Semantic analysis does not occur in the absence of awareness induced by interocular suppression

No electrophysiological evidence for semantic processing during inattentional blindness

A long-standing question concerns whether sensory input can reach semantic stages of processing in the absence of attention and awareness. Here, we examine whether the N400, an event related potential associated with semantic processing, can occur under conditions of inattentional blindness. By employing a novel three-phase inattentional blindness paradigm designed to maximise the opportunity for detecting an N400, we found no evidence for it when participants were inattentionally blind to the eliciting stimuli (related and unrelated word pairs). In contrast, participants noticed the same task-irrelevant word pairs when minimal attention was allocated to them, and a small N400 became evident. When the same stimuli were fully attended and relevant to the task, a robust N400 was observed. In addition to univariate ERP measures, multivariate decoding analyses were unable to classify related from unrelated word pairs when observers were inattentionally blind to the words, with decoding reaching above-chance levels only when the words were (at least minimally) attended. By comparison, decoding reached above-chance levels when contrasting word pairs with non-word stimuli, even when participants were inattentionally blind to these stimuli. Our results also replicated several previous studies by finding a "visual awareness negativity" (VAN) that distinguished task-irrelevant stimuli that participants noticed compared with those that were not perceived, and a P3b (or "late positivity") that was evident only when the stimuli were task relevant. Together, our findings suggest that semantic processing might require at least a minimal amount of attention.

Prime-induced illusion of control: The influence of unconscious priming on self-initiated actions and the role of regression to the mean

To what degree human cognition is influenced by subliminal stimuli is a controversial empirical question. One striking example was reported by Linser and Goschke (2007): participants overestimated how much control they had over objectively uncontrollable stimuli when masked congruent primes were presented immediately before the action. Critically, however, unawareness of the masked primes was established by post hoc data selection. In our preregistered study we sought to explore these findings while adjusting prime visibility based on individual thresholds, so that each participant underwent both visible and non-visible conditions. In experiment 1, N = 39 participants engaged in a control judgement task: following the presentation of a semantic prime, they freely selected between two keys, which triggered the appearance of a colored circle. The color of the circles, however, was independent of the key-press. Subsequently, participants assessed their perceived control over the circle's color, based on their key-presses, via a rating scale that ranged from 0 % (no control) to 100 % (complete control). Contrary to Linser and Goschke (2007)'s findings, this experiment demonstrated that predictive information influenced the experience of agency only when primes were consciously processed. In experiment 2, utilizing symbolic (arrow) primes, N = 35 participants had to rate their feeling of control over the effect-stimulus' identity during a two-choice identification paradigm (i.e., they were instructed to press a key corresponding to a target stimulus; with a contingency between target and effect stimulus of 75 %/25 %). The results revealed no significant influence of subliminal priming on agency perceptions. In summary, this study implies that unconscious stimuli may not exert a substantial influence on the conscious experience of agency, underscoring the need for careful consideration of methodological aspects and experimental design's impact on observed phenomena.

Feature-based interaction between masks and target in continuous flash suppression

Continuous flash suppression (CFS) has become one of the most popular tools in the study of visual processing in the absence of conscious awareness. Studies use different kinds of masks, like colorful Mondrians or random noise. Even though the use of CFS is widespread, little is known about some of the underlying neuronal mechanisms, such as the interactions between masks and stimuli. We designed a b-CFS experiment with feature-reduced targets and masks in order to investigate possible effects of feature-similarity or -orthogonality between masks and targets. Masks were pink noise patterns filtered with an orientation band pass to generate a strong directionality. Target stimuli were Gabors varying systematically in their orientational alignment with the masks. We found that stimuli whose orientational alignment was more similar to that of the masks are suppressed significantly longer. This feature-similarity (here: orientation) based enhancement of suppression duration can be overcome by feature orthogonality in another feature dimension (here: color). We conclude that mask-target interactions exist in continuous flash suppression, and the human visual system can use orthogonality within a feature dimension or across feature dimensions to facilitate the breaking of the CFS.

No effect of attentional modulation by spatial cueing in a masked numerical priming paradigm using continuous flash suppression (CFS)

One notion emerging from studies on unconscious visual processing is that different "blinding techniques" seem to suppress the conscious perception of stimuli at different levels of the neurocognitive architecture. However, even when only the results from a single suppression method are compared, the picture of the scope and limits of unconscious visual processing remains strikingly heterogeneous, as in the case of continuous flash suppression (CFS). To resolve this issue, it has been suggested that high-level semantic processing under CFS is facilitated whenever interocular suppression is attenuated by the removal of visuospatial attention. In this behavioral study, we aimed to further investigate this "CFS-attenuation-by-inattention" hypothesis in a numerical priming study using spatial cueing. Participants performed a number comparison task on a visible target number ("compare number to five"). Prime-target pairs were either congruent (both numbers smaller, or both larger than five) or incongruent. Based on the "CFS-attenuation-by-inattention" hypothesis, we predicted that reaction times (RTs) for congruent prime-target pairs should be faster than for incongruent ones, but only when the prime was presented at the uncued location. In the invisible condition, we observed no priming effects and thus no evidence in support of the "CFS-attenuation-by-inattention" hypothesis. In the visible condition, we found an inverse effect of prime-target congruency. Our results agree with the notion that the representation of CF-suppressed stimuli is fractionated, and limited to their basic, elemental features, thus precluding semantic processing.

Priming of natural scene categorization during continuous flash suppression

Continuous Flash Suppression (CFS) reduces conscious awareness of stimuli. Whether stimuli suppressed by CFS are processed at categorical or semantic levels is still debated. Here, we approached this question using a large set of indoor and outdoor scene photographs in a priming paradigm. Perceptually suppressed primes were followed by visible targets. Participants rapidly reported whether the targets showed an indoor or an outdoor scene. Responses were faster (and fast responses more accurate) when primes and targets came from a congruent superordinate category (e.g., both were outdoor scenes). During CFS, priming effects were relatively small (up to 10 ms) and modulated by prime visibility and stimulus onset asynchrony (SOA) of prime and target. Without CFS, the stimuli elicited consistent and more robust priming effects (about 24 ms). Our results imply that scene category is processed during CFS, although some residual prime visibility is likely necessary for significant priming effects to occur.

Neuroscientific Evidence for Processing Without Awareness

The extent to which we are affected by perceptual input of which we are unaware is widely debated. By measuring neural responses to sensory stimulation, neuroscientific data could complement behavioral results with valuable evidence. Here we review neuroscientific findings of processing of high-level information, as well as interactions with attention and memory. Although the results are mixed, we find initial support for processing object categories and words, possibly to the semantic level, as well as emotional expressions. Robust neural evidence for face individuation and integration of sentences or scenes is lacking. Attention affects the processing of stimuli that are not consciously perceived, and such stimuli may exogenously but not endogenously capture attention when relevant, and be maintained in memory over time. Sources of inconsistency in the literature include variability in control for awareness as well as individual differences, calling for future studies that adopt stricter measures of awareness and probe multiple processes within subjects.

Probing the attentional modulation of unconscious processing under interocular suppression in a spatial cueing paradigm

The debate about the scope and limits of unconscious visual processing under continuous flash suppression (CFS) has created a heterogeneous set of divergent findings that are yet to be reconciled. Attention has been suggested as an important factor in modulating the processing of suppressed visual information under CFS. Specifically, Eo et al. (2016) reported that semantic processing under CFS can be significantly facilitated when spatial attention is diverted away from the suppressed stimulus. Based on event-related potential (ERP) findings involving the N400, they proposed that inattention attenuates interocular suppression and thereby makes semantic processing available unconsciously, potentially reconciling conflicting evidence in the literature. In this study, we aimed to further investigate the "CFS-attenuation-by-inattention" hypothesis using functional magnetic resonance imaging (fMRI) and multivariate pattern analysis (MVPA). We tested whether the decodability of object category increases under CFS when attention is diverted away from the suppressed stimulus in a spatial cueing task. Our results provide no evidence for the "CFS-attenuation-by-inattention" hypothesis, but show higher decoding accuracies for visible stimuli than for invisible stimuli. We discuss the implications of our findings for the important endeavor of trying to reconcile the divergent reports of unconscious processing under CFS.

Dynamic face mask enhances continuous flash suppression

In continuous flash suppression (CFS), an image presented to one eye is suppressed from awareness by a dynamic image masker presented to the other eye. Previous studies report that face stimuli break out of CFS more readily when they are oriented upright and contain ecologically relevant information such as facial expressions or direct eye gaze, potentially implicating face processing in the mechanisms of interocular competition. It is unknown, however, whether face content helps to drive interocular suppression when incorporated into the dynamic masker itself, either by engaging higher-level visual mechanisms that underlie face detection or due to lower-level image features that the faces happen to contain. To investigate this, we devised a dynamic mask composed of upright faces and tested how well it suppressed detection of face or grating targets presented to the other eye. Relative contributions of higher-level and lower-level features were compared by manipulating the image properties of the mask. Results show that the dynamic face mask is strikingly effective at suppressing sensory input presented to the opposing eye, but its effectiveness is largely attributable to image texture, which can be quantified in terms of image entropy and edge density. This is because strong suppression was still observed following phase-scrambling or spatial inversion of the face elements, and while a target-selective effect was observed for the face mask, inverting the face elements to interfere with configural processing did not significantly diminish this effect. Thus, visual properties of faces, such as their image entropy and complex phase structure, predominate in driving interocular suppression rather than face detection per se.

The optimal spatial noise for continuous flash suppression masking is pink

A basic question in cognitive neuroscience is how sensory stimuli are processed within and outside of conscious awareness. In the past decade, CFS has become the most popular tool for investigating unconscious visual processing, although the exact nature of some of the underlying mechanisms remains unclear. Here, we investigate which kind of random noise is optimal for CFS masking, and whether the addition of visible edges to noise patterns affects suppression duration. We tested noise patterns of various density as well as composite patterns with added edges, and classic Mondrian masks as well as phase scrambled (edgeless) Mondrian masks for comparison. We find that spatial pink noise (1/F noise) achieved the longest suppression of the tested random noises, however classic Mondrian masks are still significantly more effective in terms of suppression duration. Further analysis reveals that global contrast and general spectral similarity between target and mask cannot account for this difference in effectiveness.

Unconscious semantic priming from pictures under backward masking and continuous flash suppression

It is debated whether the meaning of invisible pictures can be processed unconsciously. We tested whether pictures of animals or objects presented under backward masking or continuous flash suppression could prime the subsequent categorization of target words into animal or non-animal. In Experiment 1, the backward masking part failed to replicate the priming effect reported in two previous studies, despite sufficient statistical power (N = 59). Similarly, the continuous flash suppression part provided no evidence for a priming effect. In Experiment 2 (N = 65) we shortened the prime-target SOA from 290 ms to 90 ms, but again failed to obtain unconscious semantic priming under backward masking. Thus, our study did not provide evidence for unconscious semantic processing of pictures. These findings support the emerging view that unconscious processing is rather limited in scope.

Novel procedure for generating continuous flash suppression: Seurat meets Mondrian

Continuous flash suppression (CFS) entails presentation of a stationary target to one eye and an animated sequence of arrays of geometric figures, the mask, to the other eye. The prototypical CFS sequence comprises different sized rectangles of various colors, dubbed Mondrians. Presented as a rapid, changing sequence to one eye, Mondrians or other similarly constructed textured arrays can abolish awareness of the target viewed by the other eye for many seconds at a time, producing target suppression durations much longer than those associated with conventional binocular rivalry. We have devised an animation technique that replaces meaningless Mondrian figures with recognizable visual objects and scenes as inducers of CFS, allowing explicit manipulation of the visual semantic content of those masks. By converting each image of these CFS sequences into successively presented objects or scenes each comprised of many small, circular patches of color, we create pointillist CFS sequences closely matched in terms of their spatio-temporal power spectra. Randomly rearranging the positions of the pointillist patches scrambles the images so they are no longer recognizable. CFS sequences comprising a stream of different objects produces more robust interocular suppression than do sequences comprising a stream of different scenes, even when the two categories of CFS are matched in root mean square contrast and spatial frequency content. Factors promoting these differences in CFS potency could range from low-level, image-based features to high-level factors including attention and recognizability. At the same time, object- and scene-based CFS sequences, when themselves suppressed from awareness, do not differ in their durations of suppression, implying that semantic content of those images comprising CFS sequences are not registered during suppression. The pointillist technique itself offers a potentially useful means for examining the impact of high-level image meaning on aspects of visual perception other than interocular suppression.

Emotion in Chinese Words Could Not Be Extracted in Continuous Flash Suppression

Previous studies have demonstrated the automatic vigilance effect for faces and pictures and have attributed it to the brain's prioritized unconscious evaluation of early evolutionary stimuli that are critical to survival. Whether this effect exists for evolutionarily more recent stimuli, such as written words, has become the center of much debate. Apparently contradicting results have been reported in different languages, such as Hebrew, English, and Traditional Chinese (TC), with regard to the unconscious processing of emotional words in breaking continuous flash suppression (b-CFS). Our current study used two experiments (with two-character words or single-character words) to verify whether the emotional valence or the length of Simplified Chinese (SC) words would modulate conscious access in b-CFS. We failed to replicate the findings reported in Yang and Yeh (2011) using TC, but found that complex high-level emotional information could not be extracted from interocularly suppressed words regardless of their length. Our findings comply with the distinction between subliminal and preconscious states in Global Neuronal Workspace Theory and support the current notion that preconsciousness or partial awareness may be indispensable for high-level cognitive tasks such as reading comprehension.

Tactile priming accelerates conscious access to continuous flash-suppressed characters

NEW FINDINGS

What is the central question of this study? Research has reported that some sensory input, such as auditory and olfactory input, can affect subliminal visual processing. However, it is important to address whether tactile input, another form of elementary sensory input, could influence the interocular rivalry process. What is the main finding and its importance? We present several pieces of evidence regarding the influences of familiar tactile shapes and temperature on continuous flash suppression. Our findings provide support for the hypothesis that there is a cross-modal effect on subconscious visual semantic processing of Chinese characters. More specifically, tactile sensations affect subliminal processing of visual information.

ABSTRACT

Tactile and visual sensations are the most vital human functions for obtaining environmental information. However, whether tactile information influences visual processing remains unclear. In this study, a breaking continuous flash suppression (b-CFS) protocol was used to measure the extent to which tactile sensations facilitate visual processing subconsciously. In experiment 1, finger stimulation with cold and hot temperatures served as primers for the words 'cold' and 'hot', which were in turn suppressed by CFS. In experiment 2, subjects viewed the upright or inverted word 'cell phone', with or without tactile priming of holding a cell phone in their hand. Results demonstrated that the tactile primer significantly shortened the reaction time in the touch group compared with the control group in both experiments. Thus, the tactile sensation of a familiar article and/or temperature appears to facilitate corresponding visual semantic recognition to break CFS earlier.

No language unification without neural feedback: How awareness affects sentence processing

How does the human brain combine a finite number of words to form an infinite variety of sentences? According to the Memory, Unification and Control (MUC) model, sentence processing requires long-range feedback from the left inferior frontal cortex (LIFC) to left posterior temporal cortex (LPTC). Single word processing however may only require feedforward propagation of semantic information from sensory regions to LPTC. Here we tested the claim that long-range feedback is required for sentence processing by reducing visual awareness of words using a masking technique. Masking disrupts feedback processing while leaving feedforward processing relatively intact. Previous studies have shown that masked single words still elicit an N400 ERP effect, a neural signature of semantic incongruency. However, whether multiple words can be combined to form a sentence under reduced levels of awareness is controversial. To investigate this issue, we performed two experiments in which we measured electroencephalography (EEG) while 40 subjects performed a masked priming task. Words were presented either successively or simultaneously, thereby forming a short sentence that could be congruent or incongruent with a target picture. This sentence condition was compared with a typical single word condition. In the masked condition we only found an N400 effect for single words, whereas in the unmasked condition we observed an N400 effect for both unmasked sentences and single words. Our findings suggest that long-range feedback processing is required for sentence processing, but not for single word processing.

Individual differences in continuous flash suppression: Potency and linkages to binocular rivalry dynamics

Binocular rivalry (BR) and continuous flash suppression (CFS) are compelling psychophysical phenomena involving interocular suppression. Using an individual differences approach we assessed whether interocular suppression induced by CFS is predictable in potency from characteristics of BR that are plausibly governed by interocular inhibition. We found large individual differences in BR dynamics and, in addition, in the strength of CFS as gauged by the incidence and durations of breakthroughs in CFS during an extended viewing periods. CFS's potency waned with repeated trials, but stable individual differences persisted despite these mean shifts. We also discovered large individual differences in the strength of the post-CFS shift in BR dominance produced by interocular suppression. While CFS breakthroughs were significantly negatively correlated with shifts in BR dominance after CFS, there were no significant associations between individual differences in alternation rate during pre-CFS binocular rivalry and either breakthroughs during CFS or post-CFS dominance shifts. Bayesian hypothesis tests and highest posterior density intervals confirmed the weak association between these two forms of interocular suppression. Thus, our findings suggest that the substantial individual differences in BR dynamics and CFS effectiveness are modestly related but not entirely mediated by one common neural substrate.

Pupil Size as a Gateway Into Conscious Interpretation of Brightness

Although retinal illumination is the main determinant of pupil size, evidence indicates that extra-retinal factors, including attention and contextual information, also modulate the pupillary response. For example, stimuli that evoke the idea of brightness (e.g., pictures of the sun) induce pupillary constriction compared to control stimuli of matched luminance. Is conscious appraisal of these stimuli necessary for the pupillary constriction to occur? Participants' pupil diameter was recorded while sun pictures and their phase-scrambled versions were shown to the left eye. A stream of Mondrian patterns was displayed to the right eye to produce continuous flash suppression, which rendered the left-eye stimuli invisible on some trials. Results revealed that when participants were aware of the sun pictures their pupils constricted relative to the control stimuli. This was not the case when the pictures were successfully suppressed from awareness, demonstrating that pupil size is highly sensitive to the contents of consciousness.

Conscious awareness is necessary for affective faces to influence social judgments

A growing body of research claims that stimuli presented outside conscious awareness can influence affect, speech perception, decision-making, eating behavior, and social judgments. However, research has shown that conscious awareness is a continuous phenomenon. Using a continuous flash suppression (CFS) paradigm to suppress awareness of affective faces (smiling and scowling), we demonstrate that some awareness of suppressed stimuli is required for the stimuli to influence social judgments. We discovered this using a rigorous within-participants psychophysics method that allowed us to assess awareness at very low levels, which is difficult using traditional methods. Our findings place boundary conditions on claims (made previously by us and others) that stimuli presented completely outside conscious awareness influence judgments. This work contributes to the literature highlighting the need to study conscious awareness as a continuous phenomenon and provides a framework for researchers to ask and answer questions regarding conscious awareness and its relation to judgment and behavior.

Unconscious influence over executive control: Absence of conflict detection and adaptation

Executive control and its modulation of attentional mechanisms allow us to detect and adapt to conflicting information. According to recent studies, executive control functions may be modulated by unconsciously perceived information, although the available evidence is not consistent. In this study, we used a Flanker Task and employed Chromatic Flicker Fusion, a suppression technique that has been proposed as more adequate to elicit executive control functions, to assess conflict and conflict adaptation effects. Our results showed that, when suppressed, flankers did not evoke conflict related effects on performance. However, in trials where most flankers were incongruent, longer response times in congruent trials were observed, consistent with orienting responses. Our results help to support earlier theories regarding the inherent limitations of unconsciously perceived information, though future studies should further investigate why and under which conditions is the executive control system modulated by unconscious information.

Role of consciousness in temporal integration of semantic information

Previous studies found that word meaning can be processed unconsciously. Yet it remains unknown whether temporally segregated words can be integrated into a holistic meaningful phrase without consciousness. The first four experiments were designed to examine this by sequentially presenting the first three words of Chinese four-word idioms as prime to one eye and dynamic Mondrians to the other (i.e., the continuous flash suppression paradigm; CFS). An unmasked target word followed the three masked words in a lexical decision task. Results from such invisible (CFS) condition were compared with the visible condition where the preceding words were superimposed on the Mondrians and presented to both eyes. Lower performance in behavioral experiments and larger N400 event-related potentials (ERP) component for incongruent- than congruent-ending words were found in the visible condition. However, no such congruency effect was found in the invisible condition, even with enhanced statistical power and top-down attention, and with several potential confounding factors (contrast-dependent processing, long interval, no conscious training) excluded. Experiment 5 demonstrated that familiarity of word orientation without temporal integration can be processed unconsciously, excluding the possibility of general insensitivity of our paradigm. The overall result pattern therefore suggests that consciousness plays an important role in semantic temporal integration in the conditions we tested.

Unconscious response priming during continuous flash suppression

Continuous flash suppression (CFS) has become a popular tool for studying unconscious processing, but the level at which unconscious processing of visual stimuli occurs under CFS is not clear. Response priming is a robust and well-understood phenomenon, in which the prime stimulus facilitates overt responses to targets if the prime and target are associated with the same response. We used CFS to study unconscious response priming of shape: arrows with left or right orientation served as primes and targets. The prime was presented near the limen of consciousness and each trial was followed by subjective rating of visibility and a forced-choice response concerning the orientation of the prime in counterbalanced order. In trials without any reported awareness of the presence of the prime, discrimination of the prime's orientation was at chance level. However, priming was elicited in such unconscious trials. Unconscious priming was not influenced by the prime-target onset-asynchrony (SOA)/prime duration, whereas conscious processing, as indicated by the enhanced discriminability of the prime's orientation and conscious priming, increased at the longest SOAs/prime durations. These results show that conscious and unconscious processes can be dissociated with CFS and that CFS-masking does not completely suppress unconscious visual processing of shape.

Less Is More: Semantic Information Survives Interocular Suppression When Attention Is Diverted

The extent of unconscious semantic processing has been debated. It is well established that semantic information is registered in the absence of awareness induced by inattention. However, it has been debated whether semantic information of invisible stimuli is processed during interocular suppression, a procedure that renders one eye's view invisible by presenting a dissimilar stimulus to the other eye. Inspired by recent evidence demonstrating that reduced attention attenuates interocular suppression, we tested a counterintuitive hypothesis that attention withdrawn from the suppressed target location facilitates semantic processing in the absence of awareness induced by interocular suppression. We obtained an electrophysiological marker of semantic processing (N400 component) while human participants' spatial attention was being manipulated with a cueing paradigm during interocular suppression. We found that N400 modulation was absent when participants' attention was directed to the target location, but present when diverted elsewhere. In addition, the correlation analysis across participants indicated that the N400 amplitude was reduced with more attention being directed to the target location. Together, these results indicate that inattention attenuates interocular suppression and thereby makes semantic processing available unconsciously, reconciling conflicting evidence in the literature. We discuss a tight link among interocular suppression, attention, and conscious awareness.

SIGNIFICANCE STATEMENT

Interocular suppression offers a powerful means of studying the extent of unconscious processing by rendering a salient stimulus presented to one eye invisible. Here, we provide evidence that attention is a determining factor for unconscious semantic processing. An electrophysiological marker for semantic processing (N400 component) was present when attention was diverted away from the suppressed stimulus but absent when attention was directed to that stimulus, indicating that inattention facilitates unconscious semantic processing during the interocular suppression. Although contrary to the common sense assumption that attention facilitates information processing, this result is in accordance with recent studies showing that attention modulates interocular suppression but is not necessary for semantic processing. Our finding reconciles the conflicting evidence and advances theories of consciousness.

Opposite ERP effects for conscious and unconscious semantic processing under continuous flash suppression

We examined whether semantic processing occurs without awareness using continuous flash suppression (CFS). In two priming tasks, participants were required to judge whether a target was a word or a non-word, and to report whether the masked prime was visible. Experiment 1 manipulated the lexical congruency between the prime-target pairs and Experiment 2 manipulated their semantic relatedness. Despite the absence of behavioral priming effects (Experiment 1), the ERP results revealed that an N4 component was sensitive to the prime-target lexical congruency (Experiment 1) and semantic relatedness (Experiment 2) when the prime was rendered invisible under CFS. However, these results were reversed with respect to those that emerged when the stimuli were perceived consciously. Our findings suggest that some form of lexical and semantic processing can occur during CFS-induced unawareness, but are associated with different electrophysiological outcomes.

Automatic Processing of Emotional Words in the Absence of Awareness: The Critical Role of P2

It has been long debated to what extent emotional words can be processed in the absence of awareness. Behavioral studies have shown that the meaning of emotional words can be accessed even without any awareness. However, functional magnetic resonance imaging studies have revealed that emotional words that are unconsciously presented do not activate the brain regions involved in semantic or emotional processing. To clarify this point, we used continuous flash suppression (CFS) and event-related potential (ERP) techniques to distinguish between semantic and emotional processing. In CFS, we successively flashed some Mondrian-style images into one participant's eye steadily, which suppressed the images projected to the other eye. Negative, neutral, and scrambled words were presented to 16 healthy participants for 500 ms. Whenever the participants saw the stimuli—in both visible and invisible conditions—they pressed specific keyboard buttons. Behavioral data revealed that there was no difference in reaction time to negative words and to neutral words in the invisible condition, although negative words were processed faster than neutral words in the visible condition. The ERP results showed that negative words elicited a larger P2 amplitude in the invisible condition than in the visible condition. The P2 component was enhanced for the neutral words compared with the scrambled words in the visible condition; however, the scrambled words elicited larger P2 amplitudes than the neutral words in the invisible condition. These results suggest that the emotional processing of words is more sensitive than semantic processing in the conscious condition. Semantic processing was found to be attenuated in the absence of awareness. Our findings indicate that P2 plays an important role in the unconscious processing of emotional words, which highlights the fact that emotional processing may be automatic and prioritized compared with semantic processing in the absence of awareness.

Interplay between Narrative and Bodily Self in Access to Consciousness: No Difference between Self- and Non-self Attributes

The construct of the “self” is conceived as being fundamental in promoting survival. As such, extensive studies have documented preferential processing of self-relevant stimuli. For example, attributes that relate to the self are better encoded and retrieved, and are more readily consciously perceived. The preferential processing of self-relevant information, however, appears to be especially true for physical (e.g., faces), as opposed to psychological (e.g., traits), conceptions of the self. Here, we test whether semantic attributes that participants judge as self-relevant are further processed unconsciously than attributes that were not judged as self-relevant. In Experiment 1, a continuous flash suppression paradigm was employed with “self” and “non-self” attribute words being presented subliminally, and we asked participants to categorize unseen words as either self-related or not. In a second experiment, we attempted to boost putative preferential self-processing by relation to its physical conception, that is, one’s own body. To this aim, we repeated Experiment 1 while administrating acoustic stimuli either close or far from the body, i.e., within or outside peripersonal space. Results of both Experiment 1 and 2 demonstrate no difference in breaking suppression for self and non-self words. Additionally, we found that while participants were able to process the physical location of the unseen words (above or below fixation) they were not able to categorize these as self-relevant or not. Finally, results showed that sounds presented in the extra-personal space elicited a more stringent response criterion for “self” in the process of categorizing unseen visual stimuli. This shift in criterion as a consequence of sound location was restricted to the self, as no such effect was observed in the categorization of attributes occurring above or below fixation. Overall, our findings seem to indicate that subliminally presented stimuli are not semantically processed, at least inasmuch as to be categorized as self-relevant or not. However, we do demonstrate that the distance at which acoustic stimuli are presented may alter the balance between self- and non-self biases.

Invisible own- and other-race faces presented under continuous flash suppression produce affective response biases

One triumph of the human mind is the ability to place the multitudinous array of people we encounter into in- and out-group members based on racial characteristics. One fundamental question that remains to be answered is whether invisible own- and other-race faces can nevertheless influence subsequent affective judgments. Here, we employed continuous flash suppression (CFS) to render own- and other-race faces unperceivable in an affective priming task. Both on-line and off-line awareness checks were employed to provide more stringent control of partial awareness. Results revealed that relative to own-race faces, imperceptible other-race faces significantly facilitated participants' identification of negative words, suggesting an other-race derogation bias. When faces were presented consciously, we found that not only other-race faces facilitated detection of negative words, but also own-race faces facilitated detection of positive words. These findings together provide novel and strong evidence suggesting that invisible racial faces can bias affective responses.

Differential Visual Processing of Animal Images, with and without Conscious Awareness

The human visual system can quickly and efficiently extract categorical information from a complex natural scene. The rapid detection of animals in a scene is one compelling example of this phenomenon, and it suggests the automatic processing of at least some types of categories with little or no attentional requirements (Li et al., 2002, 2005). The aim of this study is to investigate whether the remarkable capability to categorize complex natural scenes exist in the absence of awareness, based on recent reports that “invisible” stimuli, which do not reach conscious awareness, can still be processed by the human visual system (Pasley et al., 2004; Williams et al., 2004; Fang and He, 2005; Jiang et al., 2006, 2007; Kaunitz et al., 2011a). In two experiments, we recorded event-related potentials (ERPs) in response to animal and non-animal/vehicle stimuli in both aware and unaware conditions in a continuous flash suppression (CFS) paradigm. Our results indicate that even in the “unseen” condition, the brain responds differently to animal and non-animal/vehicle images, consistent with rapid activation of animal-selective feature detectors prior to, or outside of, suppression by the CFS mask.

Blindsight and Unconscious Vision: What They Teach Us about the Human Visual System

Damage to the primary visual cortex removes the major input from the eyes to the brain, causing significant visual loss as patients are unable to perceive the side of the world contralateral to the damage. Some patients, however, retain the ability to detect visual information within this blind region; this is known as blindsight. By studying the visual pathways that underlie this residual vision in patients, we can uncover additional aspects of the human visual system that likely contribute to normal visual function but cannot be revealed under physiological conditions. In this review, we discuss the residual abilities and neural activity that have been described in blindsight and the implications of these findings for understanding the intact system.

The temporal frequency tuning of continuous flash suppression reveals peak suppression at very low frequencies

Continuous flash suppression (CFS) is a psychophysical technique where a rapidly changing Mondrian pattern viewed by one eye suppresses the target in the other eye for several seconds. Despite the widespread use of CFS to study unconscious visual processes, the temporal tuning of CFS suppression is currently unknown. In the present study we used spatiotemporally filtered dynamic noise as masking stimuli to probe the temporal characteristics of CFS. Surprisingly, we find that suppression in CFS peaks very prominently at approximately 1 Hz, well below the rates typically used in CFS studies (10 Hz or more). As well as a strong bias to low temporal frequencies, CFS suppression is greater for high spatial frequencies and increases with increasing masker contrast, indicating involvement of parvocellular/ventral mechanisms in the suppression process. These results are reminiscent of binocular rivalry, and unifies two phenomenon previously thought to require different explanations.

Time for Awareness: The Influence of Temporal Properties of the Mask on Continuous Flash Suppression Effectiveness

Visual processing is not instantaneous, but instead our conscious perception depends on the integration of sensory input over time. In the case of Continuous Flash Suppression (CFS), masks are flashed to one eye, suppressing awareness of stimuli presented to the other eye. One potential explanation of CFS is that it depends, at least in part, on the flashing mask continually interrupting visual processing before the stimulus reaches awareness. We investigated the temporal features of masks in two ways. First, we measured the suppression effectiveness of a wide range of masking frequencies (0-32Hz), using both complex (faces/houses) and simple (closed/open geometric shapes) stimuli. Second, we varied whether the different frequencies were interleaved within blocks or separated in homogenous blocks, in order to see if suppression was stronger or weaker when the frequency remained constant across trials. We found that break-through contrast differed dramatically between masking frequencies, with mask effectiveness following a skewed-normal curve peaking around 6Hz and little or no masking for low and high temporal frequencies. Peak frequency was similar for trial-randomized and block randomized conditions. In terms of type of stimulus, we found no significant difference in peak frequency between the stimulus groups (complex/simple, face/house, closed/open). These findings suggest that temporal factors play a critical role in perceptual awareness, perhaps due to interactions between mask frequency and the time frame of visual processing.

Perceptual overloading reveals illusory contour perception without awareness of the inducers

Unconscious perception is frequently examined by restricting visual input (e.g., using short stimulus durations followed by masking) to prevent that information from entering visual awareness. Failures to demonstrate perception without awareness may thus be a consequence of this restricted input rather than of limitations in unconscious perception. Here, we demonstrate a novel method that circumvents these significant drawbacks inherent in other methods. Using this new perceptual overloading technique (POT), in which stimuli are repeatedly presented in alternation with a stream of variable masks, we demonstrate illusory contour perception and modal completion even when subjects are completely unaware of the inducing elements. In addition to demonstrating a powerful new method to study consciousness by effectively gating robust visual input from visual awareness, we show that more complex contextual effects, previously considered to be a privilege only of conscious vision, can occur without awareness.

Unconscious Processing of Negative Animals and Objects: Role of the Amygdala Revealed by fMRI

Previous studies have shown that emotional stimuli can be processed through the amygdala without conscious awareness. The amygdala is also involved in processing animate and social information. However, it is unclear whether different categories of pictures (e.g., animals, objects) elicit different activity in the amygdale and other regions without conscious awareness. The objective of this study was to explore whether the factors of category, emotion and picture context modulate brain activation for unconscious processing. Pictures denoting different nonhuman animals and objects in negative and neutral emotional valences were presented using a sandwich-masking paradigm. Half of them were presented with human-related information in the contexts, and half were not. Our results showed significant interaction among category, emotion and context in the amygdala and subcortical regions. Specifically, negative animals elicited stronger activation in these regions than negative objects, especially with human contexts. In addition, there were different correlation patterns between the amygdala and cortical regions according to whether they included human context. There were limited activations in cortical category-related networks. These results suggest that the amygdala and subcortical regions dominantly process negative animals, and contextual information modulates their activities, making threatening stimuli that are most relevant to human survival preferentially processed without conscious awareness.

Colour Terms Affect Detection of Colour and Colour-Associated Objects Suppressed from Visual Awareness

The idea that language can affect how we see the world continues to create controversy. A potentially important study in this field has shown that when an object is suppressed from visual awareness using continuous flash suppression (a form of binocular rivalry), detection of the object is differently affected by a preceding word prime depending on whether the prime matches or does not match the object. This may suggest that language can affect early stages of vision. We replicated this paradigm and further investigated whether colour terms likewise influence the detection of colours or colour-associated object images suppressed from visual awareness by continuous flash suppression. This method presents rapidly changing visual noise to one eye while the target stimulus is presented to the other. It has been shown to delay conscious perception of a target for up to several minutes. In Experiment 1 we presented greyscale photos of objects. They were either preceded by a congruent object label, an incongruent label, or white noise. Detection sensitivity (d') and hit rates were significantly poorer for suppressed objects preceded by an incongruent label compared to a congruent label or noise. In Experiment 2, targets were coloured discs preceded by a colour term. Detection sensitivity was significantly worse for suppressed colour patches preceded by an incongruent colour term as compared to a congruent term or white noise. In Experiment 3 targets were suppressed greyscale object images preceded by an auditory presentation of a colour term. On congruent trials the colour term matched the object's stereotypical colour and on incongruent trials the colour term mismatched. Detection sensitivity was significantly poorer on incongruent trials than congruent trials. Overall, these findings suggest that colour terms affect awareness of coloured stimuli and colour- associated objects, and provide new evidence for language-perception interaction in the brain.

When audiovisual correspondence disturbs visual processing

Multisensory integration is known to create a more robust and reliable perceptual representation of one's environment. Specifically, a congruent auditory input can make a visual stimulus more salient, consequently enhancing the visibility and detection of the visual target. However, it remains largely unknown whether a congruent auditory input can also impair visual processing. In the current study, we demonstrate that temporally congruent auditory input disrupts visual processing, consequently slowing down visual target detection. More importantly, this cross-modal inhibition occurs only when the contrast of visual targets is high. When the contrast of visual targets is low, enhancement of visual target detection is observed, consistent with the prediction based on the principle of inverse effectiveness (PIE) in cross-modal integration. The switch of the behavioral effect of audiovisual interaction from benefit to cost further extends the PIE to encompass the suppressive cross-modal interaction.

Emotion biases voluntary vertical action only with visible cues

Emotional information influences our bodily experiences according to the space-valence metaphor (positive/negative is up/down). In the present study, we examined whether visible and invisible emotional stimuli could also modulate voluntary action. After observing an emotional image (e.g., positive, neutral, or negative), participants used a joystick to arbitrarily position a dot stimulus in a display. The emotional image was either masked (masked condition) or not (unmasked condition) via a continuous flash suppression technique, i.e., dynamic interocular masking. We found that in the unmasked condition, the placed position of the dot was significantly higher after observing the positive image compared with the negative image, but this difference was not present in the masked condition. Our findings suggest that conscious emotional information is necessary for activating sensorimotor representations of vertical directions, and voluntary action is performed based on these activations.

Visual Awareness Is Limited by the Representational Architecture of the Visual System

Visual perception and awareness have strict limitations. We suggest that one source of these limitations is the representational architecture of the visual system. Under this view, the extent to which items activate the same neural channels constrains the amount of information that can be processed by the visual system and ultimately reach awareness. Here, we measured how well stimuli from different categories (e.g., faces and cars) blocked one another from reaching awareness using two distinct paradigms that render stimuli invisible: visual masking and continuous flash suppression. Next, we used fMRI to measure the similarity of the neural responses elicited by these categories across the entire visual hierarchy. Overall, we found strong brain–behavior correlations within the ventral pathway, weaker correlations in the dorsal pathway, and no correlations in early visual cortex (V1–V3). These results suggest that the organization of higher level visual cortex constrains visual awareness and the overall processing capacity of visual cognition.

Interocular suppression prevents interference in a flanker task

Executive control of attention refers to processes that detect and resolve conflict among competing thoughts and actions. Despite the high-level nature of this faculty, the role of awareness in executive control of attention is not well understood. In this study, we used interocular suppression to mask the flankers in an arrow flanker task, in which the flankers and the target arrow were presented simultaneously in order to elicit executive control of attention. Participants were unable to detect the flanker arrows or to reliably identify their direction when masked. There was a typical conflict effect (prolonged reaction time and increased error rate under flanker-target incongruent condition compared to congruent condition) when the flanker arrows were unmasked, while the conflict effect was absent when the flanker arrows were masked with interocular suppression. These results suggest that blocking awareness of competing stimuli with interocular suppression prevents the involvement of executive control of attention.

Semantic-based crossmodal processing during visual suppression

To reveal the mechanisms underpinning the influence of auditory input on visual awareness, we examine, (1) whether purely semantic-based multisensory integration facilitates the access to visual awareness for familiar visual events, and (2) whether crossmodal semantic priming is the mechanism responsible for the semantic auditory influence on visual awareness. Using continuous flash suppression, we rendered dynamic and familiar visual events (e.g., a video clip of an approaching train) inaccessible to visual awareness. We manipulated the semantic auditory context of the videos by concurrently pairing them with a semantically matching soundtrack (congruent audiovisual condition), a semantically non-matching soundtrack (incongruent audiovisual condition), or with no soundtrack (neutral video-only condition). We found that participants identified the suppressed visual events significantly faster (an earlier breakup of suppression) in the congruent audiovisual condition compared to the incongruent audiovisual condition and video-only condition. However, this facilitatory influence of semantic auditory input was only observed when audiovisual stimulation co-occurred. Our results suggest that the enhanced visual processing with a semantically congruent auditory input occurs due to audiovisual crossmodal processing rather than semantic priming, which may occur even when visual information is not available to visual awareness.

Definitely maybe: can unconscious processes perform the same functions as conscious processes?

Hassin recently proposed the “Yes It Can” (YIC) principle to describe the division of labor between conscious and unconscious processes in human cognition. According to this principle, unconscious processes can carry out every fundamental high-level cognitive function that conscious processes can perform. In our commentary, we argue that the author presents an overly idealized review of the literature in support of the YIC principle. Furthermore, we point out that the dissimilar trends observed in social and cognitive psychology, with respect to published evidence of strong unconscious effects, can better be explained by the way how awareness is defined and measured in both research fields. Finally, we show that the experimental paradigm chosen by Hassin to rule out remaining objections against the YIC principle is unsuited to verify the new default notion that all high-level cognitive functions can unfold unconsciously.

Psychophysical "blinding" methods reveal a functional hierarchy of unconscious visual processing

Numerous non-invasive experimental "blinding" methods exist for suppressing the phenomenal awareness of visual stimuli. Not all of these suppressive methods occur at, and thus index, the same level of unconscious visual processing. This suggests that a functional hierarchy of unconscious visual processing can in principle be established. The empirical results of extant studies that have used a number of different methods and additional reasonable theoretical considerations suggest the following tentative hierarchy. At the highest levels in this hierarchy is unconscious processing indexed by object-substitution masking. The functional levels indexed by crowding, the attentional blink (and other attentional blinding methods), backward pattern masking, metacontrast masking, continuous flash suppression, sandwich masking, and single-flash interocular suppression, fall at progressively lower levels, while unconscious processing at the lowest levels is indexed by eye-based binocular-rivalry suppression. Although unconscious processing levels indexed by additional blinding methods is yet to be determined, a tentative placement at lower levels in the hierarchy is also given for unconscious processing indexed by Troxler fading and adaptation-induced blindness, and at higher levels in the hierarchy indexed by attentional blinding effects in addition to the level indexed by the attentional blink. The full mapping of levels in the functional hierarchy onto cortical activation sites and levels is yet to be determined. The existence of such a hierarchy bears importantly on the search for, and the distinctions between, neural correlates of conscious and unconscious vision.

To see or not to see--thalamo-cortical networks during blindsight and perceptual suppression

Even during moments when we fail to be fully aware of our environment, our brains never go silent. Instead, it appears that the brain can also operate in an alternate, unconscious mode. Delineating unconscious from conscious neural processes is a promising first step toward investigating how awareness emerges from brain activity. Here we focus on recent insights into the neuronal processes that contribute to visual function in the absence of a conscious visual percept. Drawing on insights from findings on the phenomenon of blindsight that results from injury to primary visual cortex and the results of experimentally induced perceptual suppression, we describe what kind of visual information the visual system analyzes unconsciously and we discuss the neuronal routing and responses that accompany this process. We conclude that unconscious processing of certain visual stimulus attributes, such as the presence of visual motion or the emotional expression of a face can occur in a geniculo-cortical circuit that runs independent from and in parallel to the predominant route through primary visual cortex. We speculate that in contrast, bidirectional neuronal interactions between cortex and the thalamic pulvinar nucleus that support large-scale neuronal integration and visual awareness are impeded during blindsight and perceptual suppression.

Brain activation to briefly presented emotional words: effects of stimulus awareness

It is unknown to what extent briefly presented emotional words can be processed without awareness. By means of two independent functional magnetic resonance imaging studies, using either a block or an event-related design, we investigated brain activation to very briefly presented threat related and neutral words during two backward masking conditions (with and without gap between target and mask). In both experiments, emotional words were perceived during the supraliminal "with gap" condition, but they were not recognized during the subliminal "without gap" condition, as indicated by signal detection theory analysis. Imaging results of both experiments showed increased activation of the amygdala, the medial prefrontal cortex and language-processing cortical areas to negative versus neutral words during supraliminal but not subliminal conditions. These results suggest that even very briefly presented emotional words are capable of triggering increased cortical and subcortical processing; however, only when awareness of these stimuli is given.

Comparing unconscious processing during continuous flash suppression and meta-contrast masking just under the limen of consciousness

Stimuli can be rendered invisible using a variety of methods and the method selected to demonstrate unconscious processing in a given study often appears to be arbitrary. Here, we compared unconscious processing under continuous flash suppression (CFS) and meta-contrast masking, using similar stimuli, tasks and measures. Participants were presented with a prime arrow followed by a target arrow. They made a speeded response to the target arrow direction and then reported on the prime’s visibility. Perception of the prime was made liminal using either meta-contrast masking (Experiment 1) or CFS (Experiments 2 and 3). Conscious perception of the prime was assessed using a sensitive visibility scale ranging from 0 to 3 and unconscious processing was measured as the priming effect on target discrimination performance of prime-target direction congruency when prime visibility was null. Crucially, in order to ensure that the critical stimuli were equally distant from the limen of consciousness, we sought stimulus and temporal parameters for which the proportion of 0-visibility trials was comparable for the two methods. We found that the method used to prevent conscious perception matters: unconscious processing was substantial with meta-contrast masking but absent with CFS. These findings suggest that CFS allows very little perceptual processing, if at all, and that previous reports of high-level and complex unconscious processing during CFS may result from partial awareness.

Perceptual and contextual awareness: methodological considerations in the search for the neural correlates of consciousness

In the last decades, the neural correlates of consciousness (NCCs) have been explored using both invasive and non-invasive recordings by comparing the brain activity elicited by seen versus unseen visual stimuli (i.e., the contrastive analysis). Here, we review a selection of these studies and discuss a set of considerations to improve the search for the NCCs using the contrastive analysis. In particular, we first argue in favor of implementing paradigms where different perceptual outputs are obtained using identical visual inputs. Second, we propose that the large disagreement in the field -in terms of the dissimilar neural patterns proposed as NCCs- is partially explained by the fact that different studies report the neural correlates of different conscious processes in the brain. More specifically, we distinguish between the perceptual awareness of a visual stimulus, associated to a boost in object-selective neural assemblies, and a more elaborate process (contextual awareness) that we argue is reflected in the firing of concept neurons in the medial temporal lobe, triggering a rich representation of the context, associations, and memories linked to the specific stimulus.