Visual Consciousness Research Articles

Electrophysiological correlates of visual consciousness and selective attention

It is not clear whether attention is necessary or not for consciousness. We studied the relationship between attention and consciousness by tracking their electrophysiological correlates. The participants attended to visual targets, ignored nontargets in the prespecified visual field and ignored all stimuli in the opposite field. Visual consciousness was varied by masking. Our results showed that the earliest electrophysiological correlate of consciousness emerged independent of the manipulations of spatial and nonspatial attention. Conversely, the electrophysiological correlate of attention, selection negativity, was elicited regardless of the presence or absence of consciousness. Only the correlates of later, higher-level conscious processes strongly depended on attention. Thus, the electrophysiological brain responses reflecting visual consciousness and attention are initially independent of each other.

Concepts of visual consciousness and their measurement

Although visual consciousness can be manipulated easily (e.g., by visual masking), it is unresolved whether it can be assessed accurately with behavioral measures such as discrimination ability and self-report. Older theories of visual consciousness postulated a sensory threshold and distinguished between subjective and objective thresholds. In contrast, newer theories distinguish among three aspects: phenomenal, access, and reflexive consciousness. This review shows that discrimination ability and self-report differ in their sensitivity to these aspects. Therefore, both need to be assessed in the study of visual consciousness.

Sensorimotor supremacy: Investigating conscious and unconscious vision by masked priming.

According to the sensorimotor supremacy hypothesis, conscious perception draws on motor action. In the present report, we will sketch two lines of potential development in the field of masking research based on the sensorimotor supremacy hypothesis. In the first part of the report, evidence is reviewed that masked, invisible stimuli can affect motor responses, attention shifts, and semantic processes. After the review of the corresponding evidence – so-called masked priming effects – an approach based on the sensorimotor supremacy hypothesis is detailed as to how the question of a unitary mechanism of unconscious vision can be pursued by masked priming studies. In the second part of the report, different models and theories of backward masking and masked priming are reviewed. Types of models based on the sensorimotor hypothesis are discussed that can take into account ways in which sensorimotor processes (reflected in masked priming effects) can affect conscious vision under backward masking conditions.

Relative blindsight in normal observers and the neural correlate of visual consciousness

By using a paradigm based on metacontrast masking, we created experimental conditions in which the subjective report of consciousness differs but the objectively measured ability to discriminate visual targets does not. This approach allowed us to study the neural correlate of consciousness while having performance levels carefully matched in healthy human subjects. A comparison of the neural activity associated with these conditions as measured by functional MRI showed that conscious perception is associated with spatially specific activity in the mid-dorsolateral prefrontal cortex (area 46). Further analysis confirms that this activation is not only free from any performance confound, but is also not driven by differences in the timing of the physical stimuli. Our results suggest that the prefrontal cortex is important for the essentially subjective aspects of conscious perception.

Towards a true neural stance on consciousness

Consciousness is traditionally defined in mental or psychological terms. In trying to find its neural basis, introspective or behavioral observations are considered the gold standard, to which neural measures should be fitted. I argue that this poses serious problems for understanding the mind-brain relationship. To solve these problems, neural and behavioral measures should be put on an equal footing. I illustrate this by an example from visual neuroscience, in which both neural and behavioral arguments converge towards a coherent scientific definition of visual consciousness. However, to accept this definition, we need to let go of our intuitive or psychological notions of conscious experience and let the neuroscience arguments have their way. Only by moving our notion of mind towards that of brain can progress be made.

II—Naomi Eilan: On the Role of Perceptual Consciousness in Explaining the Goals and Mechanisms of Vision: A Convergence on Attention?

The strong sensorimotor account of perception gives self-induced movements two constitutive roles in explaining visual consciousness. The first says that self-induced movements are vehicles of visual awareness, and for this reason consciousness ‘does not happen in the brain only’. The second says that the phenomenal nature of visual experiences is consists in the action-directing content of vision. In response I suggest, first, that the sense in which visual awareness is active should be explained by appeal to the role of attention in visual consciousness, rather than self-induced movements; and second, that the sense in which perceptual consciousness does not happen in the brain only should be explained by appeal to the relational nature of perceptual consciousness, appeal to which also shows why links with action cannot exhaust phenomenal content.

Similar Frontal and Distinct Posterior Cortical Regions Mediate Visual and Auditory Perceptual Awareness

Activity in ventral visual cortex is a consistent neural correlate of visual consciousness. However, activity in this area seems insufficient to produce awareness without additional involvement of frontoparietal regions. To test the generality of the frontoparietal response, neural correlates of auditory awareness were investigated in a paradigm that previously has revealed frontoparietal activity during conscious visual perception. A within-experiment comparison showed that frontal regions were related to both visual and auditory awareness, whereas parietal activity was correlated with visual awareness and superior temporal activity with auditory awareness. These results indicate that frontal regions interact with specific posterior regions to produce awareness in different sensory modalities.

Seeing Invisible Motion: A Human fMRI Study

A common view about visual consciousness is that it could arise when and where activity reaches some higher level of processing along the cortical hierarchy. Reports showing that activity in striate cortex can be dissociated from awareness , whereas the latter modulates activity in higher areas , point in this direction. In the specific case of visual motion, a central, "perceptual" role has been assigned to area V5: several human and monkey studies have shown V5 activity to correlate with the motion percept. Here we show that activity in this and other higher cortical areas can be also dissociated from perception and follow the physical stimulus instead. The motion information in a peripheral grating modulated fMRI responses, despite being invisible to human volunteers: under crowding conditions , areas V3A, V5, and parietal cortex still showed increased activity when the grating was moving compared to when it was flickering. We conclude that stimulus-specific activation of higher cortical areas does not necessarily result in awareness of the underlying stimulus.

The Malaise of Islam

��� magine that the senior religious authorities, the most experienced Sharia court judges, and the most prominent Islamist politicians, dissidents, and modernizing reformers from Egypt, Iran, Saudi Arabia, Pakistan, Indonesia, and Algeria are gathered in a conference hall with the intention of defining an Islamic state. Their assignment is to write a universal constitution for Muslim societies, creating a governance model by which all Muslims would live, in the Islamic state to which they profess to aspire. The constitution is to be specific, laying out details as to form of government, choice of leader and succession, economic structure, courts and judiciary, and the role of the state—if any—in family and personal status matters. It is to specify whether the zakat is to be collected by the state or left to volunteer contributions, and whether the imams of mosques are to be employees of the state. It is to spell out who is to have the authority to say whether traditional practices in some Muslim communities such as honor killings, female genital mutilation, and death for apostasy are required, permitted, or prohibited, and how such rulings are to be enforced. The delegates have no deadline, but are to stay in conference until they reach consensus on a document that will put an end to centuries of disputation about the proper way for Muslims to conduct their affairs. In truth, they are unlikely ever to emerge because the probability that such a consensus could be achieved is close to zero. Nearly 14 centuries after the death of the Prophet Muhammad and decades after the end of colonialism, the world’s Muslims have failed to achieve what the scholar Fazlur Rahman called “a conscious vision of an Islamic sociopolitical order.” 1 The tortured quest for constitutional consensus in Iraq demonstrated anew the difficulties Muslims face in defining the terms of their governance, and it is still too early to predict whether that country’s people—mostly

S2f2-1 Stochastic dynamics of visual consciousness and global network of the brain(S2-f2: "Dynamism of the Brain States (II): Towards a new paradigm of understanding",Symposia,Abstract,Meeting Program of EABS & BSJ 2006)

S2f2-1 Stochastic dynamics of visual consciousness and global network of the brain(S2-f2: "Dynamism of the Brain States (II): Towards a new paradigm of understanding",Symposia,Abstract,Meeting Program of EABS & BSJ 2006)

2P376 Common cortical network associated with spontaneous changes of visual consciousness(44. Neuro-biophysics,Poster Session,Abstract,Meeting Program of EABS & BSJ 2006)

2P376 Common cortical network associated with spontaneous changes of visual consciousness(44. Neuro-biophysics,Poster Session,Abstract,Meeting Program of EABS & BSJ 2006)

Gut reactions: a perceptual theory of emotion

Gut Reactions is an interdisciplinary defense of the claim that emotions are perceptions of changes in the body. This thesis, pioneered by William James and resuscitated by Antonio Damasio, has been widely criticized for failing to acknowledge that emotions are meaningful insofar as they represent concerns, not respiratory function and blood pressure. Fear represents danger, sadness represents loss. To explain this fact, many researchers conclude that emotions must involve judgments regarding one's relationship to the environment. Prinz offers a new unified account of the emotions that reconciles these two theories. He argues that emotions are embodied appraisals-they are perceptions of the body, but, through the body, they also allow us to literally perceive danger, loss, and other matters of concern. The basic idea behind embodied appraisal theory is captured in the familiar notion of a gut reaction, which has been overlooked by much emotion research. Using recent work in semantics, Prinz show how emotions can be meaningful without incorporating judgments or other cognitive states. Criticizing those who think that some emotions are social constructions, while others can be explained by evolutionary psychology, Prinz argues that all emotions are the same kind of phenomena, involving both nature and nurture. Prinz also distinguishes emotions from other affective states, such as motivations and moods, and offers a theory of emotional valence (what makes some emotions good and others bad). Ultimately, his theory of emotion consciousness is inspired by recent research on the neural correlates of conscious vision. Drawing a parallel between emotion consciousness and visual consciousness, Prinz shows that emotion is a form of perception in the fullest sense. Where vision reveals the identity of objects in a given situation, emotion reveals how that situation bears on our well-being.

Unconscious processing of orientation and color without primary visual cortex.

In humans, the primary visual cortex (V1) is essential for conscious vision. However, even without V1 and in the absence of awareness, some preserved ability to accurately respond to visual inputs has been demonstrated, a phenomenon referred to as blindsight. We used transcranial magnetic stimulation (TMS) to deactivate V1, producing transient blindness for visual targets presented in a foveal, TMS-induced scotoma. Despite unawareness of these targets, performance on forced choice discrimination tasks for orientation (experiment 1) and color (experiment 2) were both significantly above chance. In addition to demonstrating that TMS can be successfully used to induce blindsight within a normal population, these results suggest a functioning geniculoextrastriate visual pathway that bypasses V1 and can process orientation and color in the absence of conscious awareness.

Continuous flash suppression reduces negative afterimages

Illusions that produce perceptual suppression despite constant retinal input are used to manipulate visual consciousness. Here we report on a powerful variant of existing techniques, continuous flash suppression. Distinct images flashed successively at approximately 10 Hz into one eye reliably suppress an image presented to the other eye. The duration of perceptual suppression is at least ten times greater than that produced by binocular rivalry. Using this tool we show that the strength of the negative afterimage of an adaptor was reduced by half when it was perceptually suppressed by input from the other eye. The more completely the adaptor was suppressed, the more strongly the afterimage intensity was reduced. Paradoxically, trial-to-trial visibility of the adaptor did not correlate with the degree of reduction. Our results imply that formation of afterimages involves neuronal structures that access input from both eyes but that do not correspond directly to the neuronal correlates of perceptual awareness.

The Ferrier Lecture 1995 behind the seen: the functional specialization of the brain in space and time.

The visual brain consists of many different visual areas, which are functionally specialized to process and perceive different attributes of the visual scene. However, the time taken to process different attributes varies; consequently, we see some attributes before others. It follows that there is a perceptual asynchrony and hierarchy in visual perception. Because perceiving an attribute is tantamount to becoming conscious of it, it follows that we become conscious of different attributes at different times. Visual consciousness is therefore distributed in time. Given that we become conscious of different visual attributes because of activity at different, functionally specialized, areas of the visual brain, it follows that visual consciousness is also distributed in space. Therefore, visual consciousness is not a single unified entity, but consists of many microconsciousnesses.

Independence of Visual Awareness from the Scope of Attention: an Electrophysiological Study

Recent brain imaging studies have revealed that increased neural activity along the ventral visual stream and parietal and frontal areas is associated with visual awareness. In order to study the time-course and temporal aspects of awareness, we examined electrophysiological correlates of conscious vision in two masking experiments. The differences in event-related potentials (ERPs) between unmasked (consciously recognized) and masked (unrecognized) stimuli were considered to be electrophysiological correlates of awareness. Two attentional conditions (global, local) were included to examine the relationship between the scope of attention and awareness. Two ERP-deflections were found to correlate with awareness. First, awareness was associated with a posterior negative amplitude shift 130-320 ms after the stimulus. This effect was present in both attention conditions, suggesting that it emerges independent of the scope of attention. Second, ERPs to unmasked stimuli became more positive as compared with masked stimuli around 400 ms, peaking at parietal sites. This effect was attenuated in the local attention condition, although the participants were aware of the stimuli, suggesting that the late positivity does not directly correlate with visual awareness. The results imply that the earlier negativity is the earliest and most direct correlate of visual awareness.

Beyond localization: from hodology to function.

A century on, Campbell's largely forgotten 1905 monograph on the localization of cerebral function has a distinctly contemporary feel. Although his map of cortical fields has been eclipsed by Brodmann's later contribution, Campbell's project went beyond cytoarchitectonic cartography, attempting to integrate clinical, anatomical and physiological evidence to provide a guide to function. A key component of Campbell's integrative, functional anatomical approach was hodology--the pattern of white matter connections between cortical areas--foreshadowing a recently developed functional anatomical technique: diffusion tensor tractography. Here, we revisit Campbell's model of the human visual system using tractography to illustrate prominent white matter connections within the occipital lobe and from occipital to frontal, parietal and temporal regions. Campbell used his integrative approach to support the view that vision consisted of a "visuo-sensory" and a "visuo-psychic" stage, combining hodological, cytoarchitectonic, physiological and clinicopathological evidence to locate the former within the calcarine cortex and the latter within the cortical field surrounding it. Speaking directly to contemporary debates surrounding the neurobiology of conscious vision and providing a framework with which to shape future developments in tractography, Campbell's integrative functional anatomical approach is as relevant today as it was 100 years ago.

Imagery, art and biological aspects of visual consciousness

The last 20 years have seen an unprecedented progress in our understanding of the brain. Having come to the end of the decade of the brain, neurobiology has provided an extraordinary wealth of knowledge in complementary and opposite directions. New tools in electrophysiology are making it possible to analyse in minute detail the transmission of signals between neurones. Molecular biology is complementing our understanding of the signalling between cells and how the neurone interacts with its DNA content. At the other end of the spectrum, brain-imaging techniques are making it possible to correlate the activity of whole regions of the brain with subjective conscious experience.

Spatio-temporal dynamics of the visual system revealed in binocular rivalry

From the evolutionary viewpoint, animals need to monitor the surrounding environment and capture salient features, such as motion, for survival. The visual system is highly developed for monitoring a wide area of visual field and capturing such salient features. In humans and primates, there is a wide binocular field, suggesting a necessity of integrating the images from the two eyes. Binocular rivalry [R. Blake, A neural theory of binocular rivalry, Psychol. Rev. 96 (1989) 145–167; R. Blake, N.K. Logothetis, Visual competition, Nat. Rev. Neurosci. 3 (2002) 13–21], where incompatible inputs from the two eyes compete to emerge in the subject's visual percept, has been shown to exhibit highly adaptive behavior [I. Kovacs, T.V. Parathomas, M. Yang, A. Feher, When the brain changes its mind: interocular grouping during binocular rivalry. Proc. Natl. Acad. Sci. U.S.A. 93 (1996) 15508–15511; N.K. Logothetis, Single units and conscious vision, Philos. Trans. R. Soc. Lond. B. Biol. Sci. 353 (1998) 1801–1818]. Here we investigated the spatio-temporal dynamics of the ocular dominance pattern in binocular rivalry under conditions where conflicting salient features were presented in a temporally varying manner. We found a striking example of the detailed structure of the dominance wave propagation, by using a spatio-temporal sampling method. The data show in detail the ability of the visual system to dynamically adapt to the changing stimuli in the context of the massively parallel visual field. We show by model prediction that the globally coherent dominance change in the presence of multiple stimuli can be explained by a mechanism based on local saliency comparison.

The Anesthetic Cascade

The Anesthetic Cascade