Neural Correlates of Visual Consciousness

According to proponents of the sensorimotor contingency theory of perception (Hurley & Noë 2003, Noë 2004, O’Regan 2011), active control of camera movement is necessary for the emergence of distal attribution in tactile-visual sensory substitution (TVSS) because it enables the subject to acquire knowledge of the way stimulation in the substituting modality varies as a function of self-initiated, bodily action. This chapter, by contrast, approaches distal attribution as a solution to a causal inference problem faced by the subject’s perceptual systems. (...) Given all of the available endogenous and exogenous evidence available to those systems, what is the most probable source of stimulation in the substituting modality? From this perspective, active control over the camera’s movements matters for rather different reasons. Most importantly, it generates proprioceptive and efference-copy based information about the camera’s body-relative position necessary to make use of the spatial cues present in the stimulation that the subject receives for purposes of egocentric object localization. (shrink)

Do perceptual experiences always inherit the content of their neural correlates? Most scientists and philosophers working on perception say ‘yes’. They hold the view that an experience’s content just is (i.e. is identical to) the content of its neural correlate. This paper presses back against this view, while trying to retain as much of its spirit as possible. The paper argues that type-2 blindsight experiences are plausible cases of experiences which lack the content of their neural correlates. They are not (...) experiences of the stimuli or stimulus properties prompting them, but their neural correlates represent these stimulus properties. The argument doesn’t depend on any special view of what it is for an experience to be of a stimulus or stimulus property. The upshot is that, even assuming there is a deep relationship between experiential content and neural content, that relationship is more complex than simple identity. (shrink)

Whether the prefrontal cortex is part of the neural substrates of consciousness is currently debated. Against prefrontal theories of consciousness, many have argued that neural activity in the prefrontal cortex does not correlate with consciousness but with subjective reports. We defend prefrontal theories of consciousness against this argument. We surmise that the requirement for reports is not a satisfying explanation of the difference in neural activity between conscious and unconscious trials, and that prefrontal theories of consciousness come out of this (...) debate unscathed. (shrink)

In this chapter, we discuss a selection of current views of the neural correlates of consciousness (NCC). We focus on the different predictions they make, in particular with respect to the role of prefrontal cortex (PFC) during visual experiences, which is an area of critical interest and some source of contention. Our discussion of these views focuses on the level of functional anatomy, rather than at the neuronal circuitry level. We take this approach because we currently understand more about experimental (...) evidence at this coarse level and because these results are appropriate for arbitrating between current theoretical frameworks. We discuss the Two-Visual-Systems Hypothesis (Milner & Goodale 1995; 2006), Local Recurrency (Lamme 2010; Lamme 2006), Higher Order (Lau 2008; Lau & Rosenthal 2011) and Global Workspace theories (Baars 1997; Baars 2005; Dehaene & Naccache 2001; Dehaene 2014). Despite the apparent stark differences between conscious and unconscious perceptual processing, available evidence suggests that their neural substrates must be largely shared. This indicates that the difference in neural activity between conscious and unconscious perceptual processing is likely to be subtle and highly specialized. We argue that current experimental evidence about the involvement of specific activity in prefrontal cortex supports the higher order neural theory of consciousness. In consequence, imaging techniques that focus only on marked differences between conscious and unconscious level of activity are likely to be insensitive to the relevant neural activity patterns that underlie conscious experiences. Finally, it follows from the evidence we discuss that the functional advantages of conscious over unconscious perceptual processing may be more limited than commonly thought. (shrink)

Mind &Language, Volume 34, Issue 4, Page 423-444, September 2019.

We discuss cases where subjects seem to enjoy conscious experience when the relevant first-order perceptual representations are either missing or too weak to account for the experience. Though these cases are originally considered to be theoretical possibilities that may be problematical for the higher-order view of consciousness, careful considerations of actual empirical examples suggest that this strategy may backfire; these cases may cause more trouble for first-order theories instead. Specifically, these cases suggest that (I) recurrent feedback loops to V1 are (...) most likely not the neural correlate of first-order representations for conscious experience, (II) first-order views seem to have a problem accounting for the phenomenology in these cases, and either (III) a version of the ambitious higher-order approach is superior in that it is the simplest theory that can account for all results at face value, or (IV) a view where phenomenology is jointly determined by both first-order and higher-order states. In our view (III) and (IV) are both live options and the decision between them may ultimately be an empirical question that cannot yet be decided. (shrink)

Consciousness scientists have not reached consensus on two of the most central questions in their field: first, on whether consciousness overflows reportability; second, on the physical basis of consciousness. I review the scientific literature of the 19th century to provide evidence that disagreement on these questions has been a feature of the scientific study of consciousness for a long time. Based on this historical review, I hypothesize that a unifying explanation of disagreement on these questions, up to this day, is (...) that scientific theories of consciousness are underdetermined by the evidence, namely, that they can be preserved “come what may” in front of (seemingly) disconfirming evidence. Consciousness scientists may have to find a way of solving the persistent underdetermination of theories of consciousness to make further progress. (shrink)

Electrical brain activity modulation in terms of changes in its intensity and spatial distribution is a function of age and task demand. However, the dynamics of brain modulation is unknown when it depends on external factors such as training. The aim of this research is to verify the effect of deductive reasoning training on the modulation in the brain activity of healthy younger and older adults ( (mean age of 21 ± 3.39) and (mean age of 68.92 ± 5.72)). The (...) analysis reveals the benefits of training, showing that it lowers cerebral activation while increasing the number of correct responses in the trained reasoning task (). The brain source generators were identified by time-averaging low-resolution brain electromagnetic tomography (sLORETA) current density images. In both groups, a bilateral overactivation associated with the task and not with age was identified. However, while the profile of bilateral activation in younger adults was symmetrical in anterior areas, in the older ones, the profile was located asymmetrically in anterior and posterior areas. Consequently, bilaterality may be a marker of how the brain adapts to maintain cognitive function in demanding tasks in both age groups. However, the differential bilateral locations across age groups indicate that the tendency to brain modulation is determined by age. (shrink)

: This commentary focuses on the scientific status of perceptual projection-a central feature of Pereira’s projective theory of consciousness. In his target article, he draws on my own earlier work to develop an explanatory framework for integrating first-person viewable conscious experience with the third-person viewable neural correlates and antecedent causes that form conscious experience into a bipolar structure that contains both a sense of self and a sense of the world. I stress that perceptual projection is a psychological effect and (...) list many of the ways it has been studied within experimental psychology, for example in studies of depth perception in vision and audition and experiences of depth arising from cues arranged on two-dimensional surfaces in stereoscopic pictures, 3D cinemas, holograms, and virtual realities. I then juxtapose Pereira’s explanatory model with two other models that have similar aims and background assumptions but different orientations, Trehub’s Retinoid model, which focuses largely on the neural functioning of the visual system, and Rudrauf et al’s Projective Consciousness Model, which draws largely on projective geometries to specify the requirements of organisms that need to navigate a three-dimensional world, and how these might be implemented in human information processing. Together, these models illustrate both converging and diverging approaches to understanding the role of projective processes in human consciousness. Resumo: Este comentário enfoca o status científico da projeção perceptiva - uma característica central da teoria da consciência de Pereira. Em seu artigo alvo, ele recorre ao meu trabalho anterior para desenvolver uma estrutura explicativa para integrar a experiência consciente visível em primeira pessoa com os correlatos neurais visíveis de terceira pessoa e as causas antecedentes que formam a experiência consciente em uma estrutura bipolar que contém tanto um sentido de eu e um sentido do mundo. Enfatizo que a projeção perceptiva é um efeito psicológico e relaciono modalidades de seu estudo na psicologia experimental, por exemplo em estudos de percepção de profundidade em visão e audição, experiências de profundidade em superfícies bidimensionais em imagens estereoscópicas, cinemas 3D, hologramas e realidades virtuais. Eu então justaponho o modelo explicativo de Pereira com dois outros modelos que têm objetivos e suposições similares, mas orientações diferentes, o modelo do sistema retinóide de Trehub, que se concentra principalmente no funcionamento neural do sistema visual, e o modelo de consciência projetiva de Rudrauf et al., que estabelece raciocínios geométricos para especificar os requisitos de organismos que precisam navegar em um mundo tridimensional, e como eles podem ser implementados no processamento de informação no contexto humano. Juntos, esses modelos ilustram abordagens convergentes e divergentes para compreender o papel dos processos projetivos na consciência humana. (shrink)

Back when researchers thought about the various forms that color vision could take, the focus was primarily on the retinal mechanisms. Since that time, research on human color vision has shifted from an interest in retinal mechanisms to cortical color processing. This has allowed color research to provide insight into questions that are not limited to early vision but extend to cognition. Direct cortical connections from higher-level areas to lower-level areas have been found throughout the brain. One of the classic (...) questions in cognitive science is whether perception is influenced, and if so to what extent, by cognition and whether a clear distinction can be drawn between perception and cognition. Since perception is seen as providing justification for our beliefs about properties in the external world, these questions also have metaphysical and epistemological significance. The aim of this paper is to highlight some of the areas where research on color perception can shed new light on questions in the cognitive sciences. A further aim of the paper is to raise some questions about color research that are in dire need of further reflection and investigation. (shrink)

A reply to commentators -- Jake Beck, Nico Orlandi and Aaron Franklin, and Ian Phillips -- on our paper "Does perceptual consciousness overflow cognitive access?".

Research into the neural correlates of consciousness (NCC) aims to identify not just those brain areas that are NCC, but also those that are not. In the received method for ruling out a brain area from being an NCC, this is accomplished by showing a brain area’s content to be consistently absent from subjects’ reports about what they are experiencing. This paper points out how this same absence can be used to infer that the brain area’s content is cognitively inaccessible, (...) in which case we would expect its content to be absent from subjects’ reports whether its content is (phenomenally) conscious or not. If so, such reports cannot count as evidence against that brain area being an NCC, and the received method fails. An alternative method (one suggested in Block, 2007) is considered. (shrink)

I argue for a manipulationist-mechanistic framework for content-NCC research in the case of visual consciousness (Bechtel 2008; Neisser 2012). Reference to mechanisms is common in the NCC research. Furthermore, recent developments in non-invasive brain stimulation techniques (NIBS) lend support to a manipulationist standpoint. The crucial question is to understand what is changed after manipulation of a brain mechanism. In the second part of the paper I review the literature on intentionalism, and argue that intervention on the neural mechanism is likely (...) to change the intentional content of consciousness. This urges us to shift from content-NCC to what I call “intentional mechanisms”. Such mechanisms, it is argued, should be understood as neural prerequisites of conscious visual experience. (shrink)

In this entry we give an overview of the search for the neural correlates of consciousness (NCCs). We begin with a discussion of the conceptual complexities of defining the notion of an NCC. We then discuss some of the experimental approaches used to empirically investigate the NCCs. We then consider some competing views of NCCs. Finally, we consider how the competing views of NCCs bear on different theories of consciousness. We focus on the methodological and theoretical challenges facing this line (...) of research. (shrink)

Milner and Goodale's influential account of the primate cortical visual streams involves a division of consciousness between them, for it is the ventral stream that has the responsibility for visual consciousness. Hence, the dorsal visual stream is a ‘zombie’ stream. In this article, I argue that certain information carried by the dorsal stream likely plays a central role in the egocentric spatial content of experience, especially the experience of visual spatial constancy. Thus, the dorsal stream contributes to a pervasive feature (...) of consciousness. (shrink)

In this article I raise empirical challenges for the claim tha area MT/V5 is the neural correlate for visual experience as of motion (Block 2005). In particular, I focus on the claim that there is matching content between area MT, on one hand, and visual experience as of motion, on the other hand (Chalmers 2000, Block 2007). I survey two lines of empirical evidence which challenge the claim of matching content in area MT. The first line of evidence covers new (...) results in neuroscience which emphasize the ongoing dynamics in cortical activity. The second line of evidence focuses on results regarding area MT in particular (Maier et al. 2007 and Cohen and Newsome 2008). Together, the empirical results indicate that neural processing is context sensitive in a way that challenges the attribution of content to local areas of cortex, to area MT in particular. In the final part of the article I explore alternative approaches and discuss remaining issues. (shrink)

I use recent developments in neuroscience to show how volitional mental events can be causal within a physicalist paradigm. (1) I begin by attacking the logic of Jaegwon Kim’s exclusion argument, according to which mental information cannot be causal of physical events. I argue that the exclusion argument falls apart if indeterminism is the case. If I am right, I must still build an account of how mental events are causal in the brain. To that end I take as my (...) foundation (2) a new understanding of the neural code that emphasizes rapid synaptic resetting over the traditional emphasis on neural spiking. (3) Such a neural code is an instance of ‘criterial causation,’ which requires modifying standard interventionist conceptions of causation. A synaptic reweighting neural code provides (4) a physical mechanism that accomplishes downward informational causation, (5) a middle path between determinism and randomness, and (6) a way for mind/brain events to turn out otherwise. This ‘synaptic neural code’ allows a constrained form of randomness parameterized by information realized in and set in synaptic weights, which in turn allows physical/informational criteria to be met in multiple possible ways when combined with an account of how randomness in the synapse is amplified to the level of randomness in spike timing. This new view of the neural code also provides (7) a way out of self-causation arguments against the possibility of mental causation. It leads to (8) an emphasis on deliberation and voluntary attentional manipulation as the core of volitional mental causation rather than, say, the correlates of unconscious premotor computations seen in Libet’s readiness potentials. And this new view of the neural code leads to (9) a new theory of the neural correlates of qualia as the ‘precompiled’ informational format that can be manipulated by voluntary attention, which gives qualia a causal role within a physicalist paradigm. I elaborate each of these ideas in turn below. -/- -/- (1) Countering Kim’s exclusion argument. The exclusion argument is, roughly, that the physical substrate does all the causal work that the supervenient mental state is supposed to do, so mental or informational events can play no causal role in material events. On Kim’s reductionistic view, all causation seeps away to the rootmost physical level, i.e. particles or strings. Add to that an assumption of determinism, and the laws of physics applicable at the rootmost level are sufficient to account for event outcomes at that level and every level that might supervene on that level. So informational causation, including voluntary mental causation or any type of free will that relies on it, is ruled out. -/- I argue that indeterminism undermines this sufficiency, so provides an opening whereby physically realized mental events could be downwardly causal. I argue that biological systems introduced a new kind of physical causation into the universe, one based upon triggering physical actions in response to detected spatiotemporal patterns in energy. This is a very different kind of causation than traditional Newtonian conceptions of the causal attributes of energy, such as mass, momentum, frequency or position, which seem to underlie deterministic and exclusionary intuitions. But patterns, unlike amounts of energy, lack mass and momentum and can be created and destroyed. They only become causal if there are physical detectors that respond to some pattern in energetic inputs. Basing causal chains upon successions of detected patterns in energy, rather than the transfer of energy among particles, opens the door not only to informational downward causation but to causal chains (such as mental causal chains or causal chains that might underlie a game of baseball or bridge) that are not describable by or solely explainable by the laws of physics applicable at the rootmost level. Yes, a succession of patterns must be realized in a physical causal chain that is consistent with the laws of physics, but many other possible causal chains that are also consistent with physical laws are ruled out by informational criteria imposed on indeterministic particle outcomes. Physical/informational criteria set in synaptic weights effectively sculpt informational causal chains out of the ‘substrate’ of possible physical causal chains. -/- (2) A new view of the neural code: I develop a new understanding the neural code that emphasizes rapid and dynamic synaptic weight resetting over neural firing as the core engine of information processing in the brain. The neural code is not solely a spike code, but a code where information is transmitted and transformed by flexibly and temporarily changing synaptic weights on a millisecond timescale. One metaphor is the rapid reshaping of the mouth (analogous to rapid, temporary synaptic weight resetting) that must take place just before vibrating air (analogous to spike trains) passes through, if information is to be realized and communicated. What rapid synaptic resetting allows is a moment by moment changing of the physical and informational parameters or criteria that have to be met before a neuron will fire. This dictates what information neurons will be responsive to and what they will ‘say’ to one another from moment to moment. -/- (3) Rethinking interventionist models of causation: Standard interventionist models of causation manipulate A to determine what effects, if any, there might be on B and other variables. If instead of manipulating A's output, we manipulate the criteria, parameters or conditions that B places on A's input, which must be satisfied before B changes or acts, then changes in B do not follow passively from changes in A as they would if A and B were billiard balls. Inputs from A can be identical, but in one case B changes in response to A, and in another it does not. This constant reparameterization of B is what neurons do when they change each other's synaptic weights. What I call "criterial causation" emphasizes that what can vary is either outputs from A to other nodes, or how inputs from A are decoded by receiving nodes. On this view, standard interventionist and Newtonian models of causation are a special case where B places no conditions on input from A. But the brain, if anything, emphasizes causation via reparameterization of B, by, for example, rapidly changing synaptic weights on post-synaptic neurons. -/- (4) How downward causation works: Downward causation means that events at a supervening level can influence outcomes at the rootmost level. In this context it would mean that information could influence particle paths. While it would be impossible self-causation if a supervening event changed its own present physical basis, it is not impossible that supervening events, such as mental information, could bias future particle paths. How might this work in the brain? The key pattern in the brain to which neurons respond is temporal coincidence. A neuron will only fire if it receives a certain number of coincident inputs from other neurons. Criterial causation occurs where physical criteria imposed by synaptic weights on coincident inputs in turn realize informational criteria for firing. This permits information to be downwardly causal regarding which indeterministic events at the rootmost level will be realized; Only those rootmost physical causal chains that meet physically realized informational criteria can drive a postsynaptic neuron to fire, and thus become causal at the level of information processing. Typically the only thing that the set of all possible rootmost physical causal chains that meet those criteria have in common is that they meet the informational criteria set. To try to cut information out of the causal picture here is a mistake; The only way to understand why it is that just this subset of possible physical causal chains—namely those that are also informational causal chains—can occur, is to understand that it is informational criteria that dictate that class of possible outcomes. -/- The information that will be realized when a neuron’s criteria for firing have been met is already implicit in the set of synaptic weights that impose physical criteria for firing that in turn realize informational criteria for firing. That is, the information is already implicit in these weights before any inputs arrive, just as what sound your mouth will make is implicit in its shape before vibrating air is passed through. Assuming indeterminism, many combinations of possible particle paths can satisfy given physical criteria, and many more cannot. The subset that can satisfy the physical criteria needed to make a neuron fire is also the subset that can satisfy the informational criteria for firing (such as ‘is a face’) that those synaptic weights realize. So sets of possible paths that are open to indeterministic elementary particles which do not also realize an informational causal chain are in essence “deselected” by synaptic settings by virtue of the failure of those sets of paths to meet physical/informational criteria for the release of a spike. -/- (5) Between determinism and randomness: Hume (1739) wrote “’tis impossible to admit of any medium betwixt chance and an absolute necessity.” Many other philosophers have seen no middle path to free will between the equally ‘unfree’ extremes of determinism and randomness. They have either concluded that free will does not exist, or tried to argue that a weak version of free will, namely, ‘freedom from coercion,’ is compatible with determinism. -/- A strong conception of free will, however, is not compatible with either determined or random choices, because in the determined case there are no alternative outcomes and things cannot turn out otherwise, while in the random case what happens does not happen because it was willed. A strong free will requires meeting some high demands: Beings with free will (a) must have information processing circuits that have multiple courses of physical or mental activity open to them; (b) they must really be able to choose among them; (c) they must be or must have been able to have chosen otherwise once they have chosen; and (d) the choice must not be dictated by randomness alone, but by the informational parameters realized in those circuits. This is a tough bill to fill, since it seems to require that acts of free will involve acts of self-causation. -/- Criterial causation offers a middle path between the two extremes of determinism and randomness that Hume was not in a position to see, namely, that physically realized informational criteria parameterize what class of neural activity can be causal of subsequent neural events. The information that meets preset physical/informational criteria may be random to a degree, but it must meet those criteria if it is to lead to neural firing, so is not utterly random. Preceding brain activity specifies the range of possible random outcomes to include only those that meet preset informational criteria for firing. -/- (6) How brain/mind events can turn out otherwise: The key mechanism, I argue, whereby atomic level indeterminism has its effects on macroscopic neural behavior is that it introduces randomness in spike timing. There is no need for bizarre notions such as consciousness collapsing wave packets or any other strange quantum effects beyond this. For example, quantum level noise expressed at the level of individual atoms, such as single magnesium atoms that block NMDA receptors, is amplified to the level of randomness and near chaos in neural and neural circuit spiking behavior. A single photon can even trigger neural firing in a stunning example of amplification from the quantum to macroscopic domains. The brain evolved to harness such ‘noise’ for information processing ends. Since the system is organized around coincidence detection, where spike coincidences (simultaneous arrival of spikes) are key triggers of informational realization (i.e. making neurons fire that are tuned to particular informational criteria), randomizing which incoming spike coincidences might meet a neuron's criteria for firing means informational parameters can be met in multiple ways just by chance. -/- (7) Skirting self-causation: A synaptic account of the neural code also gets around some thorny problems of self-causation that have been used to argue against the possibility of mental causation. The traditional argument is that a mental event realized in neural event x cannot change x because this would entail impossible self-causation. Criterial causation gets around this by granting that present self-causation is impossible. But it allows neurons to alter the physical realization of possible future mental events in a way that escapes the problem of self-causation of the mental upon the physical. Mental causation is crucially about setting synaptic weights. These serve as the physical grounds for the informational parameters that must be met by unpredictable future mental events. -/- (8) Voluntary attention and free will: I argue that the core circuits underlying free choice involve frontoparietal circuits that facilitate deliberation among options that are represented and manipulated in executive working memory areas. Playing out scenarios internally as virtual experience allows a superthreshold option to be chosen before specific motoric actions are planned. The chosen option can best meet criteria held in working memory, constrained by conditions of various evaluative circuits, including reward, emotional and cognitive circuits. This process also harnesses synaptic and ultimately atomic level randomness to foster the generation of novel and unforeseeable satisfactions of those criteria. Once criteria are met, executive circuits can alter synaptic weights on other circuits that will implement a planned operation or action. -/- (9) A new theory of qualia: The paradigmatic case of volitional mental control of behavior is voluntary attentional manipulation of representations in working memory such as the voluntary attentional tracking of one or a few objects among numerous otherwise identical objects. If there is a flock of indistinguishable birds, there is nothing about any individual bird that makes it more salient. But with volitional attention, any bird can be marked and kept track of. This salience is not driven by anything in the stimulus. It is voluntarily imposed on bottom-up information, and can lead to eventual motoric acts, such as shooting or pointing at the tracked bird. This leads to viewing the neural basis of attention and consciousness as not only realized in part in rapid synaptic reweighting, but also in particular patterns of spikes that serve as higher level units that traverse neural circuits and open what I call the ‘NMDA channel of communication.’ Qualia are necessary for volitional mental causation because they are the only informational format available to volitional attentional operations. Actions that follow volitional attentional operations, such as volitional tracking, cannot happen without consciousness. Qualia on this account are a ‘precompiled’ informational format made available to attentional selection and operations by earlier, unconscious information processing. -/- Conclusion: Assuming indeterminism, it is possible to be a physicalist who adheres to a strong conception of free will. On this view, mental and brain events really can turn out otherwise, yet are not utterly random. Prior neuronally realized information parameterizes what subsequent neuronally realized informational states will pass presently set physical/informational criteria for firing. This does not mean that we are utterly free to choose what we want to want. Some wants and criteria are innate, such as what smells good or bad. However, given a set of such innate parameters, the brain can generate and play out options, then select an option that adequately meets criteria, or generate further options. This process is closely tied to voluntary attentional manipulation in working memory, more commonly thought of as deliberation or imagination. Imagination is where the action is in free will. -/- - See more at: http://philosophycommons.typepad.com/flickers_of_freedom/2013/12/peter-tses-the-neural-basis-of-free -will-an-overview.html#sthash.Y5NvKvYd.dpuf. (shrink)

Is vision informationally encapsulated from cognition or is it cognitively penetrated? I shall argue that intentions penetrate vision in the experience of visual spatial constancy: the world appears to be spatially stable despite our frequent eye movements. I explicate the nature of this experience and critically examine and extend current neurobiological accounts of spatial constancy, emphasizing the central role of motor signals in computing such constancy. I then provide a stringent condition for failure of informational encapsulation that emphasizes a computational (...) condition for cognitive penetration: cognition must serve as an informational resource for visual computation. This requires proposals regarding semantic information transfer, a crucial issue in any model of informational encapsulation. I then argue that intention provides an informational resource for computation of visual spatial constancy. Hence, intention penetrates vision. (shrink)

In recent times we have seen an explosion in the amount of attention paid to the conscious brain from scientists and philosophers alike. One message that has emerged loud and clear from scientific work is that the brain is a dynamical system whose operations unfold in time. Any theory of consciousness that is going to be physically realistic must take account of the intrinsic nature of neurons and brain activity. At the same time a long discussion on consciousness among philosophers (...) has resulted in our distinguishing several kinds of consciousness. So when we ask where the place of consciousness is in nature we may mean several different things. In this chapter I will argue that it is plausible that all of the kinds of consciousness turn out to be nothing but patterns of synchronized neural activity in various frequencies against a dynamically changing chemical background. (shrink)

Relative blindsight is said to occur when different levels of subjective awareness are obtained at equality of objective performance. Using metacontrast masking, Lau and Passingham reported relative blindsight in normal observers at the shorter of two stimulus-onset asynchronies between target and mask. Experiment 1 replicated the critical asymmetry in subjective awareness at equality of objective performance. We argue that this asymmetry cannot be regarded as evidence for relative blindsight because the observers’ responses were based on different attributes of the stimuli (...) at the two SOAs. With an invariant criterion content , there was no asymmetry in subjective awareness across the two SOAs even though objective performance was the same. Experiment 3 examined the effect of criterion level on estimates of relative blindsight. Collectively, the present results question whether metacontrast masking is a suitable paradigm for establishing relative blindsight. Implications for theories of consciousness are discussed. (shrink)

One of the most important issues concerning the foundations ofconscious perception centerson thequestion of whether perceptual consciousness is rich or sparse. The overflow argument uses a form of ‘iconic memory’ toarguethatperceptual consciousnessisricher (i.e.,has a higher capacity) than cognitive access: when observing a complex scene we are conscious of more than we can report or think about. Recently, the overflow argumenthas been challenged both empirically and conceptually. This paper reviews the controversy, arguing that proponents of sparse perception are committed to the (...) postulation of (i) a peculiar kind of generic conscious representation that has no independent rationale and (ii) an unmotivated form of unconscious representation that in some cases conflicts with what we know about unconscious representation. (shrink)

Blindsight, the ability to blindly discriminate wavelength and other aspects of stimuli in a blind field, sometimes occurs in people with lesions to striate (V1) cortex. There is currently no consensus on whether qualitative color information of the sort that is normally computed by double opponent cells in striate cortex is indeed computed in blindsight but doesn?t reach awareness, perhaps owing to abnormal neuron responsiveness in striate or extra-striate cortical areas, or is not computed at all. The existence of primesight, (...) the experience of colored afterimages in blindsight, has been taken to suggest that qualitative color information is computed either in pre-striate or striate cortical areas but is not broadcast to working memory. I argue here that a recent study in which color phosphenes were induced in a blindsighter using bilateral transcranial magnetic stimulation indicates that computations necessary for conscious color vision are lost in blindsight. Owing to this loss, the neural responsiveness in extrastriate cortical areas is abnormal and hence is unable to give rise to color awareness. Blindsight is thus degraded vision in which the computations necessary for conscious color vision have been lost. (shrink)

In order to study whether there exist a period of activity in the human early visual cortex that contributes exclusively to visual awareness, we applied transcranial magnetic stimulation over the early visual cortex and measured subjective visual awareness during visual forced-choice symbol or orientation discrimination tasks. TMS produced one dip in awareness 60–120 ms after stimulus onset, while forced-choice orientation discrimination was suppressed between 60 and 90 ms and symbol discrimination between 60 and 120 ms. Thus, a time window specific (...) to visual awareness was found only in the orientation condition at 120 ms. The results imply that both conscious and unconscious perception depend on activity in early visual areas. On the basis of previous estimates of neural processing speed, we suggest that the late part of the activity period most likely involve local extrastriate–striate interactions which provide the contents for visual awareness but are not themselves sufficient for awareness to arise. (shrink)

Blindsight is classically defined as residual visual capacity, e.g., to detect and identify visual stimuli, in the total absence of perceptual awareness following lesions to V1. However, whereas most experiments have investigated what blindsight patients can and cannot do, the literature contains several, often contradictory, remarks about remaining visual experience. This review examines closer these remarks as well as experiments that directly approach the nature of possibly spared visual experiences in blindsight.

The first edition of Blindsight, written by Lawrence Weiskrantz was an important and highly cited account of studies of the phenomenon - Blindsight. The updated edition retains the original text of the first edition, but brings the book up to date with developments in this area in the past decade.

The extent to which visual processing can proceed in the visual hierarchy without awareness determines the magnitude of perceptual delay. Increasing data demonstrate that primary visual cortex (V1) is involved in consciousness, constraining the magnitude of visual delay. This makes it possible that visual delay is actually within the optimal lengths to allow sufficient computation; thus it might be unnecessary to compensate for visual delay.

Some patients with a lesion to the striate cortex (V1), when assessed through forced-choice paradigms, are able to detect stimuli presented in the blind field, despite reporting a complete lack of visual experience. This phenomenon, known as blindsight, strongly implicates V1 in visual awareness. However, the view that V1 is indispensable for conscious visual perception is challenged by a recent finding that the blindsight subject GY can be aware of visual qualia in his blind field, implying that V1may not be (...) critical under all circumstances. This apparent contradiction raises the following question: if V1 is not always necessary for phenomenal awareness, why do V1 lesions have such a detrimental effect on conscious perception? It is suggested here that this contradiction can be resolved by considering the impact of V1 lesions on the functioning of the whole visual cortex. (shrink)

Nijhawan argues convincingly that predictive mechanisms are pervasive in the central nervous system (CNS). However, scientific understanding of visual prediction requires one to formulate empirically testable neurophysiological models. The author's suggestions in this direction are to be evaluated on the basis of more realistic experimental methodologies and more plausible assumptions on the hierarchical character of the human visual cortex.

Although theories that examine direct links between behavior and brain remain incomplete, it is known that brain expansion significantly correlates with caloric and oxygen demands. Therefore, one of the principles governing evolutionary cognitive neuroscience is that cognitive abilities that require significant brain function (and/or structural support) must be accompanied by significant fitness benefit to offset the increased metabolic demands. One such capacity is self-awareness (SA), which (1) is found only in the greater apes and (2) remains unclear in terms of (...) both cortical underpinning and possible fitness benefit. In the current experiment, transcranial magnetic stimulation (TMS) was applied to the prefrontal cortex during a spatial perspective-taking task involving self and other viewpoints. It was found that delivery of TMS to the right prefrontal region disrupted self-, but not other-, perspective. These data suggest that self-awareness may have evolved in concert with other right hemisphere cognitive abilities. (shrink)

The superficial superior colliculus appears to be a primitive visual analyzer whose function has been taken over by the visual cortex, most completely in man. The phenomenon of blindsight shows that, although intact, the superior colliculus cannot by itself provide conscious perception in human patients. Is it possible that, in anencephalic children, it recovers the role it had in lower mammals? (Published Online May 1 2007).