Closing in on the constitution of consciousness

The science of consciousness is a nascent and thriving field of research that is founded on identifying the minimally sufficient neural correlates of consciousness. However, I have argued that it is the neural constitution of consciousness that science seeks to understand and that there are no evident strategies for distinguishing the correlates and constitution of (phenomenal) consciousness. Here I review this correlation/constitution distinction problem and challenge the existing foundations of consciousness science. I present the main analyses from a longer paper in press on this issue, focusing on recording, inhibition, stimulation, and combined inhibition/stimulation strategies, including proposal of the Jenga analogy to illustrate why identifying the minimally sufficient neural correlates of consciousness should not be considered the ultimate target of consciousness science. Thereafter I suggest that while combined inhibition and stimulation strategies might identify some constitutive neural activities—indeed minimally sufficient constitutive neural activities—such strategies fail to identify the whole neural constitution of consciousness and thus the correlation/constitution distinction problem is not fully solved. Various clarifications, potential objections and related scientific and philosophical issues are also discussed and I conclude by proposing new foundational claims for consciousness science.

Related papers:

title: The Search for Neural Correlates of Consciousness
abstract: The science of consciousness begins with the search for the neural correlates of consciousness. I explain this notion and give examples of research in the field. I then discuss how different conceptions of consciousness may influence the search for the neural correlates. This includes the distinctions between access and phenomenal consciousness, and between conscious states and unified conscious fields, as well as phenomenological conceptions. Finally, I discuss what finding the neural correlates may imply for the metaphysics of consciousness, in particular for the theory that conscious states are identical to brain states.

title: The neural correlates of consciousness: New experimental approaches needed?
abstract: It appears that consciousness science is progressing soundly, in particular in its search for the neural correlates of consciousness. There are two main approaches to this search, one is content-based (focusing on the contrast between conscious perception of, e.g., faces vs. houses), the other is state-based (focusing on overall conscious states, e.g., the contrast between dreamless sleep vs. the awake state). Methodological and conceptual considerations of a number of concrete studies show that both approaches are problematic: the content-based approach seems to set aside crucial aspects of consciousness; and the state-based approach seems over-inclusive in a way that is hard to rectify without losing sight of the crucial conscious-unconscious contrast. Consequently, the search for the neural correlates of consciousness is in need of new experimental paradigms.

title: Neural correlates of consciousness reconsidered
abstract: It is widely accepted among philosophers that neuroscientists are conducting a search for the neural correlates of consciousness, or NCC. Chalmers (2000) conceptualized this research program as the attempt to correlate the contents of conscious experience with the contents of representations in specific neural populations. A notable claim on behalf of this interpretation is that the neutral language of "correlates" frees us from philosophical disputes over the mind/body relation, allowing the science to move independently. But the experimental paradigms and explanatory canons of neuroscience are not neutral about the mechanical relation between consciousness and the brain. I argue that NCC research is best characterized as an attempt to locate a causally relevant neural mechanism and not as an effort to identify a discrete neural representation, the content of which correlates with some actual experience. It might be said that the first C in "NCC" should stand for "causes" rather than "correlates."

title: Consciousness: Theoretical approaches
abstract: This chapter reviews some of the central theoretical challenges confronting the search for the brain basis of consciousness and develops a conceptual framework for tackling these challenges. At the heart of the search for the neural basis of consciousness is the notion of a neural correlate of consciousness. Identifying the neural correlates of consciousness requires that we acknowledge the various aspects of consciousness, for each of the aspects of consciousness raises its own set of methodological challenges. We examine the question of whether an account of the neural correlates of consciousness can be used to ascribe consciousness to creatures that lack the capacity to report their experiences, and we ask whether it is possible to go beyond the neural correlates of consciousness by providing neurally-based explanations of consciousness.

title: Distilling the neural correlates of consciousness
abstract: Solving the problem of consciousness remains one of the biggest challenges in modern science. One key step towards understanding consciousness is to empirically narrow down neural processes associated with the subjective experience of a particular content. To unravel these neural correlates of consciousness (NCC) a common scientific strategy is to compare perceptual conditions in which consciousness of a particular content is present with those in which it is absent, and to determine differences in measures of brain activity (the so called "contrastive analysis"). However, this comparison appears not to reveal exclusively the NCC, as the NCC proper can be confounded with prerequisites for and consequences of conscious processing of the particular content. This implies that previous results cannot be unequivocally interpreted as reflecting the neural correlates of conscious experience. Here we review evidence supporting this conjecture and suggest experimental strategies to untangle the NCC from the prerequisites and consequences of conscious experience in order to further develop the otherwise valid and valuable contrastive methodology.

title: The ‘correlates’ in neural correlates of consciousness
abstract: In the search for neural correlates of consciousness (NCC), the concept of 'consciousness' remains problematic. We suggest that not only the 'consciousness' in neural correlates of consciousness is a confused term, but 'correlates' is as well. When brain events are found to covary with conscious experience, these brain events can be the neural substrates of the experience, as is often (implicitly) suggested, but they can also be neural prerequisites or neural consequences of the experience. We here disentangle these different sorts of brain processes conceptually. But we also propose a concrete multi-pronged research program that may, in near-future consciousness research, distinguish these brain processes empirically.

title: Inner Presence: Consciousness as a Biological Phenomenon
abstract: The question of consciousness is perhaps the most significant problem still unsolved by science. In Inner Presence, Antti Revonsuo proposes a novel approach to the study of consciousness that integrates findings from philosophy, psychology, and cognitive neuroscience into a coherent theoretical framework. Arguing that any fruitful scientific approach to the problem must consider both the subjective psychological reality of consciousness and the objective neurobiological reality, Revonsuo proposes that the best strategy for discovering the connection between these two realities is one of "biological realism," using tools of the empirical biological sciences. This approach, which he calls the "biological research program," provides a theoretical and philosophical foundation that contemporary study of consciousness lacks.Revonsuo coins the term "world simulation metaphor" and uses this metaphor to develop a powerful way of thinking about consciousness as a biological system in the brain. This leads him to propose that the dreaming brain and visual consciousness are ideal model systems for empirical consciousness research. He offers a comprehensive overview and critical analysis of consciousness research and defends his approach against currently popular philosophical views, in particular against approaches that deny or externalize phenomenal consciousness, or claim that brain activity is not sufficient for consciousness. He systematically examines the principal issues in the science of consciousness -- the contents of consciousness, the unity of consciousness and the binding problem, the explanatory gap and the neural correlates of consciousness, and the causal powers and function of consciousness.Revonsuo draws together empirical data from a wide variety of sources, including dream research, brain imaging, neuropsychology, and evolutionary psychology, into the theoretical framework of the biological research program, thus pointing the way toward a unified biological science of consciousness. Applying imaginative thought experiments, Inner Presence reaches beyond the current state-of-the-art, revealing how the problem of consciousness may eventually be solved by future science.

title: Binocular Rivalry and the Cerebral Hemispheres With a Note on the Correlates and Constitution of Visual Consciousness
abstract: In addressing thescientific study of consciousness, Crick and Koch state, “It is probable that at any moment some active neuronal processes in your head correlate with consciousness, while others do not: what is the difference between them?” (1998, p. 97). Evidence from electrophysiological and brain-imaging studies of binocular rivalry supports the premise of this statement and answers to some extent, the question posed. I discuss these recent developments and outline the rationale and experimental evidence for the interhemispheric switch hypothesis of perceptual rivalry. According to this model, the perceptual alternations of rivalry reflect hemispheric alternations, suggesting that visual consciousness of rivalling stimuli may be unihemispheric at any one time (Miller et al., 2000). However, in this paper, I suggest that interhemispheric switching could involve alternating unihemispheric attentional selection of neuronal processes for access to visual consciousness. On this view, visual consciousness during rivalry could be bihemispheric because the processes constitutive of attentional selection may be distinct from those constitutive of visual consciousness. This is a special case of the important distinction between the neuronalcorrelates and constitution of visual consciousness.

title: What Is It Like to Be a Bat?
abstract: Thomas Nagel was born in Belgrade, Yugoslavia, in 1937. He came to the United States in 1939 and became a naturalized citizen five years later. After completing his undergraduate degree from Cornell University in 1958, he studied at Oxford University, where he earned a Bachelor of Philosophy degree in 1960. Nagel then enrolled in the graduate program in philosophy at Harvard University, receiving his doctorate in 1963. He taught at the University of California at Berkeley until 1966 and at Princeton University for the next fourteen years. In 1980 he accepted an appointment as Professor of Philosophy at New York University. He was named Professor of Philosophy and Law in 1986, Fiorello LaGuardia Professor of Law in 2001, and University Professor in 2002. Nagel has held visiting appointments at Rockefeller University, the University of Pittsburgh, the University of Mexico, the University of Witwatersrand (Republic of South Africa), the University of California at Los Angeles, and All Souls College at Oxford University. He is a fellow of the American Academy of Arts and Sciences, a Corresponding Fellow of the British Academy, and an Honorary Fellow of Corpus Christi College at Oxford University. Nagel’s publications include The Possibility of Altruism (1970), Mortal Questions (1979; translated into 10 languages), The View from Nowhere (1986),  (1987; translated into 20 languages), Equality and Partiality (1991), The Last Word (1997), and Concealment and Exposure and Other Essays (2002). Our reading is Nagel’s 1974 article “What Is It Like to Be a Bat?,” which examines the reductionist theory that some contemporary philosophers propose as a solution to “the mindbody problem”—the problem of how the mind and body are related. Reductionism is the view that one kind of thing can be “reduced to” (explained fully in terms of) another kind of thing. With respect to the mind-body problem, reductionism holds that the mind and mental phenomena can be reduced to physical phenomena, such as neurological activity of the brain (hence the doctrine is known as physicalism or materialism). Nagel contends that the major difficulty facing reductionist, physicalist theories of mind is the phenomenon of consciousness. While there may be some way to reduce consciousness to physical states, we are far from knowing how this might be done. Nagel explains that the fact that an organism (human or nonhuman) has consciousness means that “there is something it is like to be that organism—something it is like for that organism.” Consciousness is by nature a subjective phenomenon, and as such seems impossible to analyze exhaustively in terms of objective, physical phenomena. Subjectivity implies a single point of view, while objectivity requires a more universal point of view. Taking the example of a bat as a creature very different from us, Nagel argues that it seems impossible for us to capture in objective analysis the subjective experience of a bat. I might imagine what it would be like for me to behave as a bat behaves (hanging upside down by my feet, for example), but I seem unable to know what is it like for the bat to be [a ] bat. Even if I had complete knowledge of the working of the bat’s neurological system, how could the subjective experience of the bat be reduced to this kind of objective analysis? Nagel points out that he does not claim to have disproved the physicalist claim that mental states are states of the body; he simply claims that the two terms of this alleged equation are so different that we have no idea what it means to say that a mental state “is” a physical state. —Donald Abel

title: Consciousness and neuroscience.
abstract: The main purposes of this review are to set out for neuroscientists 
one possible approach to the problem of consciousness and 
to describe the relevant ongoing experimental work. We have 
not attempted an exhaustive review of other approaches.

title: Object vision and visual awareness.
abstract: Conscious experience involves perceiving, attending, remembering, and recognizing. Recent neuroscientific research has made significant contributions to our understanding of the mechanisms that mediate such capacities. Physiological and neuropsychological investigations have provided increasingly detailed descriptions of the location and functional properties of the brain structures involved in conscious perception, in attentive behavior and working memory, and in the recognition of objects. Such studies suggest that awareness of a visual stimulus probably reflects the interconnectivity and the type of cells involved in the representation of this stimulus, rather than the activity of specific circumscribed visual areas or processing streams.

title: Direct electrical stimulation of human cortex — the gold standard for mapping brain functions?
abstract: Despite its clinical relevance, direct electrical stimulation (DES) of the human brain is surprisingly poorly understood. Although we understand several aspects of electrical stimulation at the cellular level, surface DES evokes a complex summation effect in a large volume of brain tissue, and the effect is difficult to predict as it depends on many local and remote physiological and morphological factors. The complex stimulation effects are reflected in the heterogeneity of behavioural effects that are induced by DES, which range from evocation to inhibition of responses — sometimes even when DES is applied at the same cortical site. Thus, it is a misconception that DES — in contrast to other neuroscience techniques — allows us to draw unequivocal conclusions about the role of stimulated brain areas.

title: Interhemispheric switching mediates perceptual rivalry
abstract: Binocular rivalry refers to the alternating perceptual states that occur when the images seen by the two eyes are too different to be fused into a single percept. Logothetis and colleagues have challenged suggestions that this phenomenon occurs early in the visual pathway. They have shown that, in alert monkeys, neurons in the primary visual cortex continue to respond to their preferred stimulus despite the monkey reporting its absence. Moreover, they found that neural activity higher in the visual pathway is highly correlated with the monkey's reported percept. These and other findings suggest that the neural substrate of binocular rivalry must involve high levels, perhaps the same levels involved in reversible figure alternations.



We present evidence that activation or disruption of a single hemisphere in human subjects affects the perceptual alternations of binocular rivalry. Unilateral caloric vestibular stimulation changed the ratio of time spent in each competing perceptual state. Transcranial magnetic stimulation applied to one hemisphere disrupted normal perceptual alternations when the stimulation was timed to occur at one phase of the perceptual switch, but not at the other. Furthermore, activation of a single hemisphere by caloric stimulation affected the perceptual alternations of a reversible figure, the Necker cube.



Our findings suggest that interhemispheric switching mediates perceptual rivalry. Thus, competition for awareness in both binocular rivalry and reversible figures occurs between, rather than within, each hemisphere. This interhemispheric switch hypothesis has implications for understanding the neural mechanisms of conscious experience and also has clinical relevance as the rate of both types of perceptual rivalry is slow in bipolar disorder (manic depression).

title: Binocular rivalry, brain stimulation and bipolar disorder
abstract: Mechanistic understanding of binocular rivalry (BR) has drawn upon psychophysical, electrophysiological and brain-imaging studies. The first brain stimulation approach occurred in the late 1990s and assessed a new mechanistic proposal, the interhemispheric switch (IHS) hypothesis. Both caloric vestibular stimulation (CVS) and transcranial magnetic stimulation (TMS) modulated rivalry predominance when applied unilaterally. We describe the IHS model, its genesis and the brain stimulation evidence on which it rests. We also review more recent CVS and TMS rivalry studies, and discuss the findings of slow BR in bipolar disorder (BD) and genetic contribution to individual variation in BR rate. Finally, we describe a recent Drosophila model that can shed light on genetic, molecular and neurophysiological aspects of both BR and BD.

title: The development and application of optogenetics.
abstract: Genetically encoded, single-component optogenetic tools have made a significant impact on neuroscience, enabling specific modulation of selected cells within complex neural tissues. As the optogenetic toolbox contents grow and diversify, the opportunities for neuroscience continue to grow. In this review, we outline the development of currently available single-component optogenetic tools and summarize the application of various optogenetic tools in diverse model organisms.

title: Timing, Timing, Timing: Fast Decoding of Object Information from Intracranial Field Potentials in Human Visual Cortex
abstract: The difficulty of visual recognition stems from the need to achieve high selectivity while maintaining robustness to object transformations within hundreds of milliseconds. Theories of visual recognition differ in whether the neuronal circuits invoke recurrent feedback connections or not. The timing of neurophysiological responses in visual cortex plays a key role in distinguishing between bottom-up and top-down theories. Here, we quantified at millisecond resolution the amount of visual information conveyed by intracranial field potentials from 912 electrodes in 11 human subjects. We could decode object category information from human visual cortex in single trials as early as 100 ms poststimulus. Decoding performance was robust to depth rotation and scale changes. The results suggest that physiological activity in the temporal lobe can account for key properties of visual recognition. The fast decoding in single trials is compatible with feedforward theories and provides strong constraints for computational models of human vision.

title: Bold claims for optogenetics
abstract: Arising from J. H. Lee et al. 465, 788–792 (; Lee et al. replyIn a recent Letter to Nature, Lee and colleagues combined optogenetic stimulation with functional magnetic resonance imaging (ofMRI) to examine the relationship between pyramidal-cell spiking and the blood oxygenation level dependent (BOLD) signal. To do so, they injected an adeno-associated viral vector into the primary motor cortex (M1) of adult rats to drive the expression of channelrhodopsin (ChR2) in cortical projection neurons, thus making them sensitive to light. The authors then used combined light stimulation and functional magnetic resonance imaging (fMRI) to examine the effects of selective activation of the light-sensitive pyramidal cells on the BOLD signal, as well as to probe the value of this methodology for mapping brain connectivity. They found that excitation of these neurons induced positive BOLD signals both in the injected M1 region and in remote target thalamic nuclei receiving direct projections from that region, and concluded that ofMRI reliably links positive BOLD signals with increased local neuronal excitation. However, their analysis neglects the almost immediate activation of other circuits that could lead to the generation of BOLD signals through local perisynaptic rather than spiking activity. Their experiments therefore do not pin down the identity of the specific neuronal signals that give rise to the BOLD signal.

title: Explaining the Brain: Mechanisms and the Mosaic Unity of Neuroscience
abstract: Preface Chapter 1.  Starting With Neuroscience 1 Introduction 2 Explanations in Neuroscience Describe Mechanisms. 3 Explanations in Neuroscience are Multilevel 4 Explanations in Neuroscience Integrate Multiple Fields 5 Criteria of Adequacy for an Account of Explanation Chapter 2. Explanation and Causal Relevance 1 Introduction 2 How Calcium Explains Neurotransmitter Release 3 Explanation and Representation 4 The Covering-Law Model 5 The Unification Model 6 But What About the Hodgkin and Huxley Model? 7  Chapter 3.  Relevance and Manipulation 1 Introduction 2 The Mechanism of Long-Term Potentiation 3 Causation as Transmission 3.1 Transmission and Causal Relevance 3.2 Omission and Prevention 4 Causation and Mechanical Connection 5 Manipulation and Causation 5.1 Ideal Interventions 5.2 Invariance, Fragility, and Contingency 5.3 Manipulation and Criteria for Explanation 5.4 Manipulation, Omission, and Prevention 6  Chapter 4. The Norms of Mechanistic Explanation 1 Introduction 2 Two Normative Distinctions 3 Explaining the Action Potential 4 The Explanandum Phenomenon 5 Components 6 Activities 7 Organization 8 Constitutive Relevance 8.1 Relevance and the Boundaries of Mechanisms 8.2 Interlevel Experiments and Constitutive Relevance 8.21 Interference Experiments 8.22 Stimulation Experiments 8.23 Activation Experiments 8.3 Constitutive Relevance as Mutual Manipulability 9  Chapter 5. A Field-Guide to Levels 1 Introduction 2 Levels of Spatial Memory 3 A Field-Guide to Levels 3.1 Levels of Science (Units and Products) 3.2 Levels of Nature 3.21 Causal Levels (Processing and Control) 3.22 Levels of Size 3.23 Levels of Composition 3.231 Levels of Mereology 3.232 Levels of Aggregativity 3.233 Levels of Mere Material/Spatial Containment 3.3 Levels of Mechanisms 4  Chapter 6 Nonfundamental Explanation 1 Introduction 2.  Relevance and Making a Difference 3 Contrasts and Switch-Points 4 Causal Powers at Higher Levels of Mechanisms 5 Causal Relevance among Realized Properties 6  Chapter 7. The Mosaic Unity of Neuroscience 1 Introduction 2 Reduction and the History of Neuroscience 2.1 LTP's Origins: Not a Top-Down Search but Intralevel Integration 2.2 The Mechanistic Shift 2.3 Mechanism as a Working Hypothesis 3 Intralevel Integration and the Mosaic Unity of Neuroscience 3.1 The Space of Possible Mechanisms 3.2 Specific Constraints on the Space of Possible Mechanisms 3.21 Componency Constraints 3.22 Spatial Constraints 3.23 Temporal Constraints 3.24 Active Constraints 3.3 Reduction and the Intralevel Integration of Fields 4 Interlevel Integration and the Mosaic Unity of Neuroscience 4.1 What is Interlevel Integration? 4.2 Constraints on Interlevel Integration 4.21 Accommodative Constraints 4.22 Spatial and Temporal Interlevel Constraints 4.23 Interlevel Manipulability Constraints 4.3 Mosaic Interlevel Integration 5  The Epistemic Function of the Mosaic Unity of Neuroscience

title: Long-range and local circuits for top-down modulation of visual cortex processing
abstract: You only see what you want to see We often focus on a particular item out of a thousand objects in a visual scene. This ability is called selective attention. Selective attention enhances the responses of sensory nerve cells to whatever is being observed and dampens responses to any distractions. Zhang et al. identified a region of the mouse forebrain that modulates responses in the visual cortex. This modulation improved the mouse's performance in a visual task. Science, this issue p. 660 Projections from the frontal cortex control stimulus processing in the visual system in mice. Top-down modulation of sensory processing allows the animal to select inputs most relevant to current tasks. We found that the cingulate (Cg) region of the mouse frontal cortex powerfully influences sensory processing in the primary visual cortex (V1) through long-range projections that activate local γ-aminobutyric acid–ergic (GABAergic) circuits. Optogenetic activation of Cg neurons enhanced V1 neuron responses and improved visual discrimination. Focal activation of Cg axons in V1 caused a response increase at the activation site but a decrease at nearby locations (center-surround modulation). Whereas somatostatin-positive GABAergic interneurons contributed preferentially to surround suppression, vasoactive intestinal peptide-positive interneurons were crucial for center facilitation. Long-range corticocortical projections thus act through local microcircuits to exert spatially specific top-down modulation of sensory processing.

title: Attentional Switching in Humans and Flies: Rivalry in Large and Miniature Brains
abstract: Human perception, and consequently behavior, is driven by attention dynamics. In the special case of rivalry, where attention alternates between competing percepts, such dynamics can be measured and their determinants investigated. A recent study in the fruit fly, Drosophila melanogaster, now shows that the origins of attentional rivalry may be quite ancient. Furthermore, individual variation exists in the rate of attentional rivalry in both humans and flies, and in humans this is under substantial genetic influence. In the pathophysiological realm, slowing of rivalry rate is associated with the heritable psychiatric condition, bipolar disorder. Fly rivalry may therefore prove a powerful model to examine genetic and molecular influences on rivalry rate, and may even shed light on human cognitive and behavioral dysfunction.

title: The use of tDCS and CVS as methods of non-invasive brain stimulation
abstract: Transcranial direct current stimulation (tDCS) and caloric vestibular stimulation (CVS) are safe methods for selectively modulating cortical excitability and activation, respectively, which have recently received increased interest regarding possible clinical applications. tDCS involves the application of low currents to the scalp via cathodal and anodal electrodes and has been shown to affect a range of motor, somatosensory, visual, affective and cognitive functions. Therapeutic effects have been demonstrated in clinical trials of tDCS for a variety of conditions including tinnitus, post-stroke motor deficits, fibromyalgia, depression, epilepsy and Parkinson's disease. Its effects can be modulated by combination with pharmacological treatment and it may influence the efficacy of other neurostimulatory techniques such as transcranial magnetic stimulation. CVS involves irrigating the auditory canal with cold water which induces a temperature gradient across the semicircular canals of the vestibular apparatus. This has been shown in functional brain-imaging studies to result in activation in several contralateral cortical and subcortical brain regions. CVS has also been shown to have effects on a wide range of visual and cognitive phenomena, as well as on post-stroke conditions, mania and chronic pain states. Both these techniques have been shown to modulate a range of brain functions, and display potential as clinical treatments. Importantly, they are both inexpensive relative to other brain stimulation techniques such as electroconvulsive therapy (ECT) and transcranial magnetic stimulation (TMS).

title: What electrical microstimulation has revealed about the neural basis of cognition
abstract: Neurophysiologists have shown repeatedly that neural activity in different brain structures can be correlated with specific perceptual and cognitive functions, but the causal efficacy of the observed activity has generally been a matter of conjecture. By contrast, electrical microstimulation, which allows the experimenter to manipulate the activity of small groups of neurons with spatial and temporal precision, can now be used to demonstrate causal links between neural activity and specific cognitive functions. Here, we review this growing literature, including applications to the study of attention, visual and somatosensory perception, 'read-out' mechanisms for interpreting sensory maps, and contextual effects on perception. We also discuss potential applications of microstimulation to studies of higher cognitive functions such as decision-making and subjective experience.

title: Insights into cortical mechanisms of behavior from microstimulation experiments
abstract: Even the simplest behaviors depend on a large number of neurons that are distributed across many brain regions. Because electrical microstimulation can change the activity of localized subsets of neurons, it has provided valuable evidence that specific neurons contribute to particular behaviors. Here we review what has been learned about cortical function from behavioral studies using microstimulation in animals and humans. Experiments that examine how microstimulation affects the perception of stimuli have shown that the effects of microstimulation are usually highly specific and can be related to the stimuli preferred by neurons at the stimulated site. Experiments that ask subjects to detect cortical microstimulation in the absence of other stimuli have provided further insights. Although subjects typically can detect microstimulation of primary sensory or motor cortex, they are generally unable to detect stimulation of most of cortex without extensive practice. With practice, however, stimulation of any part of cortex can become detected. These training effects suggest that some patterns of cortical activity cannot be readily accessed to guide behavior, but that the adult brain retains enough plasticity to learn to process novel patterns of neuronal activity arising anywhere in cortex.

title: Dichoptic Viewing Methods for Binocular Rivalry Research: Prospects for Large-Scale Clinical and Genetic Studies
abstract: Binocular rivalry (BR) is an intriguing phenomenon that occurs when two different images are presented, one to each eye, resulting in alternation or rivalry between the percepts. The phenomenon has been studied for nearly 200 years, with renewed and intensive investigation over recent decades. The rate of perceptual switching has long been known to vary widely between individuals but to be relatively stable within individuals. A recent twin study demonstrated that individual variation in BR rate is under substantial genetic control, a finding that also represented the first report, using a large study, of genetic contribution for any post-retinal visual processing phenomenon. The twin study had been prompted by earlier work showing BR rate was slow in the heritable psychiatric condition, bipolar disorder (BD). Together, these studies suggested that slow BR may represent an endophenotype for BD, and heralded the advent of modern clinical and genetic studies of rivalry. This new focus has coincided with rapid advances in 3D display technology, but despite such progress, specific development of technology for rivalry research has been lacking. This review therefore compares different display methods for BR research across several factors, including viewing parameters, image quality, equipment cost, compatibility with other investigative methods, subject group, and sample size, with a focus on requirements specific to large-scale clinical and genetic studies. It is intended to be a resource for investigators new to BR research, such as clinicians and geneticists, and to stimulate the development of 3D display technology for advancing interdisciplinary studies of rivalry.

title: The neuron doctrine of binocular rivalry
abstract: Ever since Horace Barlow (1972) proposed his neuron doctrine for perceptual psychology, the holy grail for neurophysiologists has been to find individual neurons or groups of neurons whose firing correlates with specific percepts or even a state of mind or consciousness. Binocular rivalry is a particularly attractive paradigm for this approach because a stable visual stimulus causes an ever-changing subjective perceptual experience. The publication of Blake’s neural theory of binocular rivalry (Blake, 1989) inspired numerous attempts to identify neurons in various areas along the central visual pathways whose firing rate might signal dominance or suppression. Collectively these studies have shown that depending on the type of stimulus, rivalry is resolved at both lower and higher levels in the visual system.

title: Functional neuroimaging of binocular rivalry
abstract: Binocular rivalry has intrigued researchers for over two centuries, but research into its neural mechanisms was until recently limited to behavioral and animal studies. The availability of functional magnetic resonance imaging since the 1990s has boosted the neuroscientific investigation of binocular rivalry in humans. Functional neuroimaging has revealed an involvement of all levels of the brain’s visual processing hierarchy in rivalry dynamics, including early subcortical and cortical stages, functionally specialized visual areas, and non-sensory frontoparietal regions. Moreover, variants of binocular rivalry have helped to elucidate the neural fate of unconscious information during binocular rivalry suppression. The findings from neuroimaging research are integrated into a comprehensive view on how different processing stages interact to resolve perceptual conflict in the human brain.

title: Investigating the structure and function of the brain: a methodological overview
abstract: Our understanding of the inner workings of the human brain is increasing exponentially. This is due in no small part to the rapidly advancing technology that has evolved from the application of engineering to neuroscience. This chapter reviews some of the more influential tools that are in widespread use in research institutes throughout the world. Beginning with the basics of electrophysiology, a broad overview of the current methods for resolving and modulating neural activity in the fields of imaging and brain stimulation is presented. Special attention is paid to temporal and spatial resolution, i.e., the capacity of the technologies to resolve small detail in time and space. The chapter concludes with a discussion of technical obstacles and future directions.

title: Progression of change following median nerve section in the cortical representation of the hand in areas 3b and 1 in adult owl and squirrel monkeys
abstract: In an earlier study (Neuroscience 8, 33-55, 1983), we found that the cortex representing the skin of the median nerve within parietal somatosensory fields 3b and 1 was completely occupied by 'new' inputs from the ulnar and radial nerves, 2-9 months after the median nerve was cut and tied in adult squirrel and owl monkeys. In this report, we describe the results of studies directed toward determining the time course and likely mechanisms underlying this remarkable plasticity. Highly detailed maps of the hand surface representation were derived in monkeys before, immediately after, and at subsequent short and intermediate time stages after median nerve section. In one monkey, maps were derived before nerve section, immediately after nerve section, and 11, 22 and 144 days later. Thus, direct comparisons in cortical map structure could be made over time in this individual monkey. In other experiments, single maps were derived at given post-section intervals. These studies revealed that: (1) large cortical sectors were 'silenced' by median nerve transection. (2) Significant inputs restricted to the dorsum of the radial hand and the dorsum of digits 1, 2 and 3 were immediately 'unmasked' by median nerve transection. (3) These immediately 'unmasked' regions were topographically crude, and represented only fragments of this dorsal skin. They were transformed, over time, into very large, highly topographic and complete representations of dorsal skin surfaces. (4) Representations of bordering glabrous skin surfaces progressively expanded to occupy larger and larger portions of the former median nerve cortical representational zone. (5) These 'expanded' representations of ulnar nerve-innervated skin surfaces sometimes moved, in entirety, into the former median nerve representational zone. (6) Almost all of the former median nerve zone was driven by new inputs in a map derived 22 days after nerve section. At shorter times (3, 6 and 11 days), 'reoccupation' was still incomplete. (7) Very significant changes in map dimensions within and outside of the former median skin cortical field were seen after the 'reoccupation' of the deprived cortex by 'new' inputs was initially completed.  Progressive changes were recorded within the original ulnar and radial nerve cortical representational zones, as skin surfaces originally overtly represented wholly within these regions expanded into the former median nerve zone. (9) Throughout the studied period, the cortical representational loci of many skin sites appeared to change continually and often markedly.  The locations of map discontinuities also shifted significantly over time. (11) Concomitant with changes in representational magnification over time, inverse changes in receptive field sizes were recorded.(ABSTRACT TRUNCATED AT 400

title: Context-dependent perceptual modulation of single neurons in primate visual cortex
abstract: Some neurons in the visual cortex alter their spiking rate according to the perceptual interpretation of an observed stimulus, rather than its physical structure alone. Experiments in monkeys have suggested that, although the proportion of neurons showing this effect differs greatly between cortical areas, this proportion remains similar across different stimuli. These findings have raised the intriguing questions of whether the same neurons always participate in the disambiguation of sensory patterns and whether such neurons might represent a special class of cortical cells that relay perceptual signals to higher cortical areas. Here we explore this question by measuring activity in the middle temporal cortex of monkeys and asking to what degree the percept-related responses of individual neurons depend upon the specific sensory input. In contrast to our expectations, we found that even small differences in the stimuli led to significant changes in the signaling of the perceptual state by single neurons. We conclude that nearly all feature-responsive neurons in this area, rather than a select subset, can contribute to the resolution of sensory conflict, and that the role of individual cells in signaling the perceptual outcome is tightly linked to the fine details of the stimuli involved.

title: Perceptual correlates of massive cortical reorganization.
abstract: Following long-term deafferentation of one upper limb in adult primates, the cortical areas corresponding to that limb become responsive to stimuli applied to the face. To explore this phenomenon, we studied some patients after upper limb amputation. In patient VQ, stimuli applied to the lower face

title: Rapid functional plasticity of the somatosensory cortex after finger amputation
abstract: Recent research indicates that areas of the primary somatosensory (SI) and primary motor cortex show massive cortical reorganization after amputation of the upper arm, forearm or fingers. Most of these studies were carried out months or several years after amputation. In the present study, we describe cortical reorganization of areas in the SI of a patient who underwent amputation of the traumatized middle and ring fingers of his right hand 10 days before cortical magnetic source imaging data were obtained. Somatosensory-evoked magnetic fields (SEF) to mechanical stimuli to the finger tips were recorded and single moving dipoles were calculated using a realistic volume conductor model. Results reveal that the dipoles representing the second and fifth fingers of the affected hand were closer together than the comparable dipoles of the unaffected hand. Our findings demonstrate that neural cell assemblies in SI which formerly represented the right middle and ring fingers of this amputee became reorganized and invaded by neighbouring cell assemblies of the index and little finger of the same hand. These results indicate that functional plasticity occurs within a period of 10 days after amputation.

title: The analysis of visual motion: a comparison of neuronal and psychophysical performance
abstract: We compared the ability of psychophysical observers and single cortical neurons to discriminate weak motion signals in a stochastic visual display. All data were obtained from rhesus monkeys trained to perform a direction discrimination task near psychophysical threshold. The conditions for such a comparison were ideal in that both psychophysical and physiological data were obtained in the same animals, on the same sets of trials, and using the same visual display. In addition, the psychophysical task was tailored in each experiment to the physiological properties of the neuron under study; the visual display was matched to each neuron's preference for size, speed, and direction of motion. Under these conditions, the sensitivity of most MT neurons was very similar to the psychophysical sensitivity of the animal observers. In fact, the responses of single neurons typically provided a satisfactory account of both absolute psychophysical threshold and the shape of the psychometric function relating performance to the strength of the motion signal. Thus, psychophysical decisions in our task are likely to be based upon a relatively small number of neural signals. These signals could be carried by a small number of neurons if the responses of the pooled neurons are statistically independent. Alternatively, the signals may be carried by a much larger pool of neurons if their responses are partially intercorrelated.

title: Neural mechanisms for forming a perceptual decision.
abstract: Cognitive and behavioral responses to environmental stimuli depend on an evaluation of sensory signals within the cerebral cortex. The mechanism by which this occurs in a specific visual task was investigated with a combination of physiological and psychophysical techniques. Rhesus monkeys discriminated among eight possible directions of motion while directional signals were manipulated in visual area MT. One directional signal was generated by a visual stimulus and a second signal was introduced by electrically stimulating neurons that encoded a specific direction of motion. The decisions made by the monkeys in response to the two signals allowed a distinction to be made between two possible mechanisms for interpreting directional signals in MT. The monkeys tended to cast decisions in favor of one or the other signal, indicating that the signals exerted independent effects on performance and that an interactive mechanism such as vector averaging of the two signals was not operative. Thus, the data suggest a mechanism in which monkeys chose the direction encoded by the largest signal in the representation of motion direction, a "winner-take-all" decision process.

title: Microstimulation of visual cortex affects the speed of perceptual decisions
abstract: Direction-selective neurons in the middle temporal visual area (MT) are crucially involved in motion perception, although it is not known exactly how the activity of these neurons is interpreted by the rest of the brain. Here we report that in a two-alternative task, the activity of MT neurons is interpreted as evidence for one direction and against the other. We measured the speed and accuracy of decisions as rhesus monkeys performed a direction-discrimination task. On half of the trials, we stimulated direction-selective neurons in area MT, thereby causing the monkeys to choose the neurons' preferred direction more often. Microstimulation quickened decisions in favor of the preferred direction and slowed decisions in favor of the opposite direction. Even on trials in which microstimulation did not induce a preferred direction choice, it still affected response times. Our findings suggest that during the formation of a decision, sensory evidence for competing propositions is compared and accumulates to a decision-making threshold.

title: Decision Making as a Window on Cognition
abstract: A decision is a commitment to a proposition or plan of action based on information and values associated with the possible outcomes. The process operates in a flexible timeframe that is free from the immediacy of evidence acquisition and the real time demands of action itself. Thus, it involves deliberation, planning, and strategizing. This Perspective focuses on perceptual decision making in nonhuman primates and the discovery of neural mechanisms that support accuracy, speed, and confidence in a decision. We suggest that these mechanisms expose principles of cognitive function in general, and we speculate about the challenges and directions before the field.

title: Towards a true neural stance on consciousness
abstract: 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.

title: How neuroscience will change our view on consciousness
abstract: Is there consciousness in machines? Or in animals? What happens to consciousness when we are asleep, or in vegetative state? These are just a few examples of the many questions about consciousness that are troubling scientists and laypersons alike. Moreover, these questions share a striking feature: They seem to have been around forever, yet neither science nor philosophy has been able to provide an answer. Why is that? In my view, the main reason is that the study of consciousness is dominated by what we know from introspection and behavior. This has fooled us into thinking that we know what we are conscious of. The scientific equivalent of this is Global Workspace theory. But in fact we don't know what we are conscious of, as I will explain from a simple experiment in visual perception. Once we acknowledge that, it is clear that we need other evidence about the presence or absence of a conscious sensation than introspection or behavior. Assuming the brain has something to do with it, I will demonstrate how arguments from neuroscience, together with theoretical and ontological arguments, can help us resolve what the exact nature of our conscious sensation is. It turns out that we see much more than we think, and that Global Workspace theory is all about access but not about seeing. The exercise is an example of how neuroscience will move us away from psychological intuitions about consciousness, and hence depict a notion of consciousness that may go against our deepest conviction: “My consciousness is mine, and mine alone.” It's not.

title: Synchrony Unbound A Critical Evaluation of the Temporal Binding Hypothesis
abstract: Singer, 1994). The basis for this argument is that it is necessary to " tag " each visual neuron to signify the object to which its activity relates. Each neuron therefore has to carry two distinct signals, one that indicates how effective a stimulus is falling on its receptive field, and a second that tags it as a member of a particular cell assembly. To make these signals distinct, von der Mals-Center for Neural Science New York University burg (1981) proposed that the " effectiveness " signal would be carried by a conventional rate code, while the New York, New York 10003 " tag " signal would be created by synchronizing the spike activity of the neuron with spikes from other neurons in Introduction the same assembly. This novel idea has led to a great deal of experimental work and to several further elabora-In the early stages of visual processing, objects and scenes are represented by neurons with small visual tions of the original theory. In this paper, we will articulate our doubts and con-receptive fields. Each neuron provides information about local features of a scene, but to describe a scene in cerns about this theory and its experimental support. We consider first whether the theory is an a priori reasonable terms of objects requires that these features be combined. Objects can cover wide areas of visual space and approach to solving the binding problem, and conclude that it is at best incomplete. We then ask whether spike be partially occluded by other objects, so the problem of binding the separate representations of parts into synchrony can plausibly be used as an informational code, and conclude that there are significant practical coherent wholes is not a simple one. This " binding problem " has received considerable attention. Gestalt and theoretical obstacles both to encoding and to decoding information in this way. We then examine the psychologists articulated a number of principles for grouping and organizing scene elements (Kö hler, 1930; experimental evidence usually adduced to support the synchrony hypothesis, and conclude that the evidence Koffka, 1935; Kanisza, 1979), and more recently the related problem of image segmentation has received a is largely indirect and has no proven relevance to the issue of binding per se. We will finish by asking whether good deal of attention in computer vision (see, for example , Pal and Pal, 1993, for a review). The …

title: Can neuroscience explain consciousness
abstract: Cognitive neuroscience aspires to explain how the brain produces conscious states. Many people think this aspiration is threatened by the subjective nature of introspective reports, as well as by certain philosophical arguments. We propose that good neuroscientfic explanations of conscious states can consolidate an interpretation of introspective reports, in spite of their subjective nature. This is because the relative quality of explanations can be evaluated on independent, methodological grounds. To illustrate, we review studies that suggest that aspects of the feeling of being in control of one's bodily movement can be explained in terms of the complex and surprising way the brain predicts movement. This is a modest type of functional, contrastive explanation. Though we do not refute the threatening philosophical arguments, we show that they do not apply to this type of explanation.

title: Explanatory Correlates of Consciousness: Theoretical and Computational Challenges
abstract: Consciousness is a key feature of mammalian cognition and revealing its underlying mechanisms is one of the most important scientific challenges for the 21st century. In this article I review how computational and theoretical approaches can facilitate a transition from correlation to explanation in consciousness science. I describe progress towards identifying ‘explanatory correlates’ underlying four fundamental properties characterizing most if not all conscious experiences: (i) the co-existence of segregation and integration in conscious scenes, (ii) the emergence of a subjective first-person perspective, (iii) the presence of affective conscious contents, either transiently (emotion) or as a background (mood) and (iv) experiences of intention and agency that are characteristic of voluntary action. I also discuss how synthetic approaches can shed additional light on possible functions of consciousness, the role of embodiment in consciousness, and the plausibility of constructing a conscious artefact.

title: MATERIALISM AND QUALIA: THE EXPLANATORY GAP
abstract: J n “Naming and Necessity”1 and “Identity and Necessity,”2 Kripke presents a version of the Cartesian argument against mate­ rialism. His argument involves two central claims: first, that all identity state­ ments using rigid designators on both sides of the identity sign are, if true at all, true in all possible worlds where the terms refer; second, that psycho-physical identity statements are conceivably false, and therefore, by the first claim, actually false. My purpose in this paper is to transform Kripke’s argument from a metaphysical one into an epistemological one. My general point is this. Kripke relies upon a particular intuition regarding conscious experience to support his second claim. I find this intuition important, not least because of its stubborn resistance to philosophical dissolution. But I don’t believe this intuition supports the meta­ physical thesis Kripke defends—namely, that pyscho-physical identity statements must be false. Rather, I think it supports a closely related epistemological thesis— namely, that psycho-physical identity statements leave a significant explanatory gap, and, as a corollary, that we don’t have any way of determining exactly which psycho-physical identity statements are true.3 One cannot conclude from my version of the argument that materialism is false, which makes my version a weaker attack than Kripke’s. Nevertheless, it does, if correct, constitute a problem for materialism, and one that I think better captures the uneasiness many philos­ ophers feel regarding that doctrine. I will present this epistemological argument by starting with Kripke’s own argument and extracting the underlying intuition. For brevity’s sake, I am going to assume knowledge of Kripke’s general position concerning necessity and the theory of reference, and concentrate only on the argument against materialism. To begin with, let us assume that we are dealing with a physicalist type-identity theory. That is, our materialist is committed to statements like: (1) Pain is the firing of C-fibers. On Kripke’s general theory, if (1) is true at all it is necessarily true. The same of course, is the case with the following statement: (2) Heat is the motion of molecules. That is, if (2) is true at all it is necessarily true. So far so good.

title: Thinking about Mechanisms
abstract: The concept of mechanism is analyzed in terms of entities and activities, organized such that they are productive of regular changes. Examples show how mechanisms work in neurobiology and molecular biology. Thinking in terms of mechanisms provides a new framework for addressing many traditional philosophical issues: causality, laws, explanation, reduction, and scientific change.