ERP and MEG correlates of visual consciousness: The second decade

Beyond task response-Pre-stimulus activity modulates contents of consciousness

The current discussion on the neural correlates of the contents of consciousness (NCCc) focuses mainly on the post-stimulus period of task-related activity. This neglects the substantial impact of the spontaneous or ongoing activity of the brain as manifest in pre-stimulus activity. Does the interaction of pre- and post-stimulus activity shape the contents of consciousness? Addressing this gap in our knowledge, we review and converge two recent lines of findings, that is, pre-stimulus alpha power and pre- and post-stimulus alpha trial-to-trial variability (TTV). The data show that pre-stimulus alpha power modulates post-stimulus activity including specifically the subjective features of conscious contents like confidence and vividness. At the same time, alpha pre-stimulus variability shapes post-stimulus TTV reduction including the associated contents of consciousness. We propose that non-additive rather than merely additive interaction of the internal pre-stimulus activity with the external stimulus in the alpha band is key for contents to become conscious. This is mediated by mechanisms on different levels including neurophysiological, neurocomputational, neurodynamic, neuropsychological and neurophenomenal levels. Overall, considering the interplay of pre-stimulus intrinsic and post-stimulus extrinsic activity across wider timescales, not just evoked responses in the post-stimulus period, is critical for identifying neural correlates of consciousness. This is well in line with both processing and especially the Temporo-spatial theory of consciousness (TTC).

A measure centrality index for systematic empirical comparison of consciousness theories

Consciousness science is marred by disparate constructs and methodologies, making it challenging to systematically compare theories. This foundational crisis casts doubts on the scientific character of the field itself. Addressing it, we propose a framework for systematically comparing consciousness theories by introducing a novel inter-theory classification interface, the Measure Centrality Index (MCI). Recognizing its gradient distribution, the MCI assesses the degree of importance a specific empirical measure has for a given consciousness theory. We apply the MCI to probe how the empirical measures of the Global Neuronal Workspace Theory (GNW), Integrated Information Theory (IIT), and Temporospatial Theory of Consciousness (TTC) would fare within the context of the other two. We demonstrate that direct comparison of IIT, GNW, and TTC is meaningful and valid for some measures like Lempel-Ziv Complexity (LZC), Autocorrelation Window (ACW), and possibly Mutual Information (MI). In contrast, it is problematic for others like the anatomical and physiological neural correlates of consciousness (NCC) due to their MCI-based differential weightings within the structure of the theories. In sum, we introduce and provide proof-of-principle of a novel systematic method for direct inter-theory empirical comparisons, thereby addressing isolated evolution of theories and confirmatory bias issues in the state-of-the-art neuroscience of consciousness.

Task-based EEG and fMRI paradigms in a multimodal clinical diagnostic framework for disorders of consciousness

Disorders of consciousness (DoC) are generally diagnosed by clinical assessment, which is a predominantly motor-driven process and accounts for up to 40 % of non-communication being misdiagnosed as unresponsive wakefulness syndrome (UWS) (previously known as prolonged/persistent vegetative state). Given the consequences of misdiagnosis, a more reliable and objective multimodal protocol to diagnosing DoC is needed, but has not been produced due to concerns regarding their interpretation and reliability. Of the techniques commonly used to detect consciousness in DoC, task-based paradigms (active paradigms) produce the most unequivocal result when findings are positive. It is well-established that command following (CF) reliably reflects preserved consciousness. Task-based electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) can detect motor-independent CF and reveal preserved covert consciousness in up to 14 % of UWS patients. Accordingly, to improve the diagnostic accuracy of DoC, we propose a practical multimodal clinical decision framework centered on task-based EEG and fMRI, and complemented by measures like transcranial magnetic stimulation (TMS-EEG).

Late Positivity Correlates with Subjective Reports: Evidence from the Low-frequency and High-frequency Reporting Tasks

Identifying the neural correlates of consciousness (NCCs) is an important way to understand the fundamental nature of consciousness. By recording event-related potentials (ERPs) using EEG, researchers have found three potential electrophysiological NCCs: early positive correlate of consciousness (enhanced P1), visual awareness negativity (VAN), and late positivity (LP). However, LP may reflect post-perceptual processing associated with subjective reports rather than consciousness per se. The present experiment investigated the relationship between LP and subjective reports. We adopted two subjective reporting tasks that differed in the requirement for subjective reports. In the low-frequency reporting task, participants needed to report whether they saw the target picture in 25% of trials, whereas in the high-frequency reporting task, participants needed to report whether they saw the target picture in each trial. Behavioral results showed that the hit rates were lower and false alarm rates were higher on reporting trials in low-frequency reporting tasks than on reporting trials in high-frequency reporting tasks. Unexpectedly, VAN was larger on reporting trials in the low-frequency reporting task than on reporting trials in the high-frequency reporting task. Importantly, our ERP results showed that LP was larger on reporting trials in the high-frequency reporting task than on reporting trials in the low-frequency reporting task. Thus, our findings indicated that when the frequency of reports was increased, the task relevance of the stimuli increased, which led to larger LP amplitudes. These findings suggest that LP correlates with subjective reports.

Electrophysiological correlates of sustained conscious perception

Previous research on the neural correlates of consciousness (NCC) in visual perception revealed an early event-related potential (ERP), the visual awareness negativity (VAN), to be associated with stimulus awareness. However, due to the use of brief stimulus presentations in previous studies, it remains unclear whether awareness-related negativities represent a transient onset-related response or correspond to the duration of a conscious percept. Studies are required that allow prolonged stimulus presentation under aware and unaware conditions. The present ERP study aimed to tackle this challenge by using a novel stimulation design. Male and female human participants (n = 62) performed a visual task while task-irrelevant line stimuli were presented in the background for either 500 or 1000 ms. The line stimuli sometimes contained a face, which needed so-called visual one-shot learning to be seen. Half of the participants were informed about the presence of the face, resulting in faces being perceived by the informed but not by the uninformed participants. Comparing ERPs between the informed and uninformed group revealed an enhanced negativity over occipitotemporal electrodes that persisted for the entire duration of stimulus presentation. Our results suggest that sustained visual awareness negativities (SVAN) are associated with the duration of stimulus presentation.

Conscious perception of fear in faces: Insights from high-density EEG and perceptual awareness scale with threshold stimuli

Contrary to the extensive research on processing subliminal and/or unattended emotional facial expressions, only a minority of studies have investigated the neural correlates of consciousness (NCCs) of emotions conveyed by faces. In the present high-density electroencephalography (EEG) study, we first employed a staircase procedure to identify each participant's perceptual threshold of the emotion expressed by the face and then compared the EEG signals elicited in trials where the participants were aware with the activity elicited in trials where participants were unaware of the emotions expressed by these, otherwise identical, faces. Drawing on existing knowledge of the neural mechanisms of face processing and NCCs, we hypothesized that activity in frontal electrodes would be modulated in relation to participants' awareness of facial emotional content. More specifically, we hypothesized that the NCC of fear seen on someone else's face could be detected as a modulation of a later and more anterior (i.e., at frontal sites) event-related potential (ERP) than the face-sensitive N170. By adopting a data-driven approach and cluster-based statistics to the analysis of EEG signals, the results were clear-cut in showing that visual awareness of fear was associated with the modulation of a frontal ERP component in a 150-300 msec interval. These insights are dissected and contextualized in relation to prevailing theories of visual consciousness and their proposed NCC benchmarks.

N2 Responses in Youths With Psychosis Risk Syndrome and Their Association With Clinical Outcomes: A Cohort Follow-Up Study Based on the Three-Stimulus Visual Oddball Paradigm

OBJECTIVE

Schizophrenia often occurs during youth, and psychosis risk syndrome occurs before the onset of psychosis. The aim of this study was to determine whether the visual event-related potential responses in youths with psychosis risk syndrome were defective in the presence of interference stimuli and associated with their clinical outcomes.

METHODS

A total of 223 participants, including 122 patients with psychosis risk syndrome, 50 patients with emotional disorders, and 51 healthy control subjects, were assessed. Baseline EEG was recorded during the three-stimulus visual oddball task. The event-related potentials induced by square pictures with different colors were measured. Almost all patients with psychosis risk syndrome were followed up for 12 months and were reclassified into three subgroups: conversion, symptomatic, and remission. The differences in baseline event-related potential responses were compared among the clinical outcome subgroups.

RESULTS

The average N2 amplitude of the psychosis risk syndrome group was significantly less negative than that in the healthy control group (d=0.53). The baseline average N2 amplitude in the conversion subgroup was significantly less negative than that in the symptomatic (d=0.58) and remission (d=0.50) subgroups and in the healthy control group (d=0.97). The average N2 amplitude did not differ significantly between the symptomatic and remission subgroups (d=0.02). However, it was significantly less negative in the symptomatic and remission subgroups than in the healthy control group (d=0.46 and d=0.38). No statistically significant results were found in the P3 response.

CONCLUSIONS

Youths with psychosis risk syndrome had significant N2 amplitude defects in attention processing with interference stimuli. N2 amplitude shows potential as a prognostic biomarker of clinical outcome in the psychosis risk syndrome.

Transcranial magnetic stimulation effects support an oscillatory model of ERP genesis

Whether prestimulus oscillatory brain activity contributes to the generation of post-stimulus-evoked neural responses has long been debated, but findings remain inconclusive. We first investigated the hypothesized relationship via EEG recordings during a perceptual task with this correlational evidence causally probed subsequently by means of online rhythmic transcranial magnetic stimulation. Both approaches revealed a close link between prestimulus individual alpha frequency (IAF) and P1 latency, with faster IAF being related to shorter latencies, best explained via phase-reset mechanisms. Moreover, prestimulus alpha amplitude predicted P3 size, best explained via additive (correlational and causal evidence) and baseline shift mechanisms (correlational evidence), each with distinct prestimulus alpha contributors. Finally, in terms of performance, faster prestimulus IAF and shorter P1 latencies were both associated with higher task accuracy, while lower prestimulus alpha amplitudes and higher P3 amplitudes were associated with higher confidence ratings. Our results are in favor of the oscillatory model of ERP genesis and modulation, shedding new light on the mechanistic relationship between prestimulus oscillations and functionally relevant evoked components.

Feedforward connectivity patterns from visual areas to the front of the brain contain information about sensory stimuli regardless of awareness or report

Current theories of consciousness can be categorized to some extent by their predictions about the putative role of the prefrontal cortex (PFC) in conscious perception. One family of the theories proposes that the PFC is necessary for conscious perception. The other postulates that the PFC is not necessary and that other areas (e.g., posterior cortical areas) are more important for conscious perception. No-report paradigms could potentially arbitrate the debate as they disentangle task reporting from conscious perception. While previous no-report paradigms tend to point to a reduction in PFC activity, they have not examined the critical role of the PFC in "monitoring" or "reading out" the patterns of activity in the sensory cortex to generate conscious perception. To address this, we reanalysed electroencephalography (EEG) data from a no-report inattentional blindness paradigm (Shafto & Pitts, 2015). We examined the role of feedforward input patterns to the PFC from sensory cortices. We employed nonparametric spectral Granger causality and quantified the amount of information that reflected the contents of consciousness using multivariate classifiers. Unexpectedly, regardless of whether the stimulus was consciously seen or not, we found that information relating to the current sensory stimulus was present in the pattern of inputs from visual areas to the PFC. In light of these findings, we suggest various theories of consciousness need to be revised to accommodate the fact that the contents of consciousness are decodable from the input patterns from posterior sensory regions to the PFC, regardless of awareness (or report).

Impairment of unconscious emotional processing after unilateral medial temporal structure resection

The role of the amygdala in unconscious emotional processing remains a topic of debate. Past lesion studies have indicated that amygdala damage leads to impaired electrodermal activity in response to subliminally presented emotional stimuli. However, electrodermal activity can reflect both emotional and nonemotional processes. To provide behavioral evidence highlighting the critical role of the amygdala in unconscious emotional processing, we examined patients (n = 16) who had undergone unilateral resection of medial temporal lobe structures, including the amygdala. We utilized the subliminal affective priming paradigm in conjunction with unilateral visual presentation. Fearful or happy dynamic facial expressions were presented in unilateral visual fields for 30 ms, serving as negative or positive primes. Subsequently, neutral target faces were displayed, and participants were tasked with rating the valence of these targets. Positive primes, compared to negative ones, enhanced valence ratings of the target to a greater extent when they stimulated the intact hemisphere (i.e., were presented in the contralateral visual field of the intact hemisphere) than when they stimulated the resected hemisphere (i.e., were presented in the contralateral visual field of the resected hemisphere). These results suggest that the amygdala is causally involved in unconscious emotional processing.

Distrust before first sight? Examining knowledge- and appearance-based effects of trustworthiness on the visual consciousness of faces

The present EEG study with 32 healthy participants investigated whether affective knowledge about a person influences the visual awareness of their face, additionally considering the impact of facial appearance. Faces differing in perceived trustworthiness based on appearance were associated with negative or neutral social information and shown as target stimuli in an attentional blink task. As expected, participants showed enhanced awareness of faces associated with negative compared to neutral social information. On the neurophysiological level, this effect was connected to differences in the time range of the early posterior negativity (EPN)-a component associated with enhanced attention and facilitated processing of emotional stimuli. The findings indicate that the social-affective relevance of a face based on emotional knowledge is accessed during a phase of attentional enhancement for conscious perception and can affect prioritization for awareness. In contrast, no clear evidence for influences of facial trustworthiness during the attentional blink was found.

Multimodal acoustic-electric trigeminal nerve stimulation modulates conscious perception

Multimodal stimulation can reverse pathological neural activity and improve symptoms in neuropsychiatric diseases. Recent research shows that multimodal acoustic-electric trigeminal-nerve stimulation (TNS) (i.e., musical stimulation synchronized to electrical stimulation of the trigeminal nerve) can improve consciousness in patients with disorders of consciousness. However, the reliability and mechanism of this novel approach remain largely unknown. We explored the effects of multimodal acoustic-electric TNS in healthy human participants by assessing conscious perception before and after stimulation using behavioral and neural measures in tactile and auditory target-detection tasks. To explore the mechanisms underlying the putative effects of acoustic-electric stimulation, we fitted a biologically plausible neural network model to the neural data using dynamic causal modeling. We observed that (1) acoustic-electric stimulation improves conscious tactile perception without a concomitant change in auditory perception, (2) this improvement is caused by the interplay of the acoustic and electric stimulation rather than any of the unimodal stimulation alone, and (3) the effect of acoustic-electric stimulation on conscious perception correlates with inter-regional connection changes in a recurrent neural processing model. These results provide evidence that acoustic-electric TNS can promote conscious perception. Alterations in inter-regional cortical connections might be the mechanism by which acoustic-electric TNS achieves its consciousness benefits.

Stimulus awareness is associated with secondary somatosensory cortex activation in an inattentional numbness paradigm

While inattentional blindness and deafness studies have revealed neural correlates of consciousness (NCC) without the confound of task relevance in the visual and auditory modality, comparable studies for the somatosensory modality are lacking. Here, we investigated NCC using functional magnetic resonance imaging (fMRI) in an inattentional numbness paradigm. Participants (N = 44) received weak electrical stimulation on the left hand while solving a demanding visual task. Half of the participants were informed that task-irrelevant weak tactile stimuli above the detection threshold would be applied during the experiment, while the other half expected stimuli below the detection threshold. Unexpected awareness assessments after the experiment revealed that altogether 10 participants did not consciously perceive the somatosensory stimuli during the visual task. Awareness was not significantly modulated by prior information. The fMRI data show that awareness of stimuli led to increased activation in the contralateral secondary somatosensory cortex. We found no significant effects of stimulus awareness in the primary somatosensory cortex or frontoparietal areas. Thus, our results support the hypothesis that somatosensory stimulus awareness is mainly based on activation in higher areas of the somatosensory cortex and does not require strong activation in extended anterior or posterior networks, which is usually seen when perceived stimuli are task-relevant.

Rapid withdrawal from a threatening animal is movement-specific and mediated by reflex-like neural processing

Responses to potentially dangerous stimuli are among the most basic animal behaviors. While research has shown that threats automatically capture the attention of human participants, research has failed to demonstrate automatic behavioral responses to threats in humans. Using a novel naturalistic paradigm, we show that two species of animals humans often report fearing trigger rapid withdrawal responses: participants withdrew their arm from photos of snakes and spiders faster, and with higher acceleration when compared to bird and butterfly stimuli. The behavior was specific to withdrawal as approach movements or button-press/release tasks failed to detect a similar difference. Moreover, between-participant differences in how aversive they found the stimuli predicted the participant's withdrawal speed, indicating that the paradigm was also sensitive to trait-level differences between individuals. Using electroencephalography (EEG), we show that the fast withdrawal was mediated by two attentional processes. First, fast withdrawal responses were associated with early amplification of sensory signals (40-110 ms after stimulus). Second, a later correlate of feature-based attention (early posterior negativity, EPN, 200-240 ms after stimulus) revealed the opposite pattern: Stronger EPN was associated with slower behavioral responses, suggesting that the deployment of attention towards the threatening stimulus features, or failure to "disengage" attention from the stimulus, was detrimental for withdrawal speed. Altogether, the results suggest that rapid behavioral withdrawal from a threatening animal is mediated by reflex-like attentional processing, and later, conscious attention to stimulus features may hinder escaping the treat.

Young children's subjective and objective thresholds and emergent processes of visual consciousness using a backward masking task

Visual consciousness studies in humans have primarily focused on adults. However, whether young children's visual consciousness is similar to or different from that of adults remains unknown. This study examined young children's and adults' subjective awareness and objective discrimination for thresholds and emergent processes of visual consciousness in two experiments. In Experiment 1, 20 5-6-year-olds and 20 adults attempted a figure discrimination task using a square or a diamond as the target stimulus and responded, using a two-point scale, to a question on subjective awareness of the target stimulus with stimulus onset asynchronies (SOA) from 20 to 260 ms. In Experiment 2, 31 5-6-year-olds and 16 adults attempted the task and responded, using a four-point scale, to a question on subjective awareness with SOA from 50 to 550 ms. We measured the discrimination accuracy and the awareness scale with SOA and fit them to the sigmoid function. The results showed that the objective accuracy threshold of young children was larger than that of adults. Moreover, young children's subjective awareness threshold was larger than that of adults in the four-point but not in the two-point scale responses. Finally, there were no age differences in the emergent process of consciousness. This study suggests that the emergent process of consciousness in young children is similar to that in adults, however, the threshold in young children is larger than that in adults.

Decoding neural patterns for the processing of fearful faces under different visual awareness conditions: A multivariate pattern analysis

Previous studies have provided mixed findings regarding the nonconscious processing of fearful faces. Here, we used multivariate pattern analysis on electroencephalography data from three backward masking experiments to examine the processing of fearful faces under different visual awareness conditions. Three groups of participants were shown pairs of face images presented very briefly (for 16 ms) or for sufficiently long (for 266 ms), and completed tasks where the faces were either relevant to the experimental task (Experiment 1) or not (Experiments 2 and 3). Three main decoding analyses were performed. First, in the visual awareness decoding, the visibility of the faces, and hence participants' awareness of them, was maximally decodable in three time windows: 158-168 ms, 235-260 ms and 400-600 ms where the earlier neural patterns were generalized to the later stage activity. Second, we found that the spatial location of a fearful face in the face pairs was decodable, however only when the faces were consciously seen and task-relevant. Finally, we successfully decoded distinct neural patterns associated with the fearful-face-present conditions, compared to the fearful-face-absent conditions, and these patterns were decodable during both short and long presentations of the faces. Together, our results suggest that, while the processing of the spatial location of fearful faces requires awareness and task-relevancy, the mere presence of fearful faces can be processed even when visual awareness is highly restricted.

Differential modulation on neural activity related to flankers during face processing: A visual crowding study

In this visual crowding study, we manipulated the perceivability of a central crowded face (a fearful or a neutral face) by varying the similarity between the central face and the surrounding flanker stimuli. We presented participants with pairs of visual clutters and recorded their electroencephalography during an emotion judgement task. In an upright flanker condition where both the central target face and flanker faces were upright faces, participants were less likely to report seeing the target face, and their P300 was weakened, compared to a scrambled flanker condition where scrambled face images were used as flankers. Additionally, at ∼ 120 ms post-stimulus, a posterior negativity was found for the upright compared to scrambled flanker condition, however only for fearful face targets. We concluded that early neural responses seem to be affected by the perceptual characteristics of both target and flanker stimuli whereas later-stage neural activity is associated with post-perceptual evaluation of the stimuli in this visual crowding paradigm.

Neural electrophysiological correlates of detection and identification awareness

Humans have conscious experiences of the events in their environment. Previous research from electroencephalography (EEG) has shown visual awareness negativity (VAN) at about 200 ms to be a neural correlate of consciousness (NCC). However, when considering VAN as an NCC, it is important to explore which particular experiences are associated with VAN. Recent research proposes that VAN is an NCC of lower-level experiences (detection) rather than higher-level experiences (identification). However, previous results are mixed and have several limitations. In the present study, the stimulus was a ring with a Gabor patch tilting either left or right. On each trial, subjects rated their awareness on a three-level perceptual awareness scale that captured both detection (something vs. nothing) and identification (identification vs. something). Separate staircases were used to adjust stimulus opacity to the detection threshold and the identification threshold. Bayesian linear mixed models provided extreme evidence (BF10 = 131) that VAN was stronger at the detection threshold than at the identification threshold. Mean VAN decreased from [Formula: see text]2.12 microV [[Formula: see text]2.86, [Formula: see text]1.42] at detection to [Formula: see text]0.46 microV [[Formula: see text]0.79, [Formula: see text]0.11] at identification. These results strongly support the claim that VAN is an NCC of lower-level experiences of seeing something rather than of higher-level experiences of specific properties of the stimuli. Thus, results are consistent with recurrent processing theory in that phenomenal visual consciousness is reflected by VAN. Further, results emphasize that it is important to consider the level of experience when searching for NCC.

Faces capture spatial attention only when we want them to: An inattentional blindness EEG study

Previous research on emotional face processing has shown that emotional faces such as fearful faces may be processed without visual awareness. However, evidence for nonconscious attention capture by fearful faces is limited. In fact, studies using sensory manipulation of awareness (e.g., backward masking paradigms) have shown that fearful faces do not attract attention during subliminal viewings nor when they were task-irrelevant. Here, we used a three-phase inattentional blindness paradigm and electroencephalography to examine whether faces (fearful and neutral) capture attention under different conditions of awareness and task-relevancy. We found that the electrophysiological marker for attention capture, the N2-posterior-contralateral (N2pc), was elicited by face stimuli only when participants were aware of the faces and when they were task-relevant (phase 3). When participants were unaware of the presence of faces (phase 1) or when the faces were irrelevant to the task (phase 2), no N2pc was observed. Together with our previous work, we concluded that fearful faces, or faces in general, do not attract attention unless we want them to.

Frontal-occipital phase synchronization predicts occipital alpha power in perceptual decision-making

Numerous studies of perceptual decision-making have shown that lower prestimulus alpha power leads to a higher hit rate in visual detection, which is believed to correlate with the top-down control. However, whether frontal-occipital phase synchronization underlying the top-down control could impact the occipital alpha power that directly affects the perceptual performance remains unclear. In this study, we used analyses of the general linear mixed model (GLMM) and event-related potentials (ERPs) to show that the prestimulus alpha power over the occipital area directly affected visual perception. Using both the univariate and multivariate methods, we found that low-frequency (4-30 Hz) frontal-occipital phase synchronization predicted the prestimulus alpha power over the occipital area. Overall, our results suggested that frontal-occipital phase synchronization could predict occipital alpha power that directly affects perceptual decision-making.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11571-022-09862-7.

Fixation-related electrical potentials during a free visual search task reveal the timing of visual awareness

It has been repeatedly claimed that emotional faces readily capture attention, and that they may be processed without awareness. Yet some observations cast doubt on these assertions. Part of the problem may lie in the experimental paradigms employed. Here, we used a free viewing visual search task during electroencephalographic recordings, where participants searched for either fearful or neutral facial expressions among distractor expressions. Fixation-related potentials were computed for fearful and neutral targets and the response compared for stimuli consciously reported or not. We showed that awareness was associated with an electrophysiological negativity starting at around 110 ms, while emotional expressions were distinguished on the N170 and early posterior negativity only when stimuli were consciously reported. These results suggest that during unconstrained visual search, the earliest electrical correlate of awareness may emerge as early as 110 ms, and fixating at an emotional face without reporting it may not produce any unconscious processing.

The Current of Consciousness: Neural Correlates and Clinical Aspects

PURPOSE OF REVIEW

In this review, we summarize the current understanding of consciousness including its neuroanatomic basis. We discuss major theories of consciousness, physical exam-based and electroencephalographic metrics used to stratify levels of consciousness, and tools used to shed light on the neural correlates of the conscious experience. Lastly, we review an expanded category of 'disorders of consciousness,' which includes disorders that impact either the level or experience of consciousness.

RECENT FINDINGS

Recent studies have revealed many of the requisite EEG, ERP, and fMRI signals to predict aspects of the conscious experience. Neurological disorders that disrupt the reticular activating system can affect the level of consciousness, whereas cortical disorders from seizures and migraines to strokes and dementia may disrupt phenomenal consciousness. The recently introduced memory theory of consciousness provides a new explanation of phenomenal consciousness that may explain better than prior theories both experimental studies and the neurologist's clinical experience. Although the complete neurobiological basis of consciousness remains a mystery, recent advances have improved our understanding of the physiology underlying level of consciousness and phenomenal consciousness.

Neural correlates of conscious processing of emotional faces: Evidence from event-related potentials

There is a theoretical debate between the early and late neural correlates of consciousness (NCCs). Previous studies using neutral face stimuli supported an early NCC and suggested that visual awareness negativity (VAN) is associated with consciousness, while late positivity (LP) reflects post-perceptual activity. However, emotional faces may help to examine the relationship between LP and consciousness due to the differences in late processing between emotional and neutral faces. To explore the effects of facial emotional information on NCCs, the present study manipulated consciousness with the inattentional blindness paradigm and used happy, fearful, and neutral faces as visual stimuli. The results showed that the conscious processing of emotional faces was correlated with VAN and LP, while the conscious processing of neutral faces was associated with VAN. First, the results suggest that VAN is an NCC, and the relationship between LP and consciousness is affected by facial emotional information. Second, VAN reflects the early perceptual experience of emotional faces, whereas LP may reflect the late conscious processing of emotional faces. Furthermore, source localization analysis showed that the LPs of emotional faces were mainly located in the frontal and parietal lobes, whereas those of neutral faces showed no significant activation. This suggests that facial emotional information may affect the brain regions associated with conscious processing. Time-frequency analysis showed that conscious processing is related to the enhancement of alpha and theta oscillation, indicating that conscious processing may be associated with the suppression of irrelevant stimuli. Overall, the present study suggests that the integration of the theories that support early and late NCCs helps explain the conscious processing of emotional faces.

Continuous Theta Burst Stimulation to the left anterior medial prefrontal cortex influences metacognitive efficiency

The contribution of the prefrontal areas to visual awareness is critical for the Global Neuronal Workspace Theory and higher-order theories of consciousness. The goal of the present study was to test the potential engagement of the anterior medial prefrontal cortex (aMPFC) in visual awareness judgements. We aimed to temporarily influence the neuronal dynamics of the left aMPFC via neuroplasticity-like mechanisms. We used different Theta Burst Stimulation (TBS) protocols in combination with a visual identification task and visual awareness ratings. Either continuous TBS (cTBS), intermittent TBS (iTBS), or sham TBS was applied prior to the experimental paradigm in a within-participant design. Compared with sham TBS, we observed an increase in participants' ability to judge their perception adequately (metacognitive efficiency) following cTBS but not iTBS. The effect was accompanied by lower visual awareness ratings in incorrect responses. No significant differences in the identification task performance were observed. We interpret these results as evidence of the involvement of PFC in the brain network that underlies metacognition. Further, we discuss whether the results of TMS studies on perceptual metacognition can be taken as evidence for PFC involvement in awareness itself.

Spatiotemporal profile of altered neural reactivity to food images in obesity: Reward system is altered automatically and predicts efficacy of weight loss intervention

Introduction

Obesity presents a significant public health problem. Brain plays a central role in etiology and maintenance of obesity. Prior neuroimaging studies have found that individuals with obesity exhibit altered neural responses to images of food within the brain reward system and related brain networks. However, little is known about the dynamics of these neural responses or their relationship to later weight change. In particular, it is unknown if in obesity, the altered reward response to food images emerges early and automatically, or later, in the controlled stage of processing. It also remains unclear if the pretreatment reward system reactivity to food images is predictive of subsequent weight loss intervention outcome.

Methods

In this study, we presented high-calorie and low-calorie food, and nonfood images to individuals with obesity, who were then prescribed lifestyle changes, and matched normal-weight controls, and examined neural reactivity using magnetoencephalography (MEG). We performed whole-brain analysis to explore and characterize large-scale dynamics of brain systems affected in obesity, and tested two specific hypotheses: (1) in obese individuals, the altered reward system reactivity to food images occurs early and automatically, and (2) pretreatment reward system reactivity predicts the outcome of lifestyle weight loss intervention, with reduced activity associated with successful weight loss.

Results

We identified a distributed set of brain regions and their precise temporal dynamics that showed altered response patterns in obesity. Specifically, we found reduced neural reactivity to food images in brain networks of reward and cognitive control, and elevated reactivity in regions of attentional control and visual processing. The hypoactivity in reward system emerged early, in the automatic stage of processing (< 150 ms post-stimulus). Reduced reward and attention responsivity, and elevated neural cognitive control were predictive of weight loss after six months in treatment.

Discussion

In summary, we have identified, for the first time with high temporal resolution, the large-scale dynamics of brain reactivity to food images in obese versus normal-weight individuals, and have confirmed both our hypotheses. These findings have important implications for our understanding of neurocognition and eating behavior in obesity, and can facilitate development of novel integrated treatment strategies, including tailored cognitive-behavioral and pharmacological therapies.

The mechanisms of selective attention in phenomenal consciousness

In three experiments we investigated the effects of selective attention in iconic memory and fragile-visual short-term memory (VSTM), which have been related to phenomenal consciousness. We used a novel retro-cue paradigm with different delays (early vs late) and object priorities (high vs equal vs low), to investigate (a) attentional costs and benefits and the role of (b) bottom-up factors and (c) fragile-VSTM in feature-based attentional selection. Experiment 1 showed that attentional costs modulate visual maintenance at longer delays, while Experiment 2 showed that by reducing the time exposure of the memory array from 250 ms to 100 ms, as a bottom-up factor, participants were not able to select the objects based on their priorities. Finally, Experiment 3 showed that a pattern mask presented before the transfer in visual working memory, attenuates the overall performance while preserving the priority effect. The implications for phenomenal consciousness before conscious access are discussed.

Neural processing of lateralised task-irrelevant fearful faces under different awareness conditions

The neural fate of task-irrelevant emotional faces under different awareness conditions is poorly understood. Here, we examined the electrophysiological activity during an experiment where the location of target information (contrast-induced line) was manipulated orthogonally to the location of task-irrelevant fearful faces, under subliminal or supraliminal viewing conditions. We found that only target lines elicited an N2-posterior-contralateral (N2pc), indexing spatial attention shifting, in the supraliminal condition. No N2pc was found for the targets in the subliminal condition or for task-irrelevant fearful faces in either conditions. However, the mere presence of a fearful face enhanced early neural activity between 200 and 300 ms only in the subliminal condition. Additionally, the presence of a target line, but not a fearful face, enhanced the P3. Our results suggest that the N2pc is dependent on visual awareness and task-relevancy of the information and that laterally-presented task-irrelevant fearful expressions can be processed without awareness during early visual processing.

Prestimulus oscillatory brain activity interacts with evoked recurrent processing to facilitate conscious visual perception

We investigated whether prestimulus alpha-band oscillatory activity and stimulus-elicited recurrent processing interact to facilitate conscious visual perception. Participants tried to perceive a visual stimulus that was perceptually masked through object substitution masking (OSM). We showed that attenuated prestimulus alpha power was associated with greater negative-polarity stimulus-evoked ERP activity that resembled the visual awareness negativity (VAN), previously argued to reflect recurrent processing related to conscious perception. This effect, however, was not associated with better perception. Instead, when prestimulus alpha power was elevated, a preferred prestimulus alpha phase was associated with a greater VAN-like negativity, which was then associated with better cue perception. Cue perception was worse when prestimulus alpha power was elevated but the stimulus occurred at a nonoptimal prestimulus alpha phase and the VAN-like negativity was low. Our findings suggest that prestimulus alpha activity at a specific phase enables temporally selective recurrent processing that facilitates conscious perception in OSM.

Neural correlates of consciousness in an attentional blink paradigm with uncertain target relevance

Several event-related potentials (ERPs) have been proposed as neural correlates of consciousness (NCC), most prominently the early visual awareness negativity (VAN) and the late P3b component. Highly influential support for the P3b comes from studies utilizing the attentional blink (AB), where conscious perception of a first visual target (T1) impairs reporting a second target (T2) presented shortly afterwards. Recent no-report studies using other paradigms suggest that the P3b component may reflect post-perceptual processes associated with decision-making rather than awareness. However, no-report studies are limited in their awareness assessment, and their conclusions have not been tested in an AB paradigm. The present study (N = 38) addressed these issues using a novel AB paradigm, which reduced decision-making processes by omitting a discrimination task on T2 stimuli and rendering their relevance uncertain. Nevertheless, awareness was assessed trial by trial. Comparing ERPs in response to seen versus unseen T2 stimuli revealed a VAN but no enhanced P3b regardless of whether they were marked as distinct from distractor stimuli or not. Our results corroborate the VAN and challenge the P3b as NCC despite rigorous trial-by-trial assessment of conscious perception. Thus, they support the idea that awareness emerges during early sensory processing.

Task requirements affect the neural correlates of consciousness

In the search for the neural correlates of consciousness, it is often assumed that there is a stable set within the relevant sensory modality. Within the visual modality, the debate has centred upon whether frontal or occipital activations are the best predictors of perceptual awareness. Although not accepted by all as definitive evidence, no-report and decoding studies have indicated that occipital activity is the most consistently correlated with perceptual awareness whereas frontal activity might be closely related to aspects of cognition typically related to reports. However, perception is rarely just passive perception of something, but more or less always perception for something. That is, the task at hand for the perceiver may influence what is being perceived. This suggests an alternative view: that consciousness is not one specific 'function' that can be localized consistently to one area or event-related component and that the specific attributes of the neural correlates of consciousness depend on the task at hand. To investigate whether and how tasks may influence the neural correlates of consciousness, we here contrasted two tasks, a perceptual task and a conceptual task, using identical stimuli in both tasks. Using magnetoencephalography, we found that the perceptual task recruited more occipital resources than the conceptual task. Furthermore, we found that between the two conditions, the amount of frontal resources recruited differed between different gradations of perceptual awareness partly in an unexpected manner. These findings support a view of task affecting the neural correlates of consciousness.

Early and late electrophysiological correlates of gradual perceptual awareness in- and outside the Attentional Blink window

There is an ongoing debate on the neural correlates of consciousness (NCC) in the attentional blink (AB). Theoretical accounts propose that NCC during the attentional blink occur late in the processing hierarchy and that this quality is specific to the AB. We investigated this question by recording event-related potentials during an AB experiment with faces as T2. We analyzed ERPs to T2 stimuli inside (short lag) and outside (long lag) the AB window after carefully calibrating T2 stimuli to ensure equal visibility ratings across lags. We found that the N170, the visual awareness negativity (VAN), and the P3b showed an increased amplitude for seen compared to unseen face stimuli regardless of stimulus lag and that all these components scale linearly with subjective visibility. These findings suggest similar early and late mechanisms of graded perceptual awareness within and outside the AB across perceptual (N170, VAN) and post-perceptual (P3b) processing stages.

The Idiosyncrasy Principle: A New Look at Qualia

In the study of consciousness, qualia, the individual subjective experience, is neglected. It remains impenetrable because the objective perspective used for scientific investigations misses its subjective nature. In 1974, Thomas Nagel suggested that studying qualia requires an “objective phenomenology method” whose goal would be to describe the subjective character of experiences in an independent manner. We introduce a corresponding theoretical and experimental framework based on the “idiosyncrasy principle.” Accordingly, subjectivity depends on the idiosyncratic composition of physical properties to qualia. This allows conceptualizing an idiosyncratic transfer function between the world and its representation. The main challenge in delineating such a transfer function is to come up with an objective measure for another person’s perspective. Numerosity, as opposed to other perceived contents of the physical world, allows reporting subjective experience in an objective manner. On the basis of this unique attribute of numerosity, we suggest a tentative neurocognitive research plan aimed at delineating such idiosyncratic transfer functions, permitting one person to adopt the perspective of another and linking qualia to its mechanism.

The Effects of Spatial Attention Focus and Visual Awareness on the Processing of Fearful Faces: An ERP Study

Previous research on the relationship between attention and emotion processing have focused essentially on consciously-viewed, supraliminal stimuli, while the attention-emotion interplay remains unexplored in situations where visual awareness is restricted. Here, we presented participants with face pairs in a backward masking paradigm and examined the electrophysiological activity in response to fearful and neutral expressions under different conditions of attention (spatially attended vs. unattended) and stimulus visibility (subliminal vs. supraliminal). We found an enhanced N2 (visual awareness negativity -VAN-) and an enhanced P3 for supraliminal compared to subliminal faces. The VAN, indexing the early perceptual awareness, was enhanced when the faces were spatially attended compared to when they were unattended, showing that the VAN does not require spatial attention focus but can be enhanced by it. Fearful relative to neutral expressions enhanced the early neural activity (N2) regardless of spatial attention but only in the supraliminal viewing condition. However, fear-related enhancements on later neural activity (P3) were found when stimuli were both attended and presented supraliminally. These findings suggest that visual awareness is needed for emotion processing during both early and late stages. Spatial attention is required for emotion processing at the later stage but not at the early stage.

Spatial attention shifting to fearful faces depends on visual awareness in attentional blink: An ERP study

It remains unclear to date whether spatial attention towards emotional faces is contingent on, or independent of visual awareness. To investigate this question, a bilateral attentional blink paradigm was used in which lateralised fearful faces were presented at various levels of detectability. Twenty-six healthy participants were presented with two rapid serial streams of human faces, while they attempted to detect a pair of target faces (T2) displayed in close or distant succession of a first target pair (T1). Spatial attention shifting to the T2 fearful faces, indexed by the N2-posterior-contralateral component, was dependent on visual awareness and its magnitude covaried with the visual awareness negativity, a neural marker of awareness at the perceptual level. Additionally, information consolidation in working memory, indexed by the sustained posterior contralateral negativity, positively correlated with the level of visual awareness and spatial attention shifting. These findings demonstrate that spatial attention shifting to fearful faces depends on visual awareness, and these early processes are closely linked to information maintenance in working memory.

Where's My Consciousness-Ometer? How to Test for the Presence and Complexity of Consciousness

Tools and tests for measuring the presence and complexity of consciousness are becoming available, but there is no established theoretical approach for what these tools are measuring. This article examines several categories of tests for making reasonable inferences about the presence and complexity of consciousness (defined as the capacity for phenomenal/subjective experience) and also suggests ways in which different theories of consciousness may be empirically distinguished. We label the various ways to measure consciousness the measurable correlates of consciousness (MCC) and include three subcategories in our taxonomy: (a) neural correlates of consciousness, (b) behavioral correlates of consciousness, and (c) creative correlates of consciousness. Finally, we reflect on how broader philosophical views about the nature of consciousness, such as materialism and panpsychism, may also be informed by the scientific process.

The neural hierarchy of consciousness

The chief undertaking in the studies of consciousness is that of unravelling "the minimal set of neural processes that are together sufficient for the conscious experience of a particular content - the neural correlates of consciousness". To this day, this crusade remains at an impasse, with a clash of two main theories: consciousness may arise either in a graded and cortically-localised fashion, or in an all-or-none and widespread one. In spite of the long-lasting theoretical debates, neurophysiological theories of consciousness have been mostly dissociated from them. Herein, a theoretical review will be put forth with the aim to change that. In its first half, we will cover the hard available evidence on the neurophysiology of consciousness, whereas in its second half we will weave a series of considerations on both theories and substantiate a novel take on conscious awareness: the levels of processing approach, partitioning the conscious architecture into lower- and higher-order, graded and nonlinear.

Sliding Scale Theory of Attention and Consciousness/Unconsciousness

Attention defined as focusing on a unit of information plays a prominent role in both consciousness and the cognitive unconscious, due to its essential role in information processing. Existing theories of consciousness invariably address the relationship between attention and conscious awareness, ranging from attention is not required to crucial. However, these theories do not adequately or even remotely consider the contribution of attention to the cognitive unconscious. A valid theory of consciousness must also be a robust theory of the cognitive unconscious, a point rarely if ever considered. Current theories also emphasize human perceptual consciousness, primarily visual, despite evidence that consciousness occurs in diverse animal species varying in cognitive capacity, and across many forms of perceptual and thought consciousness. A comprehensive and parsimonious perspective applicable to the diversity of species demonstrating consciousness and the various forms—sliding scale theory of attention and consciousness/unconsciousness—is proposed with relevant research reviewed. Consistent with the continuous organization of natural events, attention occupies a sliding scale in regards to time and space compression. Unconscious attention in the form of the “cognitive unconscious” is time and spaced diffused, whereas conscious attention is tightly time and space compressed to the present moment. Due to the special clarity derived from brief and concentrated signals, the tight time and space compression yields conscious awareness as an emergent property. The present moment enhances the time and space compression of conscious attention, and contributes to an evolutionary explanation of conscious awareness.

Fast Unconscious Processing of Emotional Stimuli in Early Stages of the Visual Cortex

Several cortical and subcortical brain areas have been reported to be sensitive to the emotional content of subliminal stimuli. However, the timing of these activations remains unclear. Our scope was to detect the earliest cortical traces of emotional unconscious processing of visual stimuli by recording event-related potentials (ERPs) from 43 participants. Subliminal spiders (emotional) and wheels (neutral), sharing similar low-level visual parameters, were presented at two different locations (fixation and periphery). The differential (peak-to-peak) amplitude from CP1 (77 ms from stimulus onset) to C2 (100 ms), two early visual ERP components originated in V1/V2 according to source localization analyses, was analyzed via Bayesian and traditional frequentist analyses. Spiders elicited greater CP1–C2 amplitudes than wheels when presented at fixation. This fast effect of subliminal stimulation—not reported previously to the best of our knowledge—has implications in several debates: 1) The amygdala cannot be mediating these effects, 2) latency of other evaluative structures recently proposed, such as the visual thalamus, is compatible with these results, 3) the absence of peripheral stimuli effects points to a relevant role of the parvocellular visual system in unconscious processing.

Modality-specific and modality-general electrophysiological correlates of visual and auditory awareness: Evidence from a bimodal ERP experiment

To date, most studies on the event-related potential (ERP) correlates of conscious perception have examined a single perceptual modality. We compared electrophysiological correlates of visual and auditory awareness in the same experiment to test whether there are modality-specific and modality-general correlates of conscious perception. We used near threshold stimulation and analyzed event-related potentials in response to aware and unaware trials in visual, auditory and bimodal conditions. The results showed modality-specific negative amplitude correlates of conscious perception between 200 and 300 ms after stimulus onset. A combination of these auditory and visual awareness negativities was observed in the bimodal condition. A later positive amplitude difference, whose early part was modality-specific, possibly reflecting access to global workspace, and later part shared modality-general features, possibly indicating higher level cognitive processing involving the decision making, was also observed.

An active inference model of conscious access: How cognitive action selection reconciles the results of report and no-report paradigms

Cognitive theories of consciousness, such as global workspace theory and higher-order theories, posit that frontoparietal circuits play a crucial role in conscious access. However, recent studies using no-report paradigms have posed a challenge to cognitive theories by demonstrating conscious accessibility in the apparent absence of prefrontal cortex (PFC) activation. To address this challenge, this paper presents a computational model of conscious access, based upon active inference, that treats working memory gating as a cognitive action. We simulate a visual masking task and show that late P3b-like event-related potentials (ERPs), and increased PFC activity, are induced by the working memory demands of self-report generation. When reporting demands are removed, these late ERPs vanish and PFC activity is reduced. These results therefore reproduce, and potentially explain, results from no-report paradigms. However, even without reporting demands, our model shows that simulated PFC activity on visible stimulus trials still crosses the threshold for reportability - maintaining the link between PFC and conscious access. Therefore, our simulations show that evidence provided by no-report paradigms does not necessarily contradict cognitive theories of consciousness.

The Perceptual Awareness Scale-recent controversies and debates

Accurate insight into subjective experience is crucial for the science of consciousness. The Perceptual Awareness Scale (PAS) was created in 2004 as a method for obtaining precise introspective reports for participants in research projects, and since then, the scale has become increasingly popular. This does not mean, of course, that no critiques have been voiced. Here, we briefly recapitulate our main thoughts on the intended PAS usage and the findings of the first decade, and we update this with the latest empirical and theoretical developments. We focus specifically on findings with relevance to whether consciousness is gradual or all-or-none phenomenon, to what should be considered conscious/unconscious, and to whether PAS is preferable to alternative measures of awareness. We respond in detail to some recent, selected articles.

Representational 'touch' and modulatory 'retouch'-two necessary neurobiological processes in thalamocortical interaction for conscious experience

Theories of consciousness using neurobiological data or being influenced by these data have been focused either on states of consciousness or contents of consciousness. These theories have occasionally used evidence from psychophysical phenomena where conscious experience is a dependent experimental variable. However, systematic catalog of many such relevant phenomena has not been offered in terms of these theories. In the perceptual retouch theory of thalamocortical interaction, recently developed to become a blend with the dendritic integration theory, consciousness states and contents of consciousness are explained by the same mechanism. This general-purpose mechanism has modulation of the cortical layer-5 pyramidal neurons that represent contents of consciousness as its core. As a surplus, many experimental psychophysical phenomena of conscious perception can be explained by the workings of this mechanism. Historical origins and current views inherent in this theory are presented and reviewed.

Early and late evoked brain responses differentially reflect feature encoding and perception in the flash-lag illusion

In the flash-lag illusion (FLI), the position of a flash presented ahead of a moving bar is mislocalized, so the flash appears to lag the bar. Currently, it is not clear whether this effect is due to early perceptual-related neural processes such as motion extrapolation or reentrant processing, or due to later feedback processing relating to postdiction, i.e., retroactively altered perception. We presented 17 participants with the FLI paradigm while recording EEG. A central flash occurred either 51 ms ("early") or 16 ms ("late") before the bar moving from left to right reached the screen center. Participants judged whether the flash appeared to the right ("no flash lag illusion") or to the left ("flash-lag illusion") of the bar. Using single-trial linear modeling, we examined the influence of timing ("early" vs. "late") and perception ("illusion" vs. "no illusion") on flash-evoked brain responses and estimated the cortical sources underlying the FLI. An earlier frontal and occipital component (200-276 ms) differentiated time-locked early vs. late stimulus presentation, indicating that early evoked brain responses reflect feature encoding in the FLI. Perception of the FLI was associated with a late window (368-452 ms) in the ERP, with larger deflections for illusion than no illusion trials, localized to the left inferior occipital gyrus. This suggests a postdiction-related reconstruction of ambiguous sensory stimulation involving late processes in the occipito-temporal cortex, previously associated with temporal integration phenomena. Our findings indicate that perception of the FLI relies on an interplay between ongoing stimulus encoding of the moving bar and feedback processing of the flash, which takes place at later integration stages.

The myth of when and where: How false assumptions still haunt theories of consciousness

Recent advances in neural sciences have uncovered countless facts about the brain. Although there is a plethora of theories of consciousness, it seems to some philosophers that there is still an explanatory gap when it comes to a scientific account of subjective experience. In what follows, I argue why some of our more commonly acknowledged theories do not at all provide us with an account of subjective experience as they are built on false assumptions. These assumptions have led us into a state of cognitive dissonance between maintaining our standard scientific practices on the one hand, and maintaining our folk notions on the other. I end by proposing Illusionism as the only option for a scientific investigation of consciousness and that even if ideas like panpsychism turn out to be holding the seemingly missing piece of the puzzle, the path to them must go through Illusionism.

ERP signatures of auditory awareness in cross-modal distractor-induced deafness

Previous research showed that dual-task processes such as the attentional blink are not always transferable from unimodal to cross-modal settings. This study investigated whether such a transfer can be stated for a distractor-induced impairment of target detection established in vision (distractor-induced blindness, DIB) and recently observed in the auditory modality (distractor-induced deafness, DID). A cross-modal DID effect was confirmed: The detection of an auditory target indicated by a visual cue was impaired if multiple auditory distractors preceded the target. Event-related potentials (ERPs) were used to identify psychophysiological correlates of target detection. A frontal negativity about 200 ms succeeded by a sustained, widespread negativity was associated with auditory target awareness. In contrast to unimodal findings, P3 amplitude was not enhanced for hits. The results support the notion that early frontal attentional processes are linked to auditory awareness, whereas the P3 does not seem to be a reliable indicator of target access.

Involvement of Visual Mismatch Negativity in Access Processing to Visual Awareness

Psychophysiological studies with electroencephalography, focusing on the dynamical aspect of neural correlate of consciousness, reported that visual awareness negativity and P3 enhancement are observed at a latency, 200–300 ms after the visual stimulus onset, when the visual stimulus is consciously perceived. However, access processing to visual awareness (APVA) immediately before conscious perception still remains at the earlier stage of visual sensory processing, though there is little known regarding this subject. The present study hypothesized that visual mismatch negativity (vMMN), which reflects automatic change detection at a latency of 130–250 ms, might be involved in the APVA. In a previous study, vMMN was reported to be evoked by the deviant stimulus that is not consciously perceived in binocular rivalry. To clarify whether the visual change detection affects APVA, we conducted a modified experiment of oddball paradigm on binocular rivalry. The results showed a significant correlation between enhancement of vMMN amplitude and facilitation of perceptual alternation when the unconscious deviant was presented. This implies that vMMN is relevant to the APVA, which is a novel role of vMMN. In early visual processing, the attentional mechanism associated with vMMN is suggested to play an important role in unconscious neural processing at an earlier stage of visual awareness.

Expanding the discussion: Revision of the fundamental assumptions framing the study of the neural correlates of consciousness

The way one asks a question is shaped by a-priori assumptions and constrains the range of possible answers. We identify and test the assumptions underlying contemporary debates, models, and methodology in the study of the neural correlates of consciousness, which was framed by Crick and Koch's seminal paper (1990). These premises create a sequential and passive conception of conscious perception: it is considered the product of resolved information processing by unconscious mechanisms, produced by a singular event in time and place representing the moment of entry. The conscious percept produced is then automatically retained to be utilized by post-conscious mechanisms. Major debates in the field, such as concern the moment of entry, the all-or-none vs graded nature, and report vs no-report paradigms, are driven by the consensus on these assumptions. We show how removing these assumptions can resolve some of the debates and challenges and prompt additional questions. The potential non-sequential nature of perception suggests new ways of thinking about consciousness as a dynamic and dispersed process, and in turn about the relationship between conscious and unconscious perception. Moreover, it allows us to present a parsimonious account for conscious perception while addressing more aspects of the phenomenon.