The nature of the relationship between the objective, material world and the subjective world of conscious experience, of phenomenal content, lies at the heart of the ancient mind-body problem. Consciousness appears as mysterious to 21st century scholars as it was to the ancient Greeks who first systematically contemplated the nature of mind. Yet modern science is in a strong position to investigate and manipulate the physical basis of consciousness. Brain scientists seek an understanding of how and why the neural basis of one particular conscious sensation is the basis of that sensation rather than another, why so many behaviors occur outside the pale of consciousness, why sensations are structured the way they are, and how they acquire meaning. Finally, cognitive neuroscience is contributing to the problem of understanding the conscious sensation of willing an action.

The scholars represented in section X take the problem of consciousness, the first-person perspective, as given and assume that brain activity is both necessary and sufficient for biological creatures to experience something. A primary goal is to identify the specific nature of the activity of brain cells that gives rise to any one specific conscious percept, the neuronal correlates of consciousness (Crick & Koch, 1995; Metzinger, 2000; Chalmers, 2000). An auxiliary goal is to determine to what extent these correlates differ from activity that influences behavior without engaging consciousness.

Most everyone has a general idea of what it means to be conscious. According to the philosopher John Searle, “Consciousness consists of those states of sentience, or feeling, or awareness, which begin in the morning when we awake from a dreamless sleep and continue throughout the day until we fall into a coma or die or fall asleep again or otherwise become unconscious” (Searle, 1997). Practically speaking, consciousness is needed for nonroutine tasks that require retention of information over seconds. Although provisional and vague, such a definition is good enough to get the process started. As the science of consciousness advances, such operational definitions will need to be refined and expressed in more fundamental neuronal terms. Until the problem is understood much better, though, a more formal definition is likely to be either misleading or overly restrictive, or both. If this statement seems evasive, try defining a gene (Keller, 2000).

The working hypothesis is that consciousness emerges from neuronal features of the brain. Emergence is used here without any new-age overtones but in the sense that the initiation and propagation of the action potential in axons, a highly nonlinear phenomenon, is the result of—and can be predicted from—the attributes of voltage-dependent ionic channels inserted into the neuronal membrane. Although consciousness is fully compatible with the laws of physics, it is not obvious how it follows from these laws. Something else might be needed.

Understanding the material basis of consciousness is unlikely to require any exotic new physics but rather a much deeper appreciation of how highly interconnected networks of a very large number of heterogeneous but highly structured neurons work. The abilities of rapidly forming and dissolving coalitions of corticothalamic neurons to learn from interactions with the environment and from their own internal activities are routinely underestimated. The individual neurons themselves are complex entities with unique morphologies and thousands of inputs and outputs. Humans have no real experience with such vast organization. Hence, even biologists struggle to appreciate the properties and power of the nervous system.

It would be contrary to evolutionary continuity to believe that consciousness is unique to humans. Most brain scientists assume that many species possess some, but not necessarily all, of the features of consciousness—that they see, hear, smell, and otherwise experience the world (Griffin, 2001). This assumption is particularly true for monkeys and apes, whose behavior, development, and brain structure are remarkably similar to those of humans. Of course, each species has its own unique sensorium, matched to its ecological niche, but that is not to deny that animals can have feeling, subjective states. To believe otherwise seems presumptuous and flies in the face of all experimental evidence. At this point in time, we have no clear idea whether animals from phyla other than Chordata have subjective states. But the nonstereotyped and adaptive behaviors of many mollusks and insects make it likely that they too share the gift of consciousness with us.

The focus of much of the empirical work in the field and in this section of the book is on visual consciousness. More than other aspects of sensation, visual awareness is amenable to empirical investigations for a variety of reasons. First, humans are visual creatures. This fact is reflected in the large amount of brain tissue dedicated to vision and in the importance of seeing in daily life. Second, images are highly structured yet easy to control using computers. Third, phenomena such as binocular rivalry, continuous flash suppression, or motion-induced blindness can be used to manipulate the relationship between retinal input and visibility—that is, between objective sensory stimulus and subjective conscious percept. Last, the neuronal basis of many visual phenomena and illusions has been investigated throughout the animal kingdom. Perceptual neuroscience has advanced to such a point that reasonably sophisticated computational models have been constructed and have proven their worth in guiding experimental agendas and summarizing the data. It is not unlikely that all the different aspects of consciousness (smell, pain, vision, self-consciousness, the feeling of willing an action, and so on) employ one or perhaps a few common mechanisms. Figuring out the neuronal basis for one modality, therefore, will probably be the breakthrough event that will help us understand all of them.

Much has changed in consciousness studies in the five years since the previous edition of this book. So much so that an almost entirely new cast of philosophers and scientists discuss their findings in these pages, the vast majority of which have been published within the past few years. Better than anything else, this demonstrates the vitality of this research endeavor.

With the preceding background, let me briefly introduce the eight chapters. In chapter 77, Block summarizes and compares the three major theoretical approaches to consciousness that take science seriously: higher order theories, the global workspace account of consciousness, and biological theories—those that postulate that consciousness is some sort of biological state of the brain. Block, a philosopher, has the distinction of having himself directly contributed to the empirical debate, by drawing subtle but crucial distinctions between selective attention and consciousness and their underlying neuronal mechanisms (Block, 2007). In chapter 78, Schiff discusses the clinical literature pertaining to global impairments of consciousness following brain injury. The persistent vegetative state (PVS) or the minimal conscious state (MCS) are clinical conditions in which the patient is either permanently unconscious or hovering at the borderline between unconscious and conscious and in which midline structures in the brain stem and thalamus are affected. Given the large number of such patients—on the order of 100,000 in the United States alone—there is a great urgency to understand these pathologies. The exploratory work of Schiff and colleagues (2007) using thalamic deep-brain stimulation is particularly promising in this regard. Chapter 79 by Koch provides the conceptual framework and empirical data for a research program dedicated to discovering the neuronal basis of the content of conscious perception. It emphasizes what is not needed for consciousness (e.g., neither sensory input nor motor output nor self-consciousness), the dissociation between attention and consciousness, and the interaction among coalitions of corticothalamic neurons that vie for dominance. Chapter 80 by Rees and chapter 81 by Macknik and Martinez-Conde focus on different aspects of vision. Rees describes the enormous contributions functional brain imaging has made to elucidate the cortical basis of visual awareness in both patients and neurologically normal people; in contrast, Macknik and Martinez-Conde concentrate on the anatomy and electrophysiology of the early stages of the visual thalamocortical system in monkeys and what they teach us about consciousness. One of the crucial conclusions is that attention and consciousness, so often conflated, are quite distinct processes. Koenigs and Adolphs' chapter 82 deals with data and theories that seek to explain the neuronal basis of the subjective states of joy, sorrow, anger, and fear. They rightfully emphasize that much emotional processing can occur outside the pale of consciousness. While most students of the mind have gotten used to unconscious visual processing, the idea of subliminal emotional processing is not (yet) as widely accepted. Lau's chapter 83 deals with the problem of the function of consciousness in the context of willed actions. What is not in doubt is the fact that most of us consciously experience a feeling of willing an action, such as raising our arm. What is more controversial is whether this feeling has any causal effect on the behavior. Is the sensation of volition simply an epiphenomenon? Lau challenges the community to come up with experiments that conclusively demonstrate that some function cannot be carried out unconsciously. It is not an easy challenge to meet. Last, the field of consciousness studies is greatly hampered by the lack of a widely accepted theory of consciousness. We need to know, on theoretical or conceptual grounds, why a particular system of interacting parts has subjective states. Is our immune system conscious? And if not, why not? What about a fetus, a newborn child, an aphasic patient, a monkey, or a honeybee? And can we replicate consciousness in silicon? These questions are addressed in chapter 84 by Tononi and Balduzzi. Proceeding from two simple phenomenological axiomatic observations, they construct an integrated theory of consciousness, including qualia space, that explains many of the known anatomical, physiological, and psychological facts about consciousness. It is a very ambitious but enormously exciting development.

Collectively, the chapters in section X signal the emergence of a science of consciousness, an ability to investigate how phenomenal feelings emerge out of excitable brain matter in a rigorous, reliable, and reproducible manner. What are needed now are invasive experiments that begin to close the gap between correlation and causation. Molecular biology is delivering optogenetic techniques to deliberately, delicately, transiently, and reversibly dissect individual components of forebrain circuits in flies, mice, and, soon, monkeys (Adamantidis, Zhang, Aravanis, Deisseroth, & de Lecea, 2007). The applications of such technologies, in combination with continuous, long-term recordings from thousands of neurons and functional imaging techniques, will do much to advance this goal.