Neural Correlates Of Visual Awareness Research Articles

Building a decoder of perceptual decisions from microsaccades and pupil size.

Many studies have reported neural correlates of visual awareness across several brain regions, including the sensory, parietal, and frontal areas. In most of these studies, participants were instructed to explicitly report their perceptual experience through a button press or verbal report. It is conceivable, however, that explicit reporting itself may trigger specific neural responses that can confound the direct examination of the neural correlates of visual awareness. This suggests the need to assess visual awareness without explicit reporting. One way to achieve this is to develop a technique to predict the visual awareness of participants based on their peripheral responses. Here, we used eye movements and pupil sizes to decode trial-by-trial changes in the awareness of a stimulus whose visibility was deteriorated due to adaptation-induced blindness (AIB). In the experiment, participants judged whether they perceived a target stimulus and rated the confidence they had in their perceptual judgment, while their eye movements and pupil sizes were recorded. We found that not only perceptual decision but also perceptual confidence can be separately decoded from the eye movement and pupil size. We discuss the potential of this technique with regard to assessing visual awareness in future neuroimaging experiments.

Neural correlates of visual awareness and task-relevance in a no-report masking paradigm

Neural correlates of visual awareness and task-relevance in a no-report masking paradigm

Deep fuzzy model for non-linear effective connectivity estimation in the intuition of consciousness correlates

The brain connectivity, as a promising technique to explore brain networks during resting-states or cognitive tasks, has been employed remarkably in recent years. The aim of this study is to propose a new approach to improve the Granger causality as one of the fundamental methods for calculation of brain effective connectivity. To this end, we utilized a deep fuzzy structure to model the multivariate autoregressive used in the Granger causality. The proposed model benefits from the hierarchical stacked structure where first – order TSK fuzzy rules are the cores of the network. In the first layer of the stacked structure, the antecedents of the fuzzy rules are extracted from the fuzzy clustering of the input space. For subsequent layers, due to the input perturbation which is caused by the previous layer output, a shuffling approach is adopted. To assess our proposed model, we applied it to two nonlinear synthetic time series and compared it with linear Granger. Results revealed that our model is superior in the detection of effective connectivity. We additionally exploit the pioneer model for one of the controversial concepts in cognitive neuroscience in recent years: the neural correlates of visual consciousness. We applied our method to detect connectivity networks of EEG in consciousness states. Our results demonstrated that the proposed nonlinear connectivity estimator was capable of detecting novel correlates: significant differences have been observed among different states of consciousness, not only in presence of attention, as the linear method detected it, but also in absence of attention.

The P3 Reflects Awareness and Can Be Modulated by Confidence.

An important question in neural correlate of consciousness (NCC) studies is whether event-related potential (ERP) component P3 reflects visual awareness or the confidence with which one reports a visual experience. In the present study, participants detected visual stimuli presented at threshold-level contrast, then rated their subjective confidence with respect to their response on a four-point scale (very confident, quite confident, slightly confident, and not confident at all). Because awareness responses in trials with rating of “not confident at all” were likely noise, we analyzed the data excluding those trials. The ERP results revealed a significant positive difference in P3 amplitude between “aware” and “unaware” trials. P3 amplitude was more positive in aware trials compared to unaware trials. Importantly, this pattern was observed for trials with combined confidence ratings of “very confident” and “quite confident,” and for trials with confidence ratings of “slightly confident,” suggesting that awareness alone can modulate P3. A significant interaction between awareness and confidence is reported, suggesting that confidence influences P3 as well. In addition, ERP results revealed that visual awareness negativity (VAN) was observed over posterior temporal and occipital electrodes and largely not influenced by confidence. This result indicated that VAN is an early neural correlate of visual awareness.

Neural correlates of visual awareness at stimulus low vs. high-levels of processing

A crucial view in the graded vs. dichotomous debate on visual awareness proposes that its graded or dichotomous nature may depend on the depth of stimulus processing (or level of processing) associated to the experimental task. In the present study, we explored the behavioral patterns and neural correlates of different degrees of awareness associated to different depths (i.e. low vs. high) of stimulus processing. The low-level stimulus condition consisted of detecting the location of the target based on its brightness characteristics, whereas the high-level stimulus condition consisted of identifying which of four possible targets (numbers/letters) had appeared. Behavioral results showed that both subjective ratings of awareness and accuracy levels increased linearly as a function of awareness and independently of the level of stimulus processing. Additionally, the electrophysiological recordings revealed two correlates of visual awareness: enhanced posterior negativity in the N200 time window (VAN, visual awareness negativity) and enhanced positivity in the P3 time window (LP, late positivity). Interestingly, we found evidence of awareness levels modulating N200/VAN amplitudes in a graded manner only for the low-level task, whereas P3/LP amplitudes were modulated in a graded manner for both low and high-level tasks. The finding that the early posterior correlate of visual awareness (VAN at 150–250 ms) was sensitive to level of processing is consistent with task effects occurring in the visual cortex and supports the view that it is mediated by attention to task-relevant features. The amplitudes of P3/LP in both tasks correlate more directly with graded awareness and behavioral accuracy.

Do early neural correlates of visual consciousness show the oblique effect? A binocular rivalry and event-related potential study.

When dissimilar images are presented one to each eye, we do not see both images; rather, we see one at a time, alternating unpredictably. This is called binocular rivalry, and it has recently been used to study brain processes that correlate with visual consciousness, because perception changes without any change in the sensory input. Such studies have used various types of images, but the most popular have been gratings: sets of bright and dark lines of orthogonal orientations presented one to each eye. We studied whether using cardinal rival gratings (vertical, 0°, and horizontal, 90°) versus oblique rival gratings (left-oblique, –45°, and right-oblique, 45°) influences early neural correlates of visual consciousness, because of the oblique effect: the tendency for visual performance to be greater for cardinal gratings than for oblique gratings. Participants viewed rival gratings and pressed keys indicating which of the two gratings they perceived, was dominant. Next, we changed one of the gratings to match the grating shown to the other eye, yielding binocular fusion. Participants perceived the rivalry-to-fusion change to the dominant grating and not to the other, suppressed grating. Using event-related potentials (ERPs), we found neural correlates of visual consciousness at the P1 for both sets of gratings, as well as at the P1-N1 for oblique gratings, and we found a neural correlate of the oblique effect at the N1, but only for perceived changes. These results show that the P1 is the earliest neural activity associated with visual consciousness and that visual consciousness might be necessary to elicit the oblique effect.

Deactivation in the posterior mid-cingulate cortex reflects perceptual transitions during binocular rivalry: Evidence from simultaneous EEG-fMRI

Binocular rivalry is a phenomenon in which perception spontaneously shifts between two different images that are dichoptically presented to the viewer. By elucidating the cortical networks responsible for these stochastic fluctuations in perception, we can potentially learn much about the neural correlates of visual awareness. We obtained concurrent EEG-fMRI data for a group of 20 healthy human subjects during the continuous presentation of dichoptic visual stimuli. The two eyes’ images were tagged with different temporal frequencies so that eye specific steady-state visual evoked potential (SSVEP) signals could be extracted from the EEG data for direct comparison with changes in fMRI BOLD activity associated with binocular rivalry. We additionally included a smooth replay condition that emulated the perceptual transitions experienced during binocular rivalry as a control stimulus. We evaluated a novel SSVEP-informed fMRI analysis in this study in order to delineate the temporal dynamics of rivalry-related BOLD activity from both an electrophysiological and behavioral perspective. In this manner, we assessed BOLD activity during rivalry that was directly correlated with peaks and crosses of the two rivaling, frequency-tagged SSVEP signals, for comparison with BOLD activity associated with subject reported perceptual transitions. Our findings point to a critical role of a right lateralized fronto-parietal network in the processing of bistable stimuli, given that BOLD activity in the right superior/inferior parietal lobules was significantly elevated throughout binocular rivalry and in particular during perceptual transitions, compared with the replay condition. Based on the SSVEP-informed analysis, rivalry was further associated with significantly enhanced BOLD suppression in the posterior mid-cingulate cortex during perceptual transitions, compared with SSVEP crosses. Overall, this work points to a careful interplay between early visual areas, the right posterior parietal cortex and the mid-cingulate cortex in mediating the spontaneous perceptual changes associated with binocular rivalry and has significant implications for future multimodal imaging studies of perception and awareness.

Neural Correlates of Subjective Awareness for Natural Scene Categorization of Color Photographs and Line-Drawings.

It remains controversial whether visual awareness is correlated with early activation indicated by VAN (visual awareness negativity), as the recurrent process hypothesis theory proposes, or with later activation indicated by P3 or LP (late positive), as suggested by global workspace theories. To address this issue, a backward masking task was adopted, in which participants were first asked to categorize natural scenes of color photographs and line-drawings and then to rate the clarity of their visual experience on a Perceptual Awareness Scale (PAS). The interstimulus interval between the scene and the mask was manipulated. The behavioral results showed that categorization accuracy increased with PAS ratings for both color photographs and line-drawings, with no difference in accuracy between the two types of images for each rating, indicating that the experience rating reflected visibility. Importantly, the event-related potential (ERP) results revealed that for correct trials, the early posterior N1 and anterior P2 components changed with the PAS ratings for color photographs, but did not vary with the PAS ratings for line-drawings, indicating that the N1 and P2 do not always correlate with subjective visual awareness. Moreover, for both types of images, the anterior N2 and posterior VAN changed with the PAS ratings in a linear way, while the LP changed with the PAS ratings in a non-linear way, suggesting that these components relate to different types of subjective awareness. The results reconcile the apparently contradictory predictions of different theories and help to resolve the current debate on neural correlates of visual awareness.

Visual suppression at the offset of binocular rivalry.

Various paradigms can make visual stimuli disappear from awareness, but they often involve stimuli that are either relatively weak, competing with other salient inputs, and/or presented for a prolonged period of time. Here we explore a phenomenon that involves controlled perceptual disappearance of a peripheral visual stimulus without these limitations. It occurs when one eye's stimulus is abruptly removed during a binocular rivalry situation. This manipulation renders the remaining stimulus, which is still being presented to the other eye, invisible for up to several seconds. Our results suggest that this perceptual disappearance depends on a visual offset-transient that promotes dominance of the eye in which it occurs regardless of whether the eye is dominant or suppressed at the moment of the transient event. Using computational modeling, we demonstrate that standard rivalry mechanisms of interocular inhibition can indeed be complemented by a hypothesized transient-driven gating mechanism to explain the phenomenon. In essence, such a system suggests that visual awareness is dominated by the eye that receives transients and "sticks with" this eye-based dominance for some time in the absence of further transient events. We refer to this phenomenon as the "disrupted rivalry effect" and suggest that it is a potentially powerful paradigm for the study of cortical suppression mechanisms and the neural correlates of visual awareness.

Subjective visual awareness emerges prior to P3.

Studies on the neural basis of visual awareness, the subjective experience of seeing, have found several potential neural correlates of visual awareness. Some of them may not directly correlate with awareness but with post-perceptual processes, such as reporting one's awareness of the stimulus. We dissociated potential electrophysiological correlates of visual awareness from those occurring during response selection and thus co-occurring with post-perceptual processing. The participants performed two GO-NOGO conditions. In the aware-GO condition they responded with a key press when they were aware of the stimulus and withheld responding when they were unaware of it. In the unaware-GO condition they withheld responding when they were aware and responded when they were not aware of the stimulus. Thus, event-related potentials could be measured to aware and unaware trials when responding was required and when not required. The results revealed that the N200 amplitude (180-280ms) over the occipital and posterior temporal cortex was enhanced in aware trials as compared with trials without awareness. This effect (visual awareness negativity, VAN) did not depend on responding. The amplitude of P3 (350-450ms) also was enhanced in aware trials as compared with unaware trials. In addition, the amplitudes in the P3 time window depended on responding: they were greater when awareness was mapped to GO-response than when not, suggesting that P3 reflects post-perceptual processing, that is, it occurs after awareness has emerged. These findings support theories of visual awareness that assume a relatively early onset of visual awareness before P3.

Identifying neural correlates of visual consciousness with ALE meta-analyses

Neural correlates of consciousness (NCC) have been a topic of study for nearly two decades. In functional imaging studies, several regions have been proposed to constitute possible candidates for NCC, but as of yet, no quantitative summary of the literature on NCC has been done. The question whether single (striate or extrastriate) regions or a network consisting of extrastriate areas that project directly to fronto-parietal regions are necessary and sufficient neural correlates for visual consciousness is still highly debated [e.g., Rees et al., 2002, Nat Rev. Neurosci 3, 261–270; Tong, 2003, Nat Rev. Neurosci 4, 219–229]. The aim of this work was to elucidate this issue and give a synopsis of the present state of the art by conducting systematic and quantitative meta-analyses across functional magnetic resonance imaging (fMRI) studies using several standard paradigms for conscious visual perception. In these paradigms, consciousness is operationalized via perceptual changes, while the visual stimulus remains invariant. An activation likelihood estimation (ALE) meta-analysis was performed, representing the best approach for voxel-wise meta-analyses to date. In addition to computing a meta-analysis across all paradigms, separate meta-analyses on bistable perception and masking paradigms were conducted to assess whether these paradigms show common or different NCC. For the overall meta-analysis, we found significant clusters of activation in inferior and middle occipital gyrus; fusiform gyrus; inferior temporal gyrus; caudate nucleus; insula; inferior, middle, and superior frontal gyri; precuneus; as well as in inferior and superior parietal lobules. These results suggest a subcortical-extrastriate-fronto-parietal network rather than a single region that constitutes the necessary NCC. The results of our exploratory paradigm-specific meta-analyses suggest that this subcortical-extrastriate-fronto-parietal network might be differentially activated as a function of the paradigms used to probe for NCC.

Robust single trial identification of conscious percepts triggered by sensory events of variable saliency.

The neural correlates of visual awareness are elusive because of its fleeting nature. Here we have addressed this issue by using single trial statistical “brain reading” of neurophysiological event related (ERP) signatures of conscious perception of visual attributes with different levels of saliency. Behavioral reports were taken at every trial in 4 experiments addressing conscious access to color, luminance, and local phase offset cues. We found that single trial neurophysiological signatures of target presence can be observed around 300 ms at central parietal sites. Such signatures are significantly related with conscious perception, and their probability is related to sensory saliency levels. These findings identify a general neural correlate of conscious perception at the single trial level, since conscious perception can be decoded as such independently of stimulus salience and fluctuations of threshold levels. This approach can be generalized to successfully detect target presence in other individuals.

Subjective Characteristics of TMS-Induced Phosphenes Originating in Human V1 and V2

One way to study the neural correlates of visual consciousness is to localize the cortical areas whose stimulation generates subjective visual sensations, called phosphenes. While there is support for the view that the stimulation of several different visual areas in the occipital lobe may produce phosphenes, it is not clear what the contribution of each area is. Here, we studied the roles of the primary visual cortex (V1) and the adjacent area V2 in eliciting phosphenes by using functional magnetic resonance imaging-guided transcranial magnetic stimulation (TMS) combined with spherical modeling of the TMS-induced electric field. Reports of the subjective visual features of phosphenes were systematically collected and analyzed. We found that selective stimulation of V1 and V2 are equally capable of generating phosphenes, as demonstrated by comparable phosphene thresholds and similar characteristics of phosphene shape, color, and texture. However, the phosphenes induced by V1 stimulation were systematically perceived as brighter than the phosphenes induced by the stimulation of V2. Thus, these results suggest that V1 and V2 have a similar capability to produce conscious percepts. Nevertheless, V1 and V2 contribute differently to brightness: neural activation originating in V1 generates a more intense sensation of brightness than similar activation originating in V2.

Does the oblique effect influence early neural correlates of visual consciousness during binocular rivalry?

Does the oblique effect influence early neural correlates of visual consciousness during binocular rivalry?

Brief review of event-related potential correlates of visual consciousness and related studies

Studies of human consciousness (especially visual consciousness) have been increased rapidly during the last decade, including those employ a wide range of paradigms such as masking, binocular rivalry, attentional blink, and change blindness and various methods such as behavior, ERP (event-related potential) and fMRI (functional Magnatic Resonance Imaging). The present paper reviewed the ERP studies that aimed to explore the neural correlates of visual consciousness and briefly introduced the ERP correlates of visual consciousness. Three primary ERP correlates of visual consciousness were introduced: the P1 around 100 ms after stimulus onset, a negativity around 200 ms (visual awareness negativity, VAN), and the late positivity (LP) in the P3 time window. Researches have shown that the amplitudes of ERP evoked by conscious stimuli are significantly larger than that evoked by unconscious stimuli on these three components. Relevant researches were briefly introduced and several research directions and ideas for future studies were proposed from the point of views of the relationship between various attention components and consciousness, the relationship between C1 component and consciousness, the ERP research on high-order consciousness, and the factors affecting the time course of visual consciousness.

Which image is in awareness during binocular rivalry? Reading perceptual status from eye movements

Binocular rivalry is a useful psychophysical tool to investigate neural correlates of visual consciousness because the alternation between awareness of the left and right eye images occurs without any accompanying change in visual input. The conventional experiments on binocular rivalry require participants to voluntarily report their perceptual state. Obtaining reliable reports from non-human primates about their subjective visual experience, however, requires long-term training, which has made electrophysiological experiments on binocular rivalry quite difficult. Here, we developed a new binocular rivalry stimulus that consists of two different object images that are phase-shifted to move in opposite directions from each other: One eye receives leftward motion while the other eye receives rightward motion, although both eyes' images are perceived to remain at the same position. Experiments on adult human participants showed that eye movements (optokinetic nystagmus, OKN) are involuntarily evoked during the observation of our stimulus. We also found that the evoked OKN can serve as a cue for accurate estimation about which object image was dominant during rivalry, since OKN follows the motion associated with the image in awareness at a given time. This novel visual presentation technique enables us to effectively explore the neural correlates of visual awareness using animal models.

Binocular rivalry for beginners.

How to create and use binocular rivalry David Carmel, Michael Arcaro, Sabine Kastner, and Uri Hasson www.jove.com/video/2030/how-to-create-and-use-binocular-rivalry Binocular rivalry is a remarkable phenomenon of vision—two dissimilar images are presented simultaneously to each eye, and perception alternates between them. For example, if the left eye views horizontal lines and the right eye views vertical lines, an observer will report seeing one of the images for a few seconds, then the other, then the first again, and so on for as long as he or she cares to look (Wheatstone, 1838). You can experience this for yourself at www.psy.vanderbilt.edu/faculty/blake/rivalry/BR.html. Binocular rivalry is a popular tool for studying perception and awareness, because perception changes even when the physical stimulus does not. For this reason, rivalry is championed as a useful tool for studying the neural correlates of visual consciousness (Crick & Koch, 1990). However, despite there being a rich literature on rivalry, there are few teaching and learning resources devoted to the subject. Carmel, Arcaro, Kastner, and Hasson (2010) address this in their video: How to create and use binocular rivalry. The video begins with a brief description of binocular rivalry. It then provides a basic tutorial on the importance of stable vergence, because it ensures the two images fall on corresponding locations on the retina, as well as how it can be maintained during a rivalry experiment. For example, vergence can be maintained by surrounding each image with an identical frame of some sort, such as a simple or patterned border, and by placing an identical fixation point or cross in the centre of each image. The video also discusses some of the advantages and disadvantages of different methods to induce binocular rivalry. For example, chromatic (e.g., red-blue, red-green, etc.) goggles are inexpensive (a few dollars) and can be used with fMRI, but are prone to bleed-through and require careful stimulus preparation to ensure that each image can pass through only one of the chromatic filters. The video provides the same level of detail for the mirror stereoscope and prism goggles. Next, the video discusses the appearance of binocular rivalry. In particular, there is a neat demonstration of continuous flash suppression, a form of rivalry where a high-contrast image dominates perception indefinitely, and piecemeal rivalry, when patches of both images are perceived. Finally, the video discusses the distribution of dominance durations where time is plotted on the x-axis of a histogram and frequency is plotted on the y-axis, and that the data can be fitted with a gamma-like function. Overall, the video is professional and easy to use: the descriptions are accurate, the described methods are clear, and the accompanying pictures and demonstrations are crisp. Indeed, the video achieves what its title promises, but it does at the cost of some omission of important features of binocular rivalry. For example, the video mentions shutter and display goggles as methods to induce binocular rivalry, but fails to provide a discussion on the advantages and disadvantages of these techniques, and there is no discussion of the characteristics that affect rivalry. Also, there is no information about how to measure rivalry, such as by pressing one of two keys to report visibility of one of the images. This is especially surprising, because the video gives advice about the statistical tests to be used when analysing such data. Despite these minor omissions, How to create and use binocular rivalry covers a good chunk of material. It is also accompanied by a paper in which some of the video's deficiencies are redressed and in which there is a useful set of references. Although binocular-rivalry enthusiasts will be well-versed in the video's content, it is unlikely that students and newcomers to the field will be, too. For this reason, I strongly endorse Carmel et al.'s (2010) video for teaching and learning purposes. Bradley N. Jack, Discipline of Psychology and Cognitive Neuroscience Research Cluster, School of Health and Human Sciences, Southern Cross University, Coffs Harbour, Australia

Spatially localized motion aftereffect disappears faster from awareness when selectively attended to according to its direction

In searching for the target-afterimage patch among spatially separate alternatives of color-afterimages the target fades from awareness before its competitors (Bachmann, T., & Murd, C. (2010). Covert spatial attention in search for the location of a color-afterimage patch speeds up its decay from awareness: Introducing a method useful for the study of neural correlates of visual awareness. Vision Research 50, 1048–1053). In an analogous study presented here we show that a similar effect is obtained when a target spatial location specified according to the direction of motion aftereffect within it is searched by covert top-down attention. The adverse effect of selective attention on the duration of awareness of sensory qualiae known earlier to be present for color and periodic spatial contrast is extended also to sensory channels carrying motion information.

Theoretical motivations for investigating the neural correlates of consciousness.

I review the major contenders for the neural correlates of visual awareness. Different views of the neural correlates turn out to support different theories, such as the global workspace theory, local visual processing theories, and the higher-order representation theory. Therefore, studying the neural correlates of consciousness may prove useful to theoretical development, although arbitrating between different interpretations of the neural results may involve conceptual issues as well. WIREs Cogn Sci 2011 2 1-7 DOI: 10.1002/wcs.93 For further resources related to this article, please visit the WIREs website.

Neural Correlates of Motion-induced Blindness in the Human Brain

Motion-induced blindness (MIB) is a visual phenomenon in which highly salient visual targets spontaneously disappear from visual awareness (and subsequently reappear) when superimposed on a moving background of distracters. Such fluctuations in awareness of the targets, although they remain physically present, provide an ideal paradigm to study the neural correlates of visual awareness. Existing behavioral data on MIB are consistent both with a role for structures early in visual processing and with involvement of high-level visual processes. To further investigate this issue, we used high field functional MRI to investigate signals in human low-level visual cortex and motion-sensitive area V5/MT while participants reported disappearance and reappearance of an MIB target. Surprisingly, perceptual invisibility of the target was coupled to an increase in activity in low-level visual cortex plus area V5/MT compared with when the target was visible. This increase was largest in retinotopic regions representing the target location. One possibility is that our findings result from an active process of completion of the field of distracters that acts locally in the visual cortex, coupled to a more global process that facilitates invisibility in general visual cortex. Our findings show that the earliest anatomical stages of human visual cortical processing are implicated in MIB, as with other forms of bistable perception.