Visual awareness negativity is an early neural correlate of awareness: A preregistered study with two Gabor sizes

Electrophysiological recordings are commonly used to study the neural correlates of consciousness in humans. Previous research is inconsistent as to whether awareness can be indexed with visual awareness negativity (VAN) at about 200 ms or if it occurs later. The present study was preregistered with two main aims: First, to provide independent evidence for or against the presence of VAN, and second, to study whether stimulus size may account for the inconsistent findings. Subjects were shown low-contrast Gaussian filtered gratings (Gabor patches) in the four visual quadrants. Gabor size (large and small) was varied in different sessions and calibrated to each subject’s threshold of visual awareness. Event-related potentials were derived from trials in which subjects localized the Gabors correctly to capture the difference between trials in which they reported awareness versus no awareness. Bayesian analyses revealed very strong evidence for the presence of VAN for both Gabor sizes. However, there was no evidence for or against an effect of stimulus size. The present findings provide evidence for VAN as an early neural correlate of awareness.     

Is auditory awareness negativity confounded by performance?

Research suggests that the electrophysiological correlates of consciousness are similar in hearing as in vision: the auditory awareness negativity (AAN) and the late positivity (LP). However, from a recently proposed signal-detection perspective, these correlates may be confounded by performance, as the strength of the internal responses differs between aware and unaware trials. Here, we tried to apply this signal-detection approach to correct for performance in an auditory discrimination and detection task (N = 28). A large proportion of subjects had to be excluded because even a small response bias distorted the correction. For the remaining subjects, the correction mainly increased noise in the measurement. Furthermore, the signal-detection approach is theoretically problematic because it may isolate post-perceptual processes and eliminate awareness-related activity. Therefore, we conclude that AAN and LP are not confounded by performance and that the contrastive analysis identifies both as correlates of awareness.     

情绪面孔的意识神经相关物及其无意识自动加工：来自事件相关电位的证据

意识的神经相关物尚有争议, 且个体能否无意识自动检测视觉环境变化尚不清楚。本研究采用非注意视盲范式操控视觉意识, 并引入具有社会信息的情绪面孔, 探讨意识的神经相关物以及视觉意识与自动检测变化机制的关系。在A阶段, 部分被试对任务无关的情绪面孔处于无意识水平; 在B阶段, 所有被试对任务无关的情绪面孔处于意识水平; 在C阶段, 所有被试对任务相关的情绪面孔处于意识水平。结果显示, 任务无关的情绪面孔的意识过程诱发视觉意识负波(visual awareness negativity, VAN)、晚期正成分(late positivity, LP)和晚期枕区正成分(late occipital positivity, LOP)。此外, 无意识的情绪面孔能诱发视觉失匹配负波(visual mismatch negativity, vMMN), 且其幅值不受意识影响, 但是受任务相关性调制。这些结果提示对情绪面孔的视觉意识在不同的时间进程上有不同的ERP指标——VAN反映早期知觉经验, 而LP和LOP反映晚期意识过程, 而且面孔情绪信息的自动加工独立于视觉意识, 但是受视觉注意调制。     

The role of selective attention in visual awareness of stimulus features: Electrophysiological studies

Attention and awareness are closely related, but the nature of their relationship is unclear. The present study explores the timing and temporal evolution of their interaction with event-related potentials. The participants attended to specific conjunctions of spatial frequency and orientation in masked (unaware) and unmasked (aware) visual stimuli. A correlate of awareness appeared 100-200 msec from stimulus onset similarly to both attended and unattended features. Selection negativity (SN), a correlate of attentional selection, emerged in response to both masked and unmasked stimuli after 200 msec. This double dissociation between correlates of awareness and SN suggests that the electrophysiological processes associated with feature-based attentional selection and visual awareness of features can be dissociated from each other at early stages of processing. In a passive task, requiring no attention to the stimuli, early electrophysiological responses (before 200 msec) related to awareness were attenuated, suggesting that focal attention modulates visual awareness earlier than does selective feature-based attention.     

Early and late mismatch negativity elicited by words and speech-like stimuli in children

In auditory perception the brain's attentional and preattentional mechanisms select certain stimuli for preferential processing and filter out irrelevant input. This study investigated nonattentive auditory processing in children. Event-related potentials (ERPs) provide a means to study neural correlates related to language and speech-sound processing. Mismatch negativity (MMN) is an ERP wave that indicates attention-independent perceptual change detection. In this study cortical ERPs were elicited by complex tones, naturally spoken words, and pseudowords, with each stimulus type containing equal acoustical elements. Tones elicited a bifurcated mismatch negativity (MMN), with early MMN (peaking at 150-200 ms) being more dominant. On the other hand, words elicited a strong late MMN, peaking at about 400-450 ms after stimulus onset. The MMN wave form was significantly weaker for pseudowords than for words. The late MMN wave, especially for word differences, was found to reflect summating MMN generators and memory trace formation on gestalt bases. Results suggest that the auditory processing, even nonattended, is highly associated with the cognitive meaning of the stimuli.     

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.     

Temporal dynamics of awareness for facial identity revealed with ERP

In this study, we investigated the scalp recorded event-related potential (ERP) responses related to visual awareness. A backward masking procedure was performed while high-density EEG recordings were carried out. Subjects were asked to detect a familiar face, presented at durations that varied parametrically between 16 and 266 ms. ERPs were computed and awareness was assessed using a sensitivity measure from signal detection theory (d'). Modifications in the electrical scalp topographies were found to reflect visual awareness of the stimulus. In particular, an early map topography was found to emerge progressively around 230 ms, showing a pattern of increase similar to the measure of visual awareness. This was followed by an increase in duration of a second, P300-like map. Source localisation for the early awareness-related topography revealed the activation of a distributed network of brain areas including frontal and temporo-occipital regions. Our results suggest that conscious experience emerges in parallel with the activation of a specific neural network that occurs in a time window beginning from about 200 ms.     

The earliest electrophysiological correlate of visual awareness?

To examine the neural correlates and timing of human visual awareness, we recorded event-related potentials (ERPs) in two experiments while the observers were detecting a grey dot that was presented near subjective threshold. ERPs were averaged for conscious detections of the stimulus (hits) and nondetections (misses) separately. Our results revealed that hits, as compared to misses, showed a negativity around 180-350 ms at occipital and posterior temporal sites. It was followed by a positive wave after 400-500 ms, peaking at parietal sites. These correlates were not affected by a manipulation of attention. The early negativity, called 'visual awareness negativity' (VAN), may be a general, primary electrophysiological correlate of visual awareness. The present data show that it can be observed in response to appearance of a stimulus in visual awareness and that it generalizes across different manipulations of stimulus visibility.     

Attentional orienting across the sensory modalities

This event-related potential study investigated (i) to what extent incongruence between attention-directing cue and cued target modality affects attentional control processes that bias the system in advance to favor a particular stimulus modality and (ii) to what extent top-down attentional control mechanisms are generalized for the type of information that is to be attended. To this end, both visual and auditory word cues were used to instruct participants to direct attention to a specific visual (color) or auditory (pitch) stimulus feature of a forthcoming multisensory target stimulus. Effects of cue congruency were observed within 200 ms post-cue over frontal scalp regions and related to processes involved in shifting attention from the cue modality to the modality of the task-relevant target feature. Both directing visual attention and directing auditory attention were associated with dorsal posterior positivity, followed by sustained fronto-central negativity. However, this fronto-central negativity appeared to have an earlier onset and was more pronounced when the visual modality was cued. Together the present results suggest that the mechanisms involved in deploying attention are to some extent determined by the modality (visual, auditory) in which attention operates, and in addition, that some of these mechanisms can also be affected by cue congruency.     

Danger and usefulness are detected early in auditory lexical processing: evidence from electroencephalography

Visual emotionally charged stimuli have been shown to elicit early electrophysiological responses (e.g., [Ihssen et al., 2007], [Schupp et al., 2003] and [Stolarova et al., 2006]). We presented isolated words to listeners, and observed, using generalized additive modeling, oscillations in the upper part of the delta range, the theta range (Bastiaansen & Hagoort, 2003), and the lower part of the alpha range related to degree of (rated) danger and usefulness (Wurm, 2007) starting around 150 ms and continuing to 350 ms post stimulus onset. A negative deflection in the oscillations tied to danger around 250-300 ms fits well with a similar negativity observed in the same time interval for visual emotion processing. Frequency and competitor effects emerged or reached maximal amplitude later, around or following the uniqueness point. The early effect of danger, long before the words’ uniqueness points, is interpreted as evidence for the dual pathway theory of LeDoux (1996).     

The electrophysiological correlates of stimulus visibility and metacontrast masking

There are conflicting views concerning the electrophysiological correlates of visual consciousness. Whereas one view considers a relatively late positive deflection (LP) as a primary correlate of consciousness, another model links consciousness with earlier negativity (VAN). The present experiment utilized metacontrast masking in investigating the electrophysiological correlates of visual consciousness. The participants were presented with target-mask sequences in three stimulus onset asynchronies. The target stimuli were followed by either a metacontrast mask or a similar-looking, but ineffective pseudomask. The results showed that the first deflection that correlated with target visibility was VAN which was followed by LP. We argue that the VAN is the primary correlate target visibility, while the LP reflects later, postperceptual processing stages.     

The role of levels of processing in disentangling the ERP signatures of conscious visual processing

We aimed to distinguish electrophysiological signatures of visual awareness from other task-related processes through manipulating the level of processing of visual stimuli. During an event-related EEG experiment, 36 subjects performed either color (low-level condition) or magnitude (high-level condition) evaluations of masked digits. Participants also assessed subjective visibility of each stimulus using the Perceptual Awareness Scale (PAS). Mean amplitude of the components of interest was analyzed (VAN − 140–240 ms; LP − 380–480 ms) with weighted regression mixed model. In the VAN component time window the mean amplitude correlated with PAS rating in both conditions. Mean amplitude in the LP time window correlated with PAS ratings in the high-level condition, but not in the low-level condition. Our results support the temporal unfolding of ERP makers of conscious processing, with an early component reflecting the initial perceptual experience and a late component being a correlate of the conscious experience of non-perceptual information.     

Toward an electrocortical biomarker of cognition for newborn infants

The event-related potential (ERP) effect of mismatch negativity (MMN) was the first electrophysiological probe to evaluate cognitive processing (change detection) in newborn infants. Initial studies of MMN predicted clinical utility for this measure in identification of infants at risk for developmental cognitive deficits. These predictions have not been realized. We hypothesized that in sleeping newborn infants, measures derived from wavelet assessment of power in the MMN paradigm would be more robust markers of the brain's response to stimulus change than the ERP-derived MMN. Consistent with this premise, we found increased power in response to unpredictable and infrequent tones compared to frequent tones. These increases were present at multiple locations on the scalp over a range of latencies and frequencies and occurred even in the absence of an ERP-derived MMN. There were two predominant effects. First, theta band power was elevated at middle and late latencies (200 to 600 ms), suggesting that neocortical theta rhythms that subserve working memory in adults are present at birth. Second, late latency (500 ms) increased power to the unpredictable and infrequent tones was observed in the beta and gamma bands, suggesting that oscillations involved in adult cognition are also present in the neonate. These findings support the expectation that frequency dependent measures, such as wavelet power, will improve the prospects for a clinically useful test of cortical function early in the postnatal period.     

ERP and MEG correlates of visual consciousness: The second decade

The first decade of event-related potential (ERP) research had established that the most consistent correlates of the onset of visual consciousness are the early visual awareness negativity (VAN), a posterior negative component in the N2 time range, and the late positivity (LP), an anterior positive component in the P3 time range. Two earlier extensive reviews ten years ago had concluded that VAN is the earliest and most reliable correlate of visual phenomenal consciousness, whereas LP probably reflects later processes associated with reflective/access consciousness. This article provides an update to those earlier reviews. ERP and MEG studies that have appeared since 2010 and directly compared ERPs between aware and unaware conditions are reviewed, and important new developments in the field are discussed. The result corroborates VAN as the earliest and most consistent signature of visual phenomenal consciousness, and casts further doubt on LP as an ERP correlate of phenomenal consciousness.     

A dissociation between selective attention and conscious awareness in the representation of temporal order information

Previous electrophysiological studies have shown that attentional selection processes are highly sensitive to the temporal order of task-relevant visual events. When two successively presented colour-defined target stimuli are separated by a stimulus onset asynchrony (SOA) of only 10 ms, the onset latencies of N2pc components to these stimuli (which reflect their attentional selection) precisely match their objective temporal separation. We tested whether such small onset differences are accessible to conscious awareness by instructing participants to report the category (letter or digit) of the first of two target-colour items that were separated by an SOA of 10, 20, or 30 ms. Performance was at chance level for the 10 ms SOA, demonstrating that temporal order information which is available to attentional control processes cannot be utilized for conscious temporal order judgments. These results provide new evidence that selective attention and conscious awareness are functionally separable, and support the hypothesis that attention and awareness operate at different stages of cognitive processing.     

Links between rapid ERP responses to fearful faces and conscious awareness

To study links between rapid ERP responses to fearful faces and conscious awareness, a backward‐masking paradigm was employed where fearful or neutral target faces were presented for different durations and were followed by a neutral face mask. Participants had to report target face expression on each trial. When masked faces were clearly visible (200 ms duration), an early frontal positivity, a later more broadly distributed positivity, and a temporo‐occipital negativity were elicited by fearful relative to neutral faces, confirming findings from previous studies with unmasked faces. These emotion‐specific effects were also triggered when masked faces were presented for only 17 ms, but only on trials where fearful faces were successfully detected. When masked faces were shown for 50 ms, a smaller but reliable frontal positivity was also elicited by undetected fearful faces. These results demonstrate that early ERP responses to fearful faces are linked to observers' subjective conscious awareness of such faces, as reflected by their perceptual reports. They suggest that frontal brain regions involved in the construction of conscious representations of facial expression are activated at very short latencies.     

Tracking the processes behind conscious perception: a review of event-related potential correlates of visual consciousness

Event-related potential (ERP) studies have attempted to discover the processes that underlie conscious visual perception by contrasting ERPs produced by stimuli that are consciously perceived with those that are not. Variability of the proposed ERP correlates of consciousness is considerable: the earliest proposed ERP correlate of consciousness (P1) coincides with sensory processes and the last one (P3) marks postperceptual processes. A negative difference wave called visual awareness negativity (VAN), typically observed around 200 ms after stimulus onset in occipitotemporal sites, gains strong support for reflecting the processes that correlate with, and possibly enable, aware visual perception. Research suggests that the early parts of conscious processing can proceed independently of top-down attention, although top-down attention may modulate visual processing even before consciousness. Evidence implies that the contents of consciousness are provided by interactions in the ventral stream, but indispensable contributions from dorsal regions influence already low-level visual responses.     

The auditory sensory memory trace decays rapidly in newborns

The present study investigated the temporal dynamics of auditory sensory memory in newborns as reflected by the mismatch negativity (MMN), a preattentive electric change-detection response. MMN was obtained from 24 full-term healthy newborns who were either awake or asleep (quiet or active sleep) during the experiments. Stimuli were 1,000 Hz tones (standards) that were occasionally replaced by 1,100 Hz tones (deviants). The constant stimulus onset asynchrony (SOA) was, in separate blocks, either 450, 800, or 1,500 ms. A prominent MMN was obtained at the 800 ms SOA in all three sleep or waking states, whereas no MMN occurred at 450 and 1,500 ms SOAs. In view of the fact that in adults MMN is elicited even with a 10s SOA, these results imply that the time span of auditory memory is considerably shorter in neonates than in adults and 8-12-year-old children.     

Acquired equivalences between auditory stimuli in dolphins (Tursiops truncatus)

This study investigated whether dolphins would show evidence of equivalence class formation between auditory stimuli. Bottlenose dolphins were trained to press one or other of two response levers depending on which one of four auditory stimuli had been previously presented. Once they had learned the initial discriminations, the stimulus-lever contingencies was repeatedly reversed. Within any given session, however, pressing of one lever always led to reward with one set of two tones and pressing the other lever led to non-reward with an alternative set of two tones. After sufficient experience with this response reversal procedure, the dolphins spontaneously chose the same levers they had first learned to be correct with one of the across-set stimulus pairs when later in the session they were presented with the other of the across-set stimulus pairs. They thus demonstrated that they had associated the tones belonging to the two sets within two separate functional classes. It is discussed why the dolphins succeeded with auditory stimuli when they had previously failed in a similar task with visual stimuli. Received: 7 December 1999 / Accepted after revision: 23 June 2000     

Pitch discrimination without awareness in congenital amusia: Evidence from event-related potentials

Congenital amusia is a lifelong disorder characterized by a difficulty in perceiving and producing music despite normal intelligence and hearing. Behavioral data have indicated that it originates from a deficit in fine-grained pitch discrimination, and is expressed by the absence of a P3b event-related brain response for pitch differences smaller than a semitone and a bigger N2b–P3b brain response for large pitch differences as compared to controls. However, it is still unclear why the amusic brain overreacts to large pitch changes. Furthermore, another electrophysiological study indicates that the amusic brain can respond to changes in melodies as small as a quarter-tone, without awareness, by exhibiting a normal mismatch negativity (MMN) brain response. Here, we re-examine the event-related N2b–P3b components with the aim to clarify the cause of the larger amplitude observed by Peretz, Brattico, and Tervaniemi (2005), by experimentally matching the number of deviants presented to the controls according to the number of deviants detected by amusics. We also re-examine the MMN component as well as the N1 in an acoustical context to investigate further the pitch discrimination deficit underlying congenital amusia. In two separate conditions, namely ignore and attend, we measured the MMN, the N1, the N2b and the P3b to tones that deviated by an eight of a tone (25 cents) or whole tone (200 cents) from a repeated standard tone. The results show a normal MMN, a seemingly normal N1, a normal P3b for the 200 cents pitch deviance, and no P3b for the small 25 cents pitch differences in amusics. These results indicate that the amusic brain responds to small pitch differences at a pre-attentive level of perception, but is unable to detect consciously those same pitch deviances at a later attentive level. The results are consistent with previous MRI and fMRI studies indicating that the auditory cortex of amusic individuals is functioning normally.