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The observation of an ambiguous figure leads to spontaneous perceptual reversals while the observed picture stays unchanged. Some ERP studies on ambiguous figures report a P300-like component correlated with perceptual reversals supporting a top-down explanation, while other studies found early visual ERP components supporting a bottom-up explanation. Based on an experimental paradigm that permits a high temporal resolution of the endogenous reversal event, we compared endogenous Necker-cube reversals with exogenously-induced reversals of unambiguous cube variants. For both reversal types, we found a chain of ERP components with the following characteristics: (1) An early occipital ERP component (130 ms) is restricted to endogenous reversals. (2) All subsequent components also appear with exogenously-induced reversals, however 40-90 ms earlier than their endogenous counterparts. (3) The latency difference between reversal types is also reflected in the timing of manual reactions, which occur 100-130 ms after P300-like components. The results suggest that the P300-like component is the same as found in other ERP studies on ambiguous figures. This component does not reflect the reversal per se, but rather its cognitive analysis, 300 ms after a change of the representation in early visual areas. The presented ERP chains integrate the different ERP results and allow to pinpoint the steps where top-down mechanisms begin to exert their influence.

How can our percept spontaneously change while the observed object stays unchanged? This happens with ambiguous figures, like the Necker cube. Explanations favor either bottom-up factors in early visual processing, or top-down factors near awareness. The EEG has a high temporal resolution, so event related potentials (ERPs) may help to throw light on these alternative explanations. However, the precise point in time of neural correlates of perceptual reversal is difficult to estimate. We developed a paradigm that overcomes this problem and found an early (120 ms) occipital ERP signal correlated with endogenous perceptual reversal. Parallels of ambiguous-figure-reversal to binocular-rivalry-reversals are explored.

Although our eyes receive incomplete and ambiguous information, our perceptual system is usually able to successfully construct a stable representation of the world. In the case of ambiguous figures, however, perception is unstable, spontaneously alternating between equally possible outcomes. The present study compared EEG responses to ambiguous figures and their unambiguous variants. We found that slight figural changes, which turn ambiguous figures into unambiguous ones, lead to a dramatic difference in an ERP ("event-related potential") component at around 400 ms. This result was obtained across two different categories of figures, namely the geometric Necker cube stimulus and the semantic Old/Young Woman face stimulus. Our results fit well into the Bayesian inference concept, which models the evaluation of a perceptual interpretation's reliability for subsequent action planning. This process seems to be unconscious and the late EEG signature may be a correlate of the outcome.

Ambiguous figures attract observers because perception alternates between different interpretations while the sensory information stays unchanged. Understanding the underlying processes is difficult because the precise time instant of this endogenous reversal event needs to be known but is difficult to measure. Presenting ambiguous figures discontinuously and using stimulus onset as estimation of the reversal event increased temporal resolution and provided a series of well-confirmed EEG signatures. In the current EEG study we used this 'onset paradigm' for the first time with Boring's old/young woman stimulus. We found an early occipital event-related potential (ERP) correlate of reversals between the perception of the old woman and the perception of the young woman that fits well with previous ERP findings. This component was not followed by the often-reported occipito-parietal Reversal Negativity at 260 ms, but instead by an occipito-temporal N170, that is typically reported with face stimuli. We interpret our results as follows: ambiguity conflicts take place during processing of stimulus elements in early visual areas roughly 130 ms after stimulus onset. The disambiguation of these elements and their assembly to object 'gestalts' result from an interplay between early visual and object-specific brain areas in a temporal window between 130 and 260 ms after stimulus onset. In the particular case of Boring's old/young woman the processes of element disambiguation and gestalt construction are already finished at 170 ms and, thus, 90 ms earlier than in the case of ambiguous geometric figures (eg Necker cube or Schroeder staircase) or of binocular rivalrous gratings.

During observation of an ambiguous Necker cube, our percept changes spontaneously although the external stimulus does not. An EEG paradigm allowing time-resolved EEG measurement during endogenous perceptual reversals recently revealed a chain of ERP correlates beginning with an early occipital positivity at around 130 ms (Reversal Positivity, "RP"). In order to better understand the functional role of this RP, we investigated its relation to the P100, which is spatiotemporally close, typically occurring 100 ms after onset of a visual stimulus at occipital electrodes. We compared the relation of the ERP amplitudes to varying sizes of ambiguous Necker cubes. The main results are: (1) The P100 amplitude increases monotonically with stimulus size but is independent of the participants' percept. (2) The RP, in contrast, is percept-related and largely unaffected by stimulus size. (3) A similar pattern to RP was found for reaction times: They depend on the percept but not on stimulus size. We speculate that the P100 reflects processing of elementary visual features, while the RP is related to a processing conflict during 3D interpretation that precedes a reversal. The present results indicate that low-level visual processing (related to stimulus size) and (relative) high-level processing (related to perceptual reversal) occur in close spatial and temporal vicinity.

Environmental information available to our senses is incomplete and to varying degrees ambiguous. It has to be disambiguated in order to construct stable and reliable percepts. Ambiguous figures are artificial examples where perception is maximally unstable and alternates between possible interpretations. Tiny low-level changes can disambiguate an ambiguous figure and thus stabilize percepts. The present study compares ERPs evoked by ambiguous stimuli and disambiguated stimulus variants across three visual categories: geometry (Necker cube), motion (stroboscopic alternative motion stimulus, SAM) and semantics (Boring's old/young woman). We found that (a) disambiguated stimulus variants cause stable percepts and evoke two huge positive ERP excursions (Cohen's effect sizes 1-2), (b) the amplitudes of these ERP effects are inversely related to the degree of stimulus ambiguity, and (c) this pattern of results is consistent across all three tested visual categories. This generality across visual categories points to mechanisms at a very abstract (cognitive) level of processing. We discuss our results in the context of a high-level Bayesian inference unit that evaluates the reliability of perceptual processing results, given a priori incomplete, ambiguous sensory information. The ERP components may reflect the outcome of this reliability estimation.

PURPOSE: We sought brain activity that predicts visual consciousness. METHODS: We used electroencephalography (EEG) to measure brain activity to a 1000-ms display of sine-wave gratings, oriented vertically in one eye and horizontally in the other. This display yields binocular rivalry: irregular alternations in visual consciousness between the images viewed by the eyes. We replaced both gratings with 200 ms of darkness, the gap, before showing a second display of the same rival gratings for another 1000 ms. We followed this by a 1000-ms mask then a 2000-ms inter-trial interval (ITI). Eleven participants pressed keys after the second display in numerous trials to say whether the orientation of the visible grating changed from before to after the gap or not. Each participant also responded to numerous non-rivalry trials in which the gratings had identical orientations for the two eyes and for which the orientation of both either changed physically after the gap or did not. RESULTS: We found that greater activity from lateral occipital-parietal-temporal areas about 180 ms after initial onset of rival stimuli predicted a change in visual consciousness more than 1000 ms later, on re-presentation of the rival stimuli. We also found that less activity from parietal, central, and frontal electrodes about 400 ms after initial onset of rival stimuli predicted a change in visual consciousness about 800 ms later, on re-presentation of the rival stimuli. There was no such predictive activity when the change in visual consciousness occurred because the stimuli changed physically. CONCLUSION: We found early EEG activity that predicted later visual consciousness. Predictive activity 180 ms after onset of the first display may reflect adaption of the neurons mediating visual consciousness in our displays. Predictive activity 400 ms after onset of the first display may reflect a less-reliable brain state mediating visual consciousness.

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.

One of the great challenges in psychiatry is finding reliable biomarkers that may allow for more accurate diagnosis and treatment of patients. Neural variability received increasing attention in recent years as a potential biomarker. In the present explorative study we investigated temporal variability in visually evoked EEG activity in a cohort of 16 adult participants with Asperger Syndrome (AS) and 19 neurotypical (NT) controls. Participants performed a visual oddball task using fine and coarse checkerboard stimuli. We investigated various measures of neural variability and found effects on multiple time scales. (1) As opposed to the previous studies, we found reduced inter-trial variability in the AS group compared to NT. (2) This effect builds up over the entire course of a 5-min experiment and (3) seems to be based on smaller variability of neural background activity in AS compared to NTs. The here reported variability effects come with considerably large effect sizes, making them promising candidates for potentially reliable biomarkers in psychiatric diagnostics. The observed pattern of universality across different time scales and stimulation conditions indicates trait-like effects. Further research with a new and larger set of participants are thus needed to verify or falsify our findings.

The information available through our senses is noisy, incomplete, and ambiguous. Our perceptual systems have to resolve this ambiguity to construct stable and reliable percepts. Previous EEG studies found large amplitude differences in two event-related potential (ERP) components 200 and 400 ms after stimulus onset when comparing ambiguous with disambiguated visual information ("ERP Ambiguity Effects"). These effects so far generalized across classical ambiguous figures from different visual categories at lower (geometry, motion) and intermediate (Gestalt perception) levels. The present study aimed to examine whether these ERP Effects are restricted to ambiguous figures or whether they also occur for different degrees of visibility. Smiley faces with low and high visibility of emotional expressions, as well as abstract figures with low and high visibility of a target curvature were presented. We thus compared ambiguity effects in geometric cube stimuli with visibility in emotional faces, and with visibility in abstract figures. ERP Effects were replicated for the geometric stimuli and very similar ERP Effects were found for stimuli with emotional face expressions but also for abstract figures. Conclusively, the ERP amplitude effects generalize across fundamentally different stimulus categories and show highly similar effects for different degrees of stimulus ambiguity and stimulus visibility. We postulate the existence of a high-level/meta-perceptual evaluation instance, beyond sensory details, that estimates the certainty of a perceptual decision. The ERP Effects may reflect differences in evaluation results.