The Boundaries of Consciousness: Neurobiology and Neuropathology

Steven Laureys

• Publisher: Elsevier Science
• Year: 2006
• ISBN:
  9780080476209
• Topic: Medicine
• Subtopic: Neurology

Consciousness is one of the most significant scientific problems today. Renewed interest in the nature of consciousness - a phenomenon long considered not to be scientifically explorable, as well as increasingly widespread availability of multimodal functional brain imaging techniques (EEG, ERP, MEG, fMRI and PET), now offer the possibility of detailed, integrated exploration of the neural, behavioral, and computational correlates of consciousness. The present volume aims to confront the latest theoretical insights in the scientific study of human consciousness with the most recent behavioral, neuroimaging, electrophysiological, pharmacological and neuropathological data on brain function in altered states of consciousness such as: brain death, coma, vegetative state, minimally conscious state, locked-in syndrome, dementia, epilepsy, schizophrenia, hysteria, general anesthesia, sleep, hypnosis, and hallucinations. The interest of this is threefold. First, patients with altered states of consciousness continue to represent a major clinical problem in terms of clinical assessment of consciousness and daily management. Second, the exploration of brain function in altered states of consciousness represents a unique lesional approach to the scientific study of consciousness and adds to the worldwide effort to identify the "neural correlate of consciousness". Third, new scientific insights in this field have major ethical and social implications regarding our care for these patients.

CHAPTER 1 What in the world is consciousness?

Adam Zeman^*[email protected]

Department of Clinical Neurosciences, Western General Hospital, University of Edinburgh, Edinburgh EH4 2XU, UK

* Corresponding author. Tel.: [phone]; Fax: [phone]

Abstract

The concept of consciousness is multifaceted, and steeped in cultural and intellectual history. This paper explores its complexities by way of a series of contrasts: (i) states of consciousness, such as wakefulness and sleep, are contrasted with awareness, a term that picks out the contents of consciousness, which range across all our psychological capacities; (ii) consciousness is contrasted to self-consciousness, which is itself a complex term embracing self-detection, self-monitoring, self-recognition theory of mind and self-knowledge; (iii) “narrow” and “broad” senses of consciousness are contrasted, the former requiring mature human awareness capable of guiding action and self-report, the latter involving the much broader capacity to acquire and exploit knowledge; (iv) an “inner” conception of consciousness, by which awareness is essentially private and beyond the reach of scientific scrutiny, is contrasted with an “outer” conception which allows that consciousness is intrinsically linked with capacities for intelligent behavior; and finally (v) “easy” and “hard” questions of consciousness are distinguished, the former involving the underlying neurobiology of wakefulness and awareness, and the latter the allegedly more mysterious process by which biological processes generate experience — the question of whether this final distinction is valid is a focus of current debate. Varied interests converge on the study of consciousness, from the sciences and the humanities, creating scope for interdisciplinary misunderstandings, but also for a fruitful dialog.

Introduction

The current surge of excitement about the science of consciousness, attested by this volume, flows from several sources — the techniques of functional imaging are enabling us to see something of what happens in the brain during experience, and during its absence in states such as coma (Laureys et al., 2001); the idea that much of the brain’s activity proceeds without giving rise to awareness is encouraging a “contrastive analysis” of the neural substrates of conscious and unconscious processes (Baars, 2002); progress in the design of intelligent machines is allowing us to glimpse what it might take to create an artificial consciousness (see Aleksander elsewhere in this volume). Interestingly, these scientific developments are also bolstering our confidence in the value and veracity of first-person testimony, as some of the most intriguing but elusive phenomena of experience — imagery (Ishai et al., 2000), hallucinations (Ffytche et al., 1998), shivers down the spine (Blood and Zatorre, 2001; Griffiths et al., 2004) — are shown to have informative, distinctive neural correlates. Against this intellectual background, everyone interested in consciousness is bound to look for ways of resolving the tension between the first- and third-person views of our lives, and ways of understanding how experience can be at once real, functional, and rooted in our physical existence (Zeman, 2001, 2002).

This endeavor is of course an ancient one. The contemporary “problem of consciousness” flags up our version of a time-honored quest for a fully satisfying explanation of the relationship between mind and body. How does what passes through our minds relate to the events occurring in our brains? How do the anatomy and physiology of the hundred billion neurons inside our heads generate awareness — what the poet Louis MacNiece described as “the sluice of hearing and seeing,” the taste of honey, a needle’s prick, the scent of burning rubber, the bite of a cold wind, the vivid procession of moments of experience that composes our conscious lives?

The belief that the key to a solution lies in a fuller understanding of the brain is also ancient, as revealed by this famous, and astonishingly prescient, passage from Hippocrates” text “On the Sacred Disease”(Jones, 1923):

Men ought to know that from the brain, and from the brain only, arise our pleasures, joys, laughters and jests, as well as our sorrows, pains, griefs and tears. Through it … we think, see, hear, and distinguish the ugly from the beautiful, the bad from the good, the pleasant from the unpleasant … sleeplessness, inopportune mistakes, aimless anxieties, absent-mindedness, and acts that are contrary to habit. These things that we suffer all come from the brain … Madness comes from its moistness.

But progress in understanding how experience “arises” from the brain has been disappointingly slow over the ensuing two and half millennia. E.O. Wilson, in 1998, could still identify the problem as a central issue in science (Wilson, 1998):

… the master unsolved problem of biology: how the hundred million nerve cells of the brain work together to create consciousness …

To make matters worse, there have always been skeptics who have doubted that we will ever find what we are seeking in the brain. Leibniz voiced such doubts in Monadology, which invites us to imagine walking into the midst of an artificial brain (Leibniz, 1714):

Perception and that which depend on it are inexplicable by mechanical causes, that is by figures and motions. And supposing there were a machine so constructed as to think, feel and have perception, we could conceive of it as enlarged and yet preserving the same proportions, so that we might enter into it as into a mill. And this granted, we should only find on visiting it, pieces which push against one another, but never anything by which to explain perception.

Such pessimism may seem unwarranted, even curmudgeonly, set against the exciting recent work I have referred to. Yet this skepticism is a recurring theme. It has at least two possible sources. The first is the thought that our experience, unlike the brain from which it “arises”, is private, invisible, immaterial, ghostly, and inaccessible to the methods of science. If so, no amount of peering and poking in the brain will allow us to find what we are seeking: in this case an orthodox scientific explanation is simply not available for consciousness, although it may be for behavior, and for events in the brain. This thought suggests that we have been seeking the wrong kind of explanation for the phenomenon of interest. But there is another possibility: that we have been looking for the right kind of explanation for the wrong kind of thing. Perhaps experience is not what we took it to be. This line of thought is currently popular. It suggests that our concept of consciousness has misled us into hunting for a theory of an illusion.

How could this happen? We need to acknowledge, as Dan Dennett has repeatedly reminded us (Dennett, 1991), that “consciousness” is not a straightforward scientific term. On the contrary, it belongs to a network of associated concepts that has evolved over centuries under the pressure of a host of cultural influences — scientific, artistic, philosophical, and religious. The network includes other terms such as “mind,” “self” (Berrios and Markova, 2003), “soul” and “spirit.” Making a scientific sense of these, and all their connotations, is a tall order. Figure 1 shows some of the beliefs of 250 undergraduates at the University of Edinburgh on aspects of the relationship of mind to brain (data submitted for publication), underlining the distance that already exists between the beliefs of most neuroscientists about consciousness and those which are held by a wider but well-educated public.

Fig. 1 The results of a survey of 250 students from several disciplines at the University of Edinburgh on attitudes to mind and brain.

(Liew et al., submitted)

All of us may need to reconceive the scientific target of the quest for “consciousness” in the light of the evidence emerging from the quest. This process will take time.

The complexity and nuances of the concept provide a reason for opening this volume with some introductory thoughts on what we mean by consciousness. I will not be able to resolve its complexities here but can try, at least, to clarify the concept, by way of a series of contrasts designed to highlight its multiple dimensions of ambiguity.

Wakefulness versus awareness

Consciousness has two key senses in colloquial English: wakefulness and awareness. Wakefulness is a state of consciousness, distinguished from other states such as sleep and coma. These states admit of degrees: we can be wide awake or half awake, lightly or deeply anesthetized. We are normally confident of our ability to judge an individual’s state of consciousness, in this first sense, with the help of objective criteria, like those of the Glasgow Coma Scale (Teasdale and Jennett, 1974): having one’s eyes open is generally an indication of wakefulness — being able to converse pretty much settles the matter.

We usually assume that anyone who is awake will also be aware — in other words, not merely conscious but conscious of something. Objective criteria are still helpful in ascertaining the presence of consciousness in this second sense — anyone who can obe...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Foreword
• Foreword
• Preface
• Acknowledgments
• Chapter 1: What in the world is consciousness?
• Chapter 2: A neuroscientific approach to consciousness
• Chapter 3: Functional neuroimaging during altered states of consciousness: how and what do we measure?
• Chapter 4: Global workspace theory of consciousness: toward a cognitive neuroscience of human experience
• Chapter 5: Skill, corporality and alerting capacity in an account of sensory consciousness
• Chapter 6: Methods for studying unconscious learning
• Chapter 7: Computational correlates of consciousness
• Chapter 8: Machine consciousness
• Chapter 9: Consciousness, information integration, and the brain
• Chapter 10: Dynamics of thalamo-cortical network oscillations and human perception
• Chapter 11: From synchronous neuronal discharges to subjective awareness?
• Chapter 12: Genes and experience shape brain networks of conscious control
• Chapter 13: Visual phenomenal consciousness: a neurological guided tour
• Chapter 14: The mental self
• Chapter 15: Posterior cingulate, precuneal and retrosplenial cortices: cytology and components of the neural network correlates of consciousness
• Chapter 16: Human cognition during REM sleep and the activity profile within frontal and parietal cortices: a reappraisal of functional neuroimaging data
• Chapter 17: General anesthesia and the neural correlates of consciousness
• Chapter 18: Brain imaging in research on anesthetic mechanisms: studies with propofol
• Chapter 19: The cognitive modulation of pain: hypnosis- and placebo-induced analgesia
• Chapter 20: Consciousness and epilepsy: why are patients with absence seizures absent?
• Chapter 21: Two aspects of impaired consciousness in Alzheimer’s disease
• Chapter 22: Functional brain imaging of symptoms and cognition in schizophrenia
• Chapter 23: Hysterical conversion and brain function
• Chapter 24: The out-of body experience: precipitating factors and neural correlates
• Chapter 25: Near-death experiences in cardiac arrest survivors
• Chapter 26: The concept and practice of brain death
• Chapter 27: The minimally conscious state: defining the borders of consciousness
• Chapter 28: Behavioral evaluation of consciousness in severe brain damage
• Chapter 29: Evoked potentials in severe brain injury
• Chapter 30: Event-related potential measures of consciousness: two equations with three unknowns
• Chapter 31: Novel aspects of the neuropathology of the vegetative state after blunt head injury
• Chapter 32: Using a hierarchical approach to investigate residual auditory cognition in persistent vegetative state
• Chapter 33: Modeling the minimally conscious state: measurements of brain function and therapeutic possibilities
• Chapter 34: The locked-in syndrome : what is it like to be conscious but paralyzed and voiceless?
• Chapter 35: Brain-computer interfaces — the key for the conscious brain locked into a paralyzed body
• Chapter 36: Neural plasticity and recovery of function
• Chapter 37: Thirty years of the vegetative state: clinical, ethical and legal problems
• Chapter 38: Assessing health-related quality of life after severe brain damage: potentials and limitations
• Chapter 39: Outcome and ethics in severe brain damage
• Chapter 40: Clinical pragmatism and the care of brain damaged patients: toward a palliative neuroethics for disorders of consciousness
• Subject Index