that it correlates with the alternation rate between the states in both experiments and models, suggesting that the alternation rate is a reflection of the uncertainty present in the system due to ambiguous stimulation.

Competition between newly recruited and pre-existing visual cues during the construction of visual appearance

During perception, conflicting visual cues often trade against each other. Recent cue recruitment experiments show that the visual system can be conditioned to use artificial visual cues during the perception of a bistable stimulus. Does the visual system treat the new cue as an independent source of information, separate from the long-trusted cues that were used to train it? If so, presence of the long-trusted cue should not be sufficient to block the new cue’s effect. Here, we show that a newly recruited cue (stimulus location) and a long-trusted, pre-existing cue (binocular disparity) trade against each other: they contribute simultaneously to the direction of perceived 3D rotation of a Necker cube. We also show that the new position cue was based primarily on retinal position, so early visual areas may mediate the cue’s effect.

Temporal Dynamics of Auditory and Visual Bistability Reveal Common Principles of Perceptual Organization

When dealing with natural scenes, sensory systems have to process an often messy and ambiguous flow of information. A stable perceptual organization nevertheless has to be achieved in order to guide behavior. The neural mechanisms involved can be highlighted by intrinsically ambiguous situations. In such cases, bistable perception occurs: distinct interpretations of the unchanging stimulus alternate spontaneously in the mind of the observer. Bistable stimuli have been used extensively for more than two centuries to study visual perception. Here we demonstrate that bistable perception also occurs in the auditory modality. We compared the temporal dynamics of percept alternations observed during auditory streaming with those observed for visual plaids and the susceptibilities of both modalities to volitional control. Strong similarities indicate that auditory and visual alternations share common principles of perceptual bistability. The absence of correlation across modalities for subject-specific biases, however, suggests that these common principles are implemented at least partly independently across sensory modalities. We propose that visual and auditory perceptual organization could rely on distributed but functionally similar neural competition mechanisms aimed at resolving sensory ambiguities.

Attentional control over either of the two competing percepts of ambiguous stimuli revealed by a two-parameter analysis: Means do not make the difference

We studied distributions of perceptual rivalry reversals, as defined by the two fitted parameters of the Gamma distribution. We did so for a variety of bi-stable stimuli and voluntary control exertion tasks. Subjects’ distributions differed from one another for a particular stimulus and control task in a systematic way that reflects a constraint on the describing parameters. We found a variety of two-parameter effects, the most important one being that distributions of subjects differ from one another in the same systematic way across different stimuli and control tasks (i.e., a fast switcher remains fast across all conditions in a parameter-specified way). The cardinal component of subject-dependent variation was not the conventionally used mean reversal rate, but a component that was oriented—for all stimuli and tasks—roughly perpendicular to the mean rate. For the Necker cube, we performed additional experiments employing specific variations in control exertion, suggesting that subjects have to a considerable extent independent control over the reversal rate of either of the two competing percepts.

Linking dynamical perceptual decisions at different levels of description in motion pattern formation: Psychophysics

The relationship between local-level motion detection and higher level pattern-forming mechanisms was investigated with the motion quartet, a bistable stimulus for which either horizontal or vertical motion patterns are perceived. Local-level perturbations in luminance contrast affected the stability of the perceived patterns and, thereby, the size of the pattern-level hysteresis obtained by gradually changing the motion quartet's aspect ratio. Briefly eliminating luminance contrast (so nonmotion was perceived during the perturbation) eliminated pattern-level hysteresis, and briefly increasing luminance contrast (so motion was perceived during the perturbation) increased pattern-level hysteresis. Partially reducing luminance contrast resulted in bistability during the perturbation; pattern-level hysteresis was maintained when motion was perceived, and eliminated when nonmotion was perceived. The results were attributed to local motion/nonmotion perceptual decisions in area V1 affecting the magnitude of the activation feeding forward to motion detectors in area MT, where the stability of pattern-level perceptual decisions is determined by activation-dependent, future-shaping interactions that inhibit soon-to-be-stimulated detectors responsive to competing motion directions.

FMRI Measures of Perceptual Filling-in in the Human Visual Cortex

Filling-in refers to the tendency of stabilized retinal stimuli to fade and become replaced by their background. This phenomenon is a good example of central brain mechanisms that can selectively add or delete information to/from the retinal input. Importantly, such cortical mechanisms may overlap with those that are used more generally in visual perception. In order to identify cortical areas that contribute to perceptual filling-in, we used functional magnetic resonance imaging to image activity in the visual cortex while subjects experienced filling-in. Nine subjects viewed an achromatic disc with slightly higher luminance than the background and indicated the presence or absence of filling-in by a keypress. The disc was placed in either the upper or lower left quadrant. Similar high-contrast stimuli were used to map out the retinotopic representation of the disc. Unexpectedly, the lower-field high-contrast stimulus produced more parietal cortex activation than the upper-field condition, indicating preferential representation of the lower field by attentional control mechanisms. During perceptual filling-in, we observed significant contralateral reductions in activation in lower-tier retinotopic areas V1 and V2. In contrast, increased activation was consistently observed in visual areas V3A and V4v, higher-level cortex in the intraparietal sulcus, posterior superior temporal sulcus, and the ventral occipital-temporal region, as well as the pulvinar. The filling-in activation pattern was remarkably similar for both the upper- and lower-field conditions. Behaviorally, filling-in was reported to be easier for the lower visual field, and filling-in periods were longer for the lower than the upper quadrant. We suggest this behavioral asymmetry may be partially due to the preferential parietal representation of the lower field. The results lead us to propose that perceptual filling-in recruits high-level control mechanisms to reconcile competing percepts, and alters the normal image-related signals at the first stages of cortical processing. Moreover, the overall pattern of activation during filling-in resembles that seen in other studies of perceptually bistable stimuli, including binocular rivalry, indicating common control mechanisms.

FMRI Measures of Perceptual Filling-in in the Human Visual Cortex

Abstract Filling-in refers to the tendency of stabilized retinal stimuli to fade and become replaced by their background. This phenomenon is a good example of central brain mechanisms that can selectively add or delete information to/from the retinal input. Importantly, such cortical mechanisms may overlap with those that are used more generally in visual perception. In order to identify cortical areas that contribute to perceptual filling-in, we used functional magnetic resonance imaging to image activity in the visual cortex while subjects experienced filling-in. Nine subjects viewed an achromatic disc with slightly higher luminance than the background and indicated the presence or absence of filling-in by a keypress. The disc was placed in either the upper or lower left quadrant. Similar high-contrast stimuli were used to map out the retinotopic representation of the disc. Unexpectedly, the lower-field high-contrast stimulus produced more parietal cortex activation than the upper-field condition, indicating preferential representation of the lower field by attentional control mechanisms. During perceptual filling-in, we observed significant contralateral reductions in activation in lower-tier retinotopic areas V1 and V2. In contrast, increased activation was consistently observed in visual areas V3A and V4v, higher-level cortex in the intraparietal sulcus, posterior superior temporal sulcus, and the ventral occipital–temporal region, as well as the pulvinar. The filling-in activation pattern was remarkably similar for both the upper- and lower-field conditions. Behaviorally, filling-in was reported to be easier for the lower visual field, and filling-in periods were longer for the lower than the upper quadrant. We suggest this behavioral asymmetry may be partially due to the preferential parietal representation of the lower field. The results lead us to propose that perceptual filling-in recruits high-level control mechanisms to reconcile competing percepts, and alters the normal image-related signals at the first stages of cortical processing. Moreover, the overall pattern of activation during filling-in resembles that seen in other studies of perceptually bistable stimuli, including binocular rivalry, indicating common control mechanisms.

Microsaccades Counteract Visual Fading during Fixation

Our eyes move continually, even while we fixate our gaze on an object. If fixational eye movements are counteracted, our perception of stationary objects fades completely, due to neural adaptation. Some studies have suggested that fixational microsaccades refresh retinal images, thereby preventing adaptation and fading. However, other studies disagree, and so the role of microsaccades remains unclear. Here, we correlate visibility during fixation to the occurrence of microsaccades. We asked subjects to indicate when Troxler fading of a peripheral target occurs, while simultaneously recording their eye movements with high precision. We found that before a fading period, the probability, rate, and magnitude of microsaccades decreased. Before transitions toward visibility, the probability, rate, and magnitude of microsaccades increased. These results reveal a direct link between suppression of microsaccades and fading and suggest a causal relationship between microsaccade production and target visibility during fixation.