Consciousness

Brain (2001), 124, 1263–1289 INVITED REVIEW Consciousness Adam Zeman Department of Clinical Neurosciences, Western General Hospital, Edinburgh, UK Correspondence to: A. Zeman, Department of Clinical Neurosciences, Western General Hospital, Edinburgh EH4 2XU, UK E-mail: Contents Summary I Introduction 1264 II (a) (b) (i) (ii) (iii) (c) (i) (ii) (iii) (iv) (v) Concepts of consciousness The etymology of ‘consciousness’ and ‘conscience’ The meanings of ‘consciousness’ Consciousness as the waking state Consciousness as experience Consciousness as mind The meanings of ‘self-consciousness’ Self-consciousness as proneness to embarrassment Self-consciousness as self-detection Self-consciousness as self-recognition Self-consciousness as awareness of awareness Self-consciousness as self-knowledge 1265 1265 1265 1265 1266 1266 1266 1266 1266 1266 1266 1266 III (a) (b) (i) (ii) (iii) (c) The science of wakefulness The electricity of the brain The control of conscious states Anatomy Physiology Pharmacology Pathologies of wakefulness 1267 1267 1268 1268 1269 1270 1271 IV (a) (b) (c) The science of awareness Exquisite correlations Implicit perception Insights from the study of memory and action 1272 1273 1275 1277 V (a) (b) (c) Theories of consciousness Neurobiological theories Information processing theories Social theories 1278 1278 1280 1281 VI (a) (b) (c) (d) The philosophy of consciousness Three intuitions about consciousness Identity theory Functionalism Dualism 1282 1282 1282 1283 1283 VII Conclusion 1284 © Oxford University Press 2001 1264 A. Zeman Summary Consciousness is topical, for reasons including its renewed respectability among psychologists, rapid progress in the neuroscience of perception, memory and action, advances in artificial intelligence and dissatisfaction with the dualistic separation of mind and body. Consciousness is an ambiguous term. It can refer to (i) the waking state; (ii) experience; and (iii) the possession of any mental state. Self-consciousness is equally ambiguous, with senses including (i) proneness to embarrassment in social settings; (ii) the ability to detect our own sensations and recall our recent actions; (iii) self-recognition; (iv) the awareness of awareness; and (v) self-knowledge in the broadest sense. The understanding of states of consciousness has been transformed by the delineation of their electrical correlates, of structures in brainstem and diencephalon which regulate the sleep–wake cycle, and of these structures’ cellular physiology and regional pharmacology. Clinical studies have defined pathologies of wakefulness: coma, the persistent vegetative state, the ‘locked-in’ syndrome, akinetic mutism and brain death. Interest in the neural basis of perceptual awareness has focused on vision. Increasingly detailed neuronal correlates of real and illusory visual experience are being defined. Experiments exploiting circumstances in which visual experience changes while external stimulation is held constant are tightening the experimental link between consciousness and its neural correlates. Work on unconscious neural processes provides a complementary approach. ‘Unperceived’ stimuli have detectable effects on neural events and subsequent action in a range of circumstances: blindsight provides the classical example. Other areas of cognitive neuroscience also promise experimental insights into consciousness, in particular the distinctions between implicit and explicit memory and deliberate and automatic action. Overarching scientific theories of consciousness include neurobiological accounts which specify anatomical or physiological mechanisms for awareness, theories focusing on the role played by conscious processes in information processing and theories envisaging the functions of consciousness in a social context. Whether scientific observation and theory will yield a complete account of consciousness remains a live issue. Physicalism, functionalism, property dualism and dual aspect theories attempt to do justice to three central, but controversial, intuitions about experience: that it is a robust phenomenon which calls for explanation, that it is intimately related to the activity of the brain and that it has an important influence on behaviour. Keywords: consciousness; awareness; perception; neuropsychology; neuroscience Abbreviations: ARAS ⫽ ascending reticular activating system; NCC ⫽ neural correlate of consciousness; REM ⫽ rapid eye movement; SWS ⫽ slow wave sleep I. Introduction The past decade has seen a rising tide of interest in consciousness, accompanied by a surge of publications, new journals and scientific meetings (Dennett, 1991; McGinn, 1991; Edelman, 1992; Flanagan, 1992; Milner and Rugg, 1992; Searle, 1992; Crick, 1994; Penrose, 1994; Metzinger, 1995; Chalmers, 1996; Velmans 1996; Weiskrantz, 1997; Hurley, 1998; Jasper et al., 1998; Rose, 1998; Velmans, 2000). The ‘problem of consciousness’ has been identified as an outstanding intellectual challenge across disciplines ranging from basic neuroscience through psychology to philosophy, although opinions vary widely on the chances of achieving a solution. The subject is unusual in drawing together scholars from both sides of the gulfs which separate the sciences and the arts, the study of the body and the study of the mind. Several factors help to explain the current fascination with consciousness. The techniques of physiological psychology, human neuropsychology and, recently, functional imaging are revealing exquisitely detailed correlations between neural processes and features of conscious experience. The recognition of unconscious or ‘implicit’ capacities which can exert an influence on behaviour, such as blindsight, has opened up the way to distinguishing the neural substrates of conscious and unconscious activity in the brain. Advances in computational science and artificial intelligence hold out the prospect of engineering conscious systems. More generally, there is a deep dissatisfaction with the Cartesian separation of body and mind, and a desire to find a place for subjective experience in the scientist’s world picture. This is a timely moment to review progress in the field. As at least part of the problem of consciousness flows from the ambiguities of the term, I shall briefly consider its various senses, and its relationship to ‘self-consciousness’, in Section II, flagging up some philosophical problems for later discussion. Sections III and IV provide a necessarily selective review of empirical research bearing on mechanisms of arousal, visual awareness, memory and volitional, or conscious, action. Section V examines a number of overarching theories of consciousness. Section VI reflects on the philosophical ‘problem of consciousness’ in the light of the Consciousness empirical advances and recent contributions to the philosophical debate. II. Concepts of consciousness (a) The etymology of ‘consciousness’ and ‘conscience (...truncated)