Preface

Reliable scientic evidence about consciousness has often been dicult to nd. This volume presents more than ve dozen signicant articles in the brain and cognitive sciences, all with a direct bearing on the subject. Our guiding idea has been to select studies that ask, What dierence does consciousness make? What are its properties, and what role could it play in the nervous system? How do conscious brain functions dier from unconscious ones? Among the published articles that satisfy this standard we have aimed for the most denitive, the pioneering, the most articulate, and the briefest. We believe they are of fundamental importance. We have thought especially about readers who are new to this fast-growing literature. Wherever possible we have looked for good introductory articles, and our own introductions are meant to highlight briey why each one is important. We are painfully aware that we have had to omit many signicant articles. Each selection should therefore be considered a point of entry to a larger body of published work. Although we emphasize breadth, we have not covered neighboring topics such as voluntary control and self. A new literature is emerging there, such as the remarkable reconstruction by Hannah Damasio and coauthors of the brain of Phineas Gage, the nineteenth-century braininjured patient who underwent a great change of personality when his frontal cortex was penetrated by an explosively driven railroad spike (1994). An equally important body of ndings is now growing on voluntary control, including work by psychologists like Daniel Wegner on agency (Wenzla and Wegner 2000) and by neurologists on brain conditions that dissociate voluntary from involuntary control. Consciousness, self, and voluntary control are indeed sister issues, but they cannot be covered in depth in a single volume. Finally, we omitted ideas that are unsupported by evidence at this time. There has been much discussion of quantum-theory claims about consciousness, for example, for which we have no

direct evidence at present. Although no hypothesis can be excluded a priori, this volume presents ideas for which we have reasonably direct evidence today. The Rediscovery of Consciousness Because of the curious history of consciousness in the twentieth century, this most fundamental human question has been oddly neglected. Yet for decades now, the evidence has been mounting in quantity and quality. The scientic community has responded with rising interest. After almost a century of neglect, consciousness has become a major focus for research. Each month new ndings appear in leading journals. In the coming century this new ferment is likely to reshape our understanding of mind and brain in the most basic way. Consciousness has long been seen as central to the human condition, with a serious literature dating to the earliest written records from Asia and the Fertile Crescent. It interpenetrates all human functions. Sensory perception, attention, and language can be usefully divided into conscious and unconscious aspects. Many memory researchers now believe that consciousness partitions their eld in crucial ways, in distinguishing between fundamental concepts like recall and recognition, for example. Motivation, action control, emotion, learning, and development all interact with consciousness in fundamental ways. With the advent of a body of scientic evidence bearing directly on the subject, a core study program seems to be emerging. In the brain sciences, psychology, and philosophy, new courses and seminars are being taught. Specialized doctoral programs have been organized in a few places, and many laboratories are refocusing their eorts. Even where the word consciousness is still treated with caution, the same questions are debated. We believe this volume can contribute to informed discussion by pre-

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senting the breadth and depth of the eld, showing the strength of the scientic foundations and highlighting pathways for the future. Our gratitude goes to the authors presented here and to those who deserved to be included, but for the limitations of space. We honor the pioneers, who often had to work in very dicult circumstances. All who care about understanding human consciousness are in their debt. Note: After the manuscript of this book was essentially complete, James Newman died unexpectedly, to the great shock and regret of his friends. Jim was a pioneer in scientic studies of consciousness. He made noteworthy contributions to the interpretation of thalamocortical mechanisms, as well as to scientic organizations and activities in the eld. He was aliated with Colorado Neurological Institute. This book is gratefully dedicated to his memory. ReferencesDamasio, H., Grabowski, T., Frank, R., Galaburda, A. M., and Damasio, A. R. (1994) The return of Phineas Gage: Clues about the brain from the skull of a famous patient. Science 264: 11021105. Gray, J. A., Wedderburn, A. A. I. (1960) Grouping strategies with simultaneous stimuli. Quarterly Journal of Experimental Psychology 12, 180184. Wenzla, R. M., Wegner, D. M. (2000) Thought suppression. Annual Review of Psychology 51: 5991.

Bernard J. Baars* The Neurosciences Institute William P. Banks Pomona College* Supported by The Neurosciences Institute and The Neurosciences Research Foundation, which are gratefully acknowledged.

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American Journal of Psychiatry, 134, 13351348. Reprinted with permission of the American Psychiatric Association. 58. S. P. LaBerge, L. E. Nagel, W. C. Dement, and V. P. Zarcone, Jr (1981) Lucid dreaming veried by volitional communication during REM sleep. Perceptual and Motor Skills, 52, 727732. s and D. Pare (1991) Commentary: Of 59. R. Llina dreaming and wakefulness. Neuroscience, 44, 512535. Reprinted with permission of Elsevier Science. 60. G. Tononi and G. M. Edelman (1998) Consciousness and complexity. Science, 282, 18461851. Reprinted with the permission of Science. 61. S. Grossberg (1999) The link between brain learning, attention, and consciousness. From Consciousness and Cognition, 8, 144. Reprinted with permission of Academic Press. 62. J. Taylor and F. N. Alavi (1993) A global competitive network for attention. Neural Network World, 5, 477502. Reprinted with the permission of International Science Publishers and Neural Network World. 63. A. R. Damasio (1989) Time-locked multiregional retroactivation: A systems-level proposal for the neural substrates of recall and recognition. Cognition, 33, 25 62. 64. W. Singer and C. M. Gray (1995) Visual feature integration and the temporal correlation hypothesis. Annual Review of Neuroscience, 18, 555586. Reprinted with permission of Annual Reviews. 65. B. J. Baars (1998) Metaphors of consciousness and attention in the brain. Trends in Neurosciences, 21, 58 62. Reprinted with permission of Elsevier Science. 66. B. J. Baars (1993) How does a serial, integrated, and very limited stream of consciousness emerge from a nervous system that is mostly unconscious, distributed, parallel, and of enormous capacity? In G. R. Bock and J. Marsh (eds.) Experimental and theoretical studies of consciousness, pp. 282303. Chichester, U.K.: Wiley. CIBA Foundation Symposium 174. 67. J. Newman, B. J. Baars, and S.-B. Cho (1997) A Neural Global Workspace model for conscious attention. Neural Networks (Special issue), 10, 11951206. Reprinted with permission of Elsevier Science. 68. S. Franklin and A. Graesser (1999) A software agent model of consciousness. Consciousness and Cognition, 8, 285301. Reprinted with permission of Academic Press.

1

Introduction: Treating Consciousness as a Variable: The Fading Taboo

Bernard J. Baars Consciousness is both the most obvious and the most hotly debated topic in psychology and brain science. All healthy humans are conscious of sights and sounds, of some mental images, of inner speech and emotional feelings, and of some of our goals and beliefs. Essentially all biopsychological experiments involve consciousness in one way or another. Yet for most of this century, scientists have been hesitant to explore the issue directly. This hesitation is historically new. More than two millenia ago, philosophers in Asia and Greece began the written record of human thought by exploring conscious experiences. Most of our basic mental concepts have their origin in this long tradition. Modern scientic psychology and neurology began in about 1800 with the study of human conscious experience, and some works from that era, such as William Jamess Principles of Psychology (1890/1983) are still widely read today. Until the twentieth century, scientists were deeply involved in eorts to understand consciousness. That receptive attitude changed radically in the years just after 1900, when a great shift occurred toward scientic physicalismthe idea that all human activities must be explained by physical brain processes or by physical stimuli and responses. In brain science this philosophy was popularized by I. P. Pavlov, and in the new discipline of psychology, by behaviorists like John B. Watson and later B. F. Skinner and many others. Although consciousness did not go away, so little physical evidence was known about it that serious scientists tended to avoid it altogether. The universal fact of human consciousness came to resemble a scientic taboo. The neglect of consciousness is now fading rapidly. After almost a century, an accelerating series of signicant papers has begun to appear in leading journals such as Science and Nature, reporting marked progress in understanding conscious vision in the cortex, conscious memories mediated by the hippocampus, and more. In all cases, conscious events are compared to unconscious ones: conscious vision is contrasted with unconscious visual activity, and conscious (explicit) memories with unconscious ones. But that is only the tip of the iceberg. Since the early 1980s, thousands of studies of conscious and unconscious processes have appeared in the brain and psychological literature, under various headings. There is little doubt that we are again looking at questions that were familiar to William James and his generation, but now with better evidence and theory than ever before. Evidence Many scientists question whether there is any evidence about conscious experience as such. In this volume we approach this issue by selecting studies that treat consciousness as a variable. They include the following comparisons: Between conscious and unconscious streams of stimulation Between conscious and unconscious elements in memory Between forms of brain damage that selectively impair conscious processes and those that do not Between wakefulness compared to deep sleep, coma, and anesthesia Between new and habituated events Many comparison cases like these have been studied. In each of them, consciousness is treated as an experimental variable, just as in any other topic of scientic study. We believe that such comparisons are the key to the evidence. Although many studies explore consciousness in this way, this fact may not be obvious because the word consciousness is

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Table 1.1 Some widely studied polarities between matched conscious and unconscious phenomena Conscious 1. Explicit cognition 2. Immediate memory 3. Novel, informative, and signicant events 4. Attended information 5. Focal contents 6. Declarative memory (facts, etc.) 7. Supraliminal stimulation 8. Eortful tasks 9. Remembering (recall) 10. Available memories 11. Strategic control 12. Grammatical strings 13. Intact reticular formation and bilateral intralaminar thalamic nuclei 14. Rehearsed items in Working Memory 15. Wakefulness and dreams (cortical arousal) 16. Explicit inferences 17. Episodic memory (autobiographical) 18. Autonoetic memory 19. Intentional learning 20. Normal vision Unconscious Implicit cognition Longer term memory Routine, predictable, and nonsignicant events Unattended information Fringe contents (e.g., familiarity) Procedural memory (skills, etc.) Subliminal stimulation Spontaneous/automatic tasks Knowing (recognition) Unavailable memories Automatic control Implicit underlying grammars Lesioned reticular formation, or bilateral intralaminar nuclei Unrehearsed items Deep sleep, coma, sedation (cortical slow waves) Automatic inferences Semantic memory (conceptual knowledge) Noetic memory Incidental learning Blindsight (cortical blindness)

sometimes still avoided. Instead, investigators talk about explicit versus implicit cognition, or attended versus unattended stimulation. Table 1.1 shows some of the popular substitutes for conscious and unconscious. Notice, by the way, that any theory of the conscious component of human cognition must somehow explain all of these contrasts. The problem is therefore very strongly bounded. One cannot simply make up a theory to explain one of the contrasts and expect it to explain the others. (See Baars 1988, 1997, and 2002 for many detailed examples). This profusion of terms tends to hide underlying similarities. All words on the left side of table 1.1 refer to reportable, broadly conscious processes. All those on the right side refer to very similar processes that are not reportable and not conscious. This simple fact is easily lost in the great variety of technical synonyms. But it is now increasingly being recognized. One aim of this volume is to call attention to such fundamental similarities. It is relatively easy to scour any major research literature for studies that compare conscious and unconscious events. For this volume we did not nd it dicult to nd seventy seminal articles that do just that. Indeed, our problem was to winnow down hundreds of candidate articles to a more practical number; many excellent articles had to be left out. Contrary to traditional opinion, therefore, our empirical knowledge about consciousness is quite extensive. (See Baars 1988, 1997, and in press.) It Has Been Historically Dicult to Think of Consciousness as a Variable Scientically it seems obvious that we can only study something as an empirical variable, comparing more of it to less of it. A number of historic breakthroughs in science emerged from the realization that some previously assumed constant, like atmospheric pressure or gravity,

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was actually a variable. The rst step is always to nd at least one comparison condition: earth gravity compared to near-zero gravity in space, or sea-level air pressure compared to an articial vacuum. Discovering comparison conditions is often a wrenching process. In the case of gravity, it required a great leap of imagination for natural philosophers in the seventeenth century to understand that all objects in the universe need not fall toward the center of the earth. It was Newtons ability to imagine variable amounts and directions of gravitational force that led to the solution of the ancient puzzle of planetary motion. Likewise, the reality of atmospheric pressure was not recognized until variations in air pressure could be observed with barometers, which were invented only a few hundred years ago. Gravity and atmospheric pressure were simply taken for granted before they were found to be variable in the sixteenth and seventeenth centuries. Most of these conceptual advances were vigorously opposed. Yet discovering comparison conditions is often the key to new insights. Biology as a science emerged from Darwins revolutionary idea that species are not xed, but variable over geologic time. Modern earth sciences emerged from the key idea that the worlds continents are not stable, but are oating fragments of earth crust. Relativistic physics and quantum theory provide other familiar examples. Perhaps all the sciences have their origins in such moments of insight, when an apparent constant is suddenly revealed to be variable. When new comparison conditions emerge, facts long hidden from view may suddenly become visible and salient. Historically, however, consciousness seemed to be dierent from all other scientic concepts. It has been extraordinarily dicult to see it as a variable. The persistent pattern over centuries has been to see our own experience as the only psychological domain that can be conceived, one that has no conceivable comparison condition. The notion that conscious experience is incommensurable with any other event may be a con-

sequence of our inability to compare our own private experience with other things. We cannot vary our own consciousness from the inside; as soon as we decrease it, we lose the ability to observe anything. And the consciousness of others is simply invisible as a direct datum. What are the natural comparison conditions for conscious events? To study consciousness as a variable, the events to be compared must be similar enough to make comparison meaningful. The evidence that unconscious brain events are often comparable to conscious ones is now extensive (Baars 1988, 1997, in press). Most readings in this book present more support for this claim. The notion that consciousness can be studied with natural comparison conditions, which cast light on the fundamental question, has now emerged in many dierent places in mind and brain science. As a result, we have a burgeoning scientic literature with much to tell us. After many years of neglect and confusion, the topic has come back into focus. Some of our existing knowledge about consciousness now seems so obvious that we rarely bother to make it explicit. There is good evidence, for example, that waking consciousness is both widespread and biologically adaptive. Sleep-waking cycles occur throughout the vertebrate phylum, associated with characteristic neuronal activity and such behavioral activities as goal-directed seeking and avoidance. Outside of the waking state, vertebrates do not feed, mate, reproduce, defend their territory or young, migrate, or carry out any other purposeful survival or reproductive activity. Physiologically, consciousness has pervasive eects: its characteristic electrical signature (fast, low voltage, and irregular) can be found throughout the waking brain, and in unconscious states like deep sleep and coma, slow and coherent waves are equally widely distributed. In these respects, consciousness is not some subtle or hard-to-observe phenomenon. It is hard to avoid. Brain and cognitive scientists all over the world have come to similar conclusions in recent

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years, so that today a new race to understand consciousness is in full swing. Most articles in this volume were published in the last decade, and the trend toward more research in consciousness appears to be accelerating. Consciousness as a Construct Indexed by Behavioral Report Many observers have pointed out that science is obliged to treat consciousness not as an observable datum but as an inferred concept based on public evidence. To each of us conscious sights and sounds appear as primary events, but as researchers dealing with public evidence, we can conrm only the reports people make about their conscious experience. Scientically, therefore, consciousness is not something we know directly; it is a theoretical construct based on shared, public observations. Edwin G. Boring (1933) summarized this view several decades ago,. . . that human consciousness is an inferred construct, a capacity as inferential as any of the other psychological realities, and that literally immediate observation, the introspection that cannot lie, does not exist. All observation is a process that takes time and is subject to error in the course of its occurrence. (p. 23)

was the gene; so was the vastness of geological time, a necessary assumption for Darwinian evolution; and other scientic constructs too numerous to list. Cognitive neuroscience applies this common-sense epistemology more explicitly than in everyday life. We can speak of meaning, thought, imagery, attention, memory, and recently, conscious and unconscious processes all inferred concepts that have been tested in careful experiments and stated in increasingly adequate theories. Operational Denitions Our standard behavioral index for consciousness is the ability people have to report their experiences, often in ways that can be checked for accuracy. More than a century of investigation into sensory processes is based on this fundamental fact. Indeed our knowledge of the senses comes largely from psychophysical research, in which we ask people to report their conscious experiences of precisely controlled sensory stimuli. Under well-dened conditions, such reports are exquisitely sensitive. Conscious processes can be operationally dened as events that: 1. can be reported and acted upon, 2. with veriable accuracy, 3. under optimal reporting conditions, 4. and which are reported as conscious. These conditions t standard practice in the study of perception, immediate memory, problem-solving, imagery, and many other phenomena. Optimal reporting conditions implies a minimum delay between the event and the report, freedom from distraction, and the like. The fourth condition is helpful to dierentiate focal conscious contents from other events that meet the rst three conditions but that are not typically reported as conscious. A noteworthy example is William Jamess fringe conscious

This is a familiar strategy in science. We now have three decades of research showing that we can make useful inferences about constructs like selective attention, working memory, imagery, and the like, based on robust observable evidence. Consciousness can be viewed as another theoretical construct, one that has the remarkable feature of reportability across a vast range of contents. In most cases this objective construct also coincides with our own experience. It cannot be overemphasized that inferred constructs are not unique to psychology and brain science. All sciences make inferences that go beyond the observations. The atom was highly inferential in its rst modern century; so

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events, such as the feeling of knowing that something is familiar or beautiful or true, without being able to pinpoint the conscious event that is the source of such feelings. (See section VIII, and below.) Reportability as an operational criterion seems to generalize to other primates. This has been studied especially well in the macaque monkey. Blindsight (cortical blindness) is a condition in which the rst cortical projection area (V1) of the primary visual pathway is damaged. In the occluded part of the visual eld, humans report a loss of conscious visual qualities like color, motion, and object identity. Yet there is excellent evidence that such properties of the visual stimulus are still processed by the visual brain. In forced-choice tasks, blindsight patients can point to a visual object, name it, and detect motion and color, while strongly denying that they have a conscious visual experience of the object. This makes blindsight an ideal case for studying visual consciousness (Weiskrantz 1986; Cowey and Stoerig, chap. 10, this volume). The macaques visual brain resembles the human one in many ways. Careful lesion studies show that the macaque behaves much like a human blindsighted subject when parts of area V1 are removed. But how can we be sure that the blindsighted macaque has lost conscious visual qualities, the qualia discussed by philosophers, such as color, motion, and texture? A remarkable experiment by Cowey and Stoerig (chap. 10, this volume) makes this case, using a behavioral index called the commentary key, which allows the macaque not merely to choose between two stimuli but also to make a metacognitive comment about its own response. Like a human blindsight subject, the blindsighted macaque can choose accurately between colors, for example. The commentary key allows it to signal whether a chosen stimulus in the occluded visual eld can also be distinguished from a blank trial in the intact eld. Cowey and Stoerig were able to show that macaques could do the rst task but not the second one. In eect, the mon-

key was saying, Yes, I can discriminate behaviorally between the two colors, but I dont really experience the dierence between colored and blank slides. The analogous human case is to perform a successful discrimination task while denying visual qualitative experience of the stimuli. Such results strengthen the case that macaques have conscious visual experiences not unlike ours. In sum, behavioral reports of conscious experience have proved to be quite reliable. Although more direct measures are desirable, reportability provides a useful public criterion for brain studies of consciousness in humans and some animals. Unconscious Events If we are to treat consciousness as a variable, we also need a way to operationally dene the unconscious comparison condition. Operationally, an event can be dened as unconscious if: 1. knowledge of its presence can be veried, even if 2. that knowledge is not claimed to be conscious; 3. and it cannot be voluntarily reported, acted on, or avoided; 4. even under optimal reporting conditions. There is again a reasonable t between this denition and existing scientic practice. The simplest example is the great multitude of memories that are currently unconscious. You may recall this mornings breakfastbut what happened to that memory before it was brought to mind? We believe it was still extant in the nervous system, though not consciously. We know, however, that unconscious memories can inuence other processes without ever coming to mind. If you had orange juice for breakfast today, you may want milk tomorrow, even without bringing todays orange juice to mind. The ob-

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servation that unconscious memories can inuence behavior without becoming conscious goes back to Hermann Ebbinghaus, who noticed that repeatedly memorizing the same word in a list produces improvements in recall, without conscious recall of earlier eorts to memorize the word. Any systematic behavioral change like this, without reportability, can be used as evidence for unconscious processes. Note that both conscious and unconscious processes involve inferences from publicly observable behavior. But although it is easy to infer consciousness from accurate reports of events, inferring unconscious ones is much trickier. Can we really be sure that an unreported event is necessarily unconscious? In some cases, apparently unconscious events may be momentarily conscious, but so quickly or vaguely that we cannot recall them even a few seconds later (e.g., Sperling, chap. 22, this volume). William James understood this problem very well and suggested in response that there may be no unconscious psychological processes at all! (See James 1890/ 1983, Baars 1988.) This is one of those tricky cases wherein the evidence for unconsciousness could retreat ever further and further beyond the grasp of diligent experimenters. Jacoby and Kelley (1992) suggest an attractive answera criterion for unconscious events that does not solve the problem exactly, but which does give a reasonable basis for consensus. Suppose, they suggest, that we ask a subject to consciously avoid reporting certain memories when they are evoked? If people can avoid reporting specic memories on cue, they must have some knowledge of the memory and must be conscious of it. If they cannot suppress a particular memory, it is presumably because they do not consciously know that it is to be avoided. As an example, take Ebbinghauss discovery that repeated words show improved recall even when we are not conscious that they were encountered before (Ebbinghaus 1885/1913). One way to test this unconscious savings hypothesis is to ask subjects to avoid saying repeated material.

If they cannot avoid repeating previously seen words, they were plausibly unconscious of the dierence between old and new material. This may not be the ultimate solution; the Jacoby and Kelley criterion only taps into what might be called functional consciousnessthe ability to act on, report, and avoid reporting a eeting mental event. But it does provide an empirical standard for separating conscious from other mental events. This may be the best we can do for the time being. In due course, improved brain measures may bring us a step closer. Fringe Conscious Events There is an interesting class of phenomena that is neither quite conscious nor unconscious, but that is nevertheless central to normal mental functioning. William James believed that such fringe conscious events were at least as important as focal conscious experiences. Fringe events include feelings of rightness, beauty, coherence, anomaly, familiarity, attraction, repulsion, and so on. Fringe states seem to be very useful. There is evidence that they are involved in accurate decision-making, predict resolution of tip-of-the-tongue states, and give a sense of availability of a memory even before it comes to mind (Mangan 1993; chap. 45, this volume). When people experience a melody as beautiful they may be quite condent of their judgment. But is the experience of beauty speciable in detail, like the sight of a red plastic toothbrush? Surely not. The combination of high condence and low experienced detail denes a fringe conscious state. Mangan (1993) has developed Jamess ideas about fringe consciousness in modern terms, suggesting that fringe phenomena may not be subject to the classical capacity limitations of consciousness. As we listen to a song, we can feel moved by it, know that it is familiar, and have a sense of rightness and t, seemingly at the same instant in time. Given that focal conscious capacity is notoriously limited to one

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consistent event at any moment, Mangan sees fringe experience as a means of circumventing that limitation. The fringe may be, in Mangans terms, a radical condensation of unconscious information in near-consciousness. Research on fringe consciousness is still in its early stages. We can, however, suggest a useful operational denition for fringe conscious eventsfor instance, the feeling of familiarity created by a well-known song or cliche. The fringe experience of familiarity: 1. is reported with veriable accuracy and high condence, 2. and can be voluntarily acted on, 3. but is not reported to have dierentiated conscious contents, 4. even under optimal reporting conditions. Note that the ability to report conscious contents as conscious (3) dierentiates these fringe criteria from those for focal conscious contents, as well as unconscious events, as described above. But Is It Really Consciousness? The Question of Subjectivity Although reportability is the best operational measure available today, we cannot forget that it is only a behavioral index to the entire rich world of subjective experience. Many philosophers and scientists have pointed out that it is subjective experience that constitutes the core of the issue. As David Chalmers has recently written:We can say that a being is conscious if there is something it is like to be that being, to use a phrase made famous by the philosopher Thomas Nagel. Similarly, a mental state is conscious if there is something it is like to be in that mental state. To put it another way, we can say that a mental state is conscious if it has a qualitative feelan associated quality of experience. Those qualitative feels are also known as phenomenal qualities, or qualia for short. The problem of explain-

ing these phenomenal qualities is just the problem of explaining consciousness. This is the really hard part of the mind-body problem. (Chalmers 1996, p. 4).

Philosophers like Thomas Nagel, Ned Block, and David Chalmers have argued that scientists can address some aspects of consciousness, but that subjectivity, the experience of redness or the grittiness of wet sand, may be inherently beyond scientic study (Nagel 1974, Block 1995). Notice that the stated denition of subjectivity does not treat consciousness as a variable. It claims that subjectivity is incommensurable with any comparison condition, except perhaps the subjective experience of others. Yet when we treat consciousness as a variable, subjectivity is necessarily included. After all, there is no serious question that all the events on the left side of table 1.1 are experienced subjectively by human beings. We can point out, therefore, that science routinely makes use of subjectivity as a source of information. Consider the following example. If you, the reader, focus xedly on a single letter on this page from about 12 inches away, you may be conscious of neighboring letters within a few degrees of visual arc of your xation point, but of no letters in your visual periphery, although we know the peripheral eld needs to process printed words in order to aim accurate eye movements in reading. There is no question that your experience of the focal contents of vision is indeed a genuine subjective experience. But it also has natural unconscious comparison conditions in the visual periphery, outside the visual focus. For another example, you were very probably unaware of the nine alternative meanings of the word focus in the previous sentence. Yet there is good evidence that some additional meanings of ambiguous words tend to be processed unconsciously in normal reading. It therefore makes sense to compare conscious and unconscious meaning representations of the same word. This comparison involves genuine subjectivity on the conscious side, but it also

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enables us to study the entire dimension empirically. From this perspective there is no conict between the deep philosophical questions about subjectivity, and standard scientic practice. We are addressing the same issue. In cognitive neuroscience we always supplement subjective reports with objectively veriable methods. For scientic purposes we prefer to use public reports of conscious experiences. But there is generally such a close correlation between objective reports and the subjective experiences they refer to, that for all intents and purposes we can talk of phenomenology, of consciousness as experienced. Thus in modern science we are practicing a kind of veriable phenomenology. The strategy of treating consciousness as a variable provides a useful empirical basis. No longer are we exclusively dependent on plausible intuitions, thought experiments, or rhetorical brilliance, the bread and butter of traditional thought. We can actually test hypotheses, and the results have a plausible bearing on longstanding questions of consciousness. Accurate reports of conscious experiences are used every day in scientic studies of sensory perception and have been for almost two centuries. The results have been wholly reliable and cumulative, just like other scientic eorts, and they often converge well with our rapidly increasing understanding of the brain. Surely there is something important in the fact that you and I can be conscious of the words in front of us right now and that we can come to substantial agreement on those experiences. In many tasks, this human rule of thumb seems to work for science as well. Limits of Consciousness Reports Sometimes reports of subjective experiences are remarkably fruitful, but sometimes they lead to predictable failure. Nisbett and Wilson (1977)

have pointed out that introspective reports are not very accurate in nding out why people make decisions. Human beings often misattribute their reasons for doing things. Yet such errors rarely occur in sensory perception, in reporting inner speech during problem solving, or in vivid mental imagery. Reports of those inner and outer senses yield extremely useful data (Ericsson and Simon 1984/1993, Farah 1989, Kosslyn 1980). Scientically this is quite normal. All operational indices have their limits. Mercury thermometers are useless for measuring the heat of the stars, and yardsticks do not help to measure the altitude of clouds. Part of the job of science is to specify such measurement limits. Under the proper conditions, objective indices of conscious events often t our own experience. This is exactly what we would expect of an empirical construct, a convergence between objective and subjective evidence when conditions are optimal. That is why perception researchers often use their own experience to understand objective experiments. Sometimes we can even serve as our own formal subjects, in randomly controlled psychophysical studies, for example. That can work very well; but in other experiments, being ones own subject guarantees bad results. Whether our own experience is a useful guide is an empirical question. The frequent convergence between subjective experience and objective measures raises questions about the behavioristic taboo against taking our own conscious experiences into account. We could even turn behavioristic skepticism about consciousness on its headwe could ask, By what scientic authority do we know that our conscious experience is useless at all times? Who is it that laid down the law against considering our own reliable, conscious experiences of color, texture, visual images, inner speech, and the like? And of course there is no such authority. The taboo against using ones own experience seems to come from the methods of physics

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and biology, reinforced by a debatable critique of introspectionism about 1900 (see Blumenthal 1979). In ordinary science, such issues are treated as purely pragmatic. Nothing I have claimed here proves that subjectivity is understandable. It is an open question that has inspired scientists since Fechner and Helmholtz. Subjectivity may forever be unknowable. But it seems more sensible to go on asking testable questions than to speculate about impassable barriers. If empirical investigation runs into a solid wall, well know it very quickly. Summary Contrary to past beliefs, many aspects of consciousness are not untestable at all, as shown by productive research traditions on topics like attention, perception, psychophysics, problemsolving, thought monitoring, imagery, dream research, and so on. All of these eorts meet the most widely used operational criterion of conscious experiencenamely, veriable report of some event described as conscious by an observer. The key, I would suggest, is to study consciousness as a variable, by seeing whether it is a dierence that makes a dierence. But do the results tell us about real consciousness? Could it be just a behavioral response, without subjectivity? In fact, most objective reports correspond well to our own experience. Investigations into conscious processes like sensation continue to cumulate well after two centuries, which suggests that they have not yet run into some insurmountable barrier. I suggest that consciousness should be treated like any other fundamental scientic question. Acknowledgment With thanks to The Neurosciences Institute and The Neurosciences Research Foundation.

ReferencesBaars, B. J. (1986) The cognitive revolution in psychology. New York: Guilford Press. Baars, B. J. (1988) A cognitive theory of consciousness. New York: Cambridge University Press. Second edition in press with MIT Press, Cambridge, Mass. Baars, B. J. (1997) In the theater of consciousness: The workspace of the mind. Oxford: Oxford University Press. Baars, B. J. (2002) The conscious access hypothesis: origins and recent evidence. Trends in Cognitive Sciences, 6 (1): 4752. Block, N. (1995) On a confusion about the function of consciousness. Behavioral and Brain Sciences, 18, 227 287. Blumenthal, A. L. (1979) Wilhelm Wundt, the founding father we never knew. Contemporary Psychology, 24 (7), 547550. Boring, E. G. (1933) The physical dimensions of consciousness. New York: Century. Boring, E. G. (1950) A history of experimental psychology. New York: Appleton-Century-Crofts. Chalmers, D. (1996) The conscious mind: In search of a fundamental theory. Oxford: Oxford University Press. Cowey, A., and P. Stoerig. (1995) Blindsight in monkeys. Nature, 373 (6511): 247249. Crick, F. (1984) Function of the thalamic reticular complex: The searchlight hypothesis. Proceedings of the National Academy of Sciences USA, 81: 45864590. Dennett, D. C. (1992) Consciousness explained. New York: Basic Books. Ebbinghaus, H. (1885/1913) Memory: A contribution to experimental psychology. (H. A. Ruger and C. E. Bussenius, trans.) New York: Teachers College. Edelman, G. (1989) The remembered present: A biological theory of consciousness. New York: Basic Books. Jacoby, L. L., and C. M. Kelley. (1992) A processdissociation framework for investigating unconscious inuences: Freudian slips, projective tests, subliminal perception, and signal detection theory. Current Directions in Psychological Science, 1 (6): 174179. James, W. (1890/1983) The principles of psychology. New York: Holt. Republished by Harvard University Press, Cambridge, Mass.

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Leopold, D. A., and N. K. Logothetis. (1996) Activity changes in early visual cortex reect monkeys percepts during binocular rivalry. Nature, 379: 549553. Mangan, B. (1993) Taking phenomenology seriously: The fringe and its implications for cognitive research. Consciousness and Cognition, 2 (2): 89108. Nagel, T. (1974) What is it like to be a bat? Philosophical Review, 4, 435450. Nisbett, R. E., and T. D. Wilson. (1977) Telling more than we can know: Verbal reports on mental processes. Psychological Review, 84, 231259. Sperling, G. (1960) The information available in brief visual presentations. Psychological Monographs, 74 (498): 129. Watson, J. B. (1913) Psychology as the behaviorist sees it. Psychological Review, 20: 158177. Weiskrantz, L. (1986) Blindsight: A case study and implications. Oxford: Clarendon Press.

I

OVERVIEW

A classic paper by George Mandler states the problem of consciousness with traditional caution: It is respectable, useful, and probably necessary. Specically,I hope to show that consciousness is respectable in the sense that it has become the object of serious and impressive experimental research; it is useful because it avoids circumlocutions as well as [articial] constructions . . . that are more easily addressed by an appeal to consciousness as part of the apparatus of cognition; and it is probably necessary because it serves to tie together many disparate but obviously related mental concepts. . . .

cortex, the rst projection area V1 may not be the locus of visual consciousness, although damage to V1 does abolish visual experience (see Weiskrantz, chap. 9). Yet perhaps their most important contribution has been simply to encourage scientists to take a serious look at a formerly taboo problem.

Mandler speaks as a cognitive scientist emerging from a strong neobehavioristic tradition. He speaks in part to undo the harm that consciousness suered during 50 years (approximately 1910 to 1960) in the oubliettes of behaviorism. It is additionally needed because so many of us have a history of collaboration with the keepers of the jail and to speak freely of the need for a concept of consciousness still ties the tongues of not a few cognitive psychologists. Some decades later that is still true, but it is clearly changing. Crick and Koch (1998) provide a broad introduction to several recent methods and results. Although their article is addressed to neuroscientists, it is of interest to a wider audience. Francis Crick has indeed been one of the great proponents of neuroscientic research into consciousness for many years. Crick and Koch have proposed several inuential hypotheses (see Crick 1984). One of them is the notion that sensory consciousness must involve binding of widely separated sensory feature neurons for color, motion, shading, and so forth, into a single coherent representation, perhaps involving temporally coordinated oscillations (see Singer and Gray, chap. 64). Another is that in visualReferences for each section introduction do not include chapters in this volume.

2

Consciousness: Respectable, Useful, and Probably Necessary

George Mandler I welcome this opportunity to act as amicus curiae on behalf of one of the central concepts of cognitive theoryconsciousness. Another statement, however imperfect, may be useful to undo the harm that consciousness suered during fty years (approximately 1910 to 1960) in the oubliettes of behaviorism. It is additionally needed because so many of us have a history of collaboration with the keepers of the jail and to speak freely of the need for a concept of consciousness still ties the tongues of not a few cognitive psychologists. I hope to show that consciousness is respectable in the sense that it has become the object of serious and impressive experimental research; it is useful because it avoids circumlocutions as well as constructions, such as short-term memory and focal attention, that are more easily addressed by an appeal to consciousness as part of the apparatus of cognition; and it is probably necessary because it serves to tie together many disparate but obviously related mental concepts, including attention, perceptual elaboration, and limited capacity notions. The Revival of Consciousness The history of consciousness is strewn with philosophical, theological, and pedestrian semantic debris; the history of unconscious concepts has, by inherited contrast, suered similarly. Having made the decision to recall the concept of consciousness to service, it is useful to start baldly with the distinction between conscious and unconscious processes. I do so with some sense of `-vis the contributions of embarrassment vis-a others. Freud, in particular, has contributed much to the ner distinctions among shadings of the unconscious. However, if we are to make a fresh start within the experimental investigation of consciousness, we shall probably also have to rediscover these distinctions within the new realm of discourse. For present purposes the distinction among preconscious, preattentitive, primary processes, and unconscious are premature. It suces to distinguish those processes that are accessible to consciousness and those that are not. We shall note that Neisser, for example, assumes that the product of preattentive (preconscious) processes are holistic, vague, and unelaborated. It is not at all certain that current research on reading and language production and comprehension will bear out this assumption. Miller (1962), following Freud, set a distinct boundary between preconscious and unconscious processes, implying that the latter are inaccessible. The evidence suggests that accessibility of unconscious processes shades from the readily accessible to the inaccessible. In what follows I shall have repeated occasions to use the various dierent terms of consciousness as they occur in context. However, the intent, from my vantage point, isat this timeto distinguish only between the contents of consciousness and unconscious processes. The latter include these that are no available to conscious experience, be they feature analyzers, deep syntactic structures, aective appraisals, computational processes, language production systems, action system of many kinds, or whatever. Much of what we know and say today about consciousness has been known and said by others before us, in the past hundred years by Wundt, the Wu rzburgers, Lashley, and many others. I only want to summarize here the high points of a modern view of consciousness as it has developed, or rather revived, during the development of a disciplined and highly structured new view of cognitive psychology. The development of this viewpoint was tentative, as one would expect it to be against the background of the established dogma of behaviorism in the United States. In 1962, George A.

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Miller, one of the prophets of the new mentalism of the 1970s, started o his discussion of consciousness by suggesting that we ban the word [consciousness] for a decade or two until we can develop more precise terms for the several uses which consciousness now obscures (p. 25). More than a decade has passed and we seem to be doing as prescribed without any intervening banishment. Most current thought on the topic was, as a matter of fact, well summarized by Miller. Following William James, Miller stressed the selective functions of consciousnessthe notion that only some part of all the possible experiences that are available at any point in time and space is selected for conscious expression. Miller also noted what we will stress again later, namely that the selective function of consciousness and the limited span of attention are complementary ways of talking about the same thing (p. 49). And, with Lashley, he reminds us that [it] is the result of thinking, not the process of thinking that appears spontaneously in consciousness (p. 56). It is well to keep that last statement in mind, because it is an important part of the new mentalism, of modern cognitive psychology, of the human information processing approach. Thinking or cognition or information processing for the psychologist is a term that refers to theoretical processes, complex transformations on internal and external objects, events, and relations. These processes are not conscious; they are, in the rst instance, constructions generated by the psychological theorist. By denition the conscious individual cannot be consciousin any acceptable sense of the termof theoretical processes involved to explain his actions. In the same sense, the term mind refers to the totality of theoretical processes that are ascribed to the individual. To accept this point of view avoids the solipsisms and sophisms of philosophies of mind. The important advances in our excursions into consciousness must come through the usual in-

terplay of empirical investigation and imaginative theory. The functions of consciousness are slowly being investigated and the beginnings of theoretical integrations of the concept of consciousness into cognitive models are emerging. As a prolegomena to theory and better understanding of private knowledge, Natsoulas (1970) has examined the content of introspective awareness. Although he does not present us with any conclusions, Natsoulas has provided a partial list of the problems that psychologists and philosophers encounter when they want to deal directly with these contents. We have, in general, not gone far beyond such a listing since it is obviously too early to argue for a specic model of private experience or consciousnessit does not exist, not even in the broadest outline. However, some of the necessary rst steps have been taken to build some of the components that such a theory must accommodate. At the same time, psychologists are becoming sensitive to the need for a critical evaluation of common sense and philosophical notions about consciousness. Many phenomenologically oriented philosophers and psychologists are still wedded to a Wundtian idea that psychology should be the study of conscious mental events, whereas unconscious (mental) mechanisms are to be left to some other world such as physiology. In modern theory, one of the most inuential books, Neissers Cognitive psychology (1967), is strangely circumspect about the problem of consciousness. Was it too early then to talk openly about the Imperial Psychologys clothes? Not that Neisser avoids the subjecthe clearly talks about consciousness though one comes upon it in circuitous ways. In his nal chapter Neisser tackles the relationship between iconic memory and consciousness; he comes to consciousness via the attentive processes in visual perception and thence to memory. He notes that the constructive processes in memory themselves never appear in consciousness, their products do (Neisser 1967, p. 301). And in rational problem solving the executive processes share

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many of the properties of focal attention in vision and of analysis-by-synthesis in hearing (p. 302). Noting the distinction between primary and secondary processes (see also Garner 1974), he asserts that rational and therefore presumably conscious thought operates as a secondary processelaborating often unconscious, probably unlearned primary process operations in the Freudian sense. The products of the primary process alone, preceding consciousness and attention, are only eetingly conscious unless elaborated by secondary processes. By implication the elaboration by secondary processes is what produces fully conscious events. By tentative implication primary processes are like preattentive processes in vision and hearing, the conscious processes are like focal attention. The secondary processes elaborate and select. We shall hear similar arguments from Posner and Shallice shortly. However, Neissers contribution to the study of consciousness is in his discussion of preattentive processes and focal attention. When we turn to these processes, the clues from the nal chapter open up a major contribution to the theory of consciousness. I apologize for the talmudic exegesis before coming to this point, but it does, I believe, illustrate the gingerly and skitterish way in which psychologists, up until very recently, have permitted themselves to talk about consciousness. In chapter 4, Neisser starts by taking the term focal attention straight from Schachtel (1959), a psychoanalyst whose history has not prevented a frank discussion of these forbidden topics. If you will, in what follows, permit the free translation of attention into consciousness, you will note why Neissers contribution is important. First, attention . . . is an allotment of analysing mechanisms to a limited region of the eld (p. 88). In other words, consciousness is a limited capacity mechanism. On the other hand, preattentive processes, that is, processes that are not in attention but precede it, have the function of forming the objects of attention. Some of these preattentive processes (the primary pro-

cesses of the nal chapter) are innate. Many actions are under such preattentive control. Walking, driving, and many others are made without the use of focal attention (p. 92). There are processes that run o outside of consciousness (unconsciously) while others do not: More permanent storage of information requires an act of attention (p. 93). Transfer to permanent storage requires consciousness. As apparently do some decisional processes. Harking back to another ear, the processes of attentive synthesis often lead to an internal verbalization (p. 103). We often talk about the contents of consciousness. In brief summary, then, Neissers interpretation is in concord with much modern speculation about the role of consciousness. It is a limitedcapacity mechanism, often synonymous with the notion of attention. The processes that make up consciousness are secondary processes, secondary in elaboration and time to primary, preattentive processes that are unconscious, sometimes innate and often the result of automatization. Consciousness is an area that permits decision processes of some types to operate, where the outputs from dierent systems may be integrated, and where transfers to long term storage systems take place.1 Among the important attempts specically to incorporate awareness notions into contemporary cognitive theory, Shallice (1972) has argued for the necessity of studying phenomena of consciousness if for no other reasons than that there exist a number of concepts in current psychological theory that require the implicit or explicit postulation of some consciousness mechanisms. Among these are the postulation of conscious rehearsal in primary memory and the frequent equation of attention and consciousness (see, e.g., Mandler 1974); others are methodological, as in experiments which require subjects to monitor private experiences (e.g., Sperling 1967). In his theoretical development Shallice argues for an isomorphism between phenomenal experience and information-processing concepts.

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Specically he develops in some detail the notion that the content of consciousness can be identied with a selector input that determines rst what particular action system will become dominant, and, second, sets the goal for the action system. Another approach, derived from problems of attention, has been mounted by Posner and his associates. They have focused on those mental operations which are characterized by interference eects or in other words, by a limited capacity mechanism which may be related to the subjective experience of the unity of consciousness (Posner and Keele 1970). By studying the processes that interfere one with another and showing how limited capacity is assigned to different functions, one can connect the operations of this limited capacity mechanism to intention, awareness, storage and other traditional functions of consciousness (Posner and Keele 1970). Both Shallice and Posner deny any attempt to specify a mechanism of consciousness or private experience that is coextensive with common language uses of the concept. The attempt is, so to speak, from the bottom up, trying to specify with some rigor some of the mechanisms that may be isomorphic with consciousness and learn more about their operation within theoretical systems. Posner and Boise (1971) in their ingenious study of the components of attention have noted that attention in the sense of central processing capacity is related to mental operations of which we are conscious, such as rehearsing or choosing a response . . . (p. 407). Conversely, they noted that the contact between input and long-term memory is not part of this attentional (conscious) process, and in fact, the conscious component of the processing mechanism occurs, as a result, rather late in the sequence of attentional events. Posner and Klein (1973) have summarized this interpretation of the use of consciousness within the context of experimental investigations. They suggest that it refers to operations

such as rehearsal or priming that require access to a limited capacity system. Although the conscious processes are usually late in the processing sequence and follow habitual or preattentive encoding, they are exible and may occur early under time pressure. Also, in a bridge to other similar views these processes are seen as setting up required responses without depending upon the actual release of the motor program (Posner and Klein 1973, p. 34).2 The recurrent theme of readiness for and choices among actions has also been invoked by Festinger et al. (1967). They have suggested that, in part, the conscious aspect of some perceptual phenomena depends on preprogrammed sets of eerent (action) programs that a particular input puts into a state of readiness for immediate use. The limited evidence from the experimental studies as well as from informed theory points to extensive preconscious processing which is, under some circumstances, followed by conscious processing. Much of behavior, however, is automatic and does not require conscious attention. Typically such actions are called habitual, automatic, or preprogrammed. Typically also they tend to be ballistic in form and run o with little variation. It is far beyond the scope of this paper to discuss the problem of automaticity and the kinds of structures that occur automatically in contrast with those that require attentional, conscious work. In reference to response sequencesor actions systems as I prefer to call them nowI have discussed the notion that cognitive structures or symbolic analogues of discrete actions systems develop during overlearning (Mandler 1962). It is well known in the integration that occurs during skill learning, for example, driving. I allude to the probem here simply to note that much of learned behavior and actions can be integrated into new central structures, which then become functional units represented cognitively as single chunks and manipulable consciously in the constructions of new plans and new actions. At the action side of consciousness

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relatively little work has been done to describe how the limited capacity system is used in integrating representation of overt actions into larger units. Just as consciousness deals with chunks of incoming information, so must it deal with chunks of eerent actions. In the arena of perceptual events the topic of automatic processing or encoding confronts us repeatedly as the converse of conscious processing. Posner and Warren (1972) have discussed the variety of such automatic processes involved in coding mechanisms. Generally what is automatic is very much like what Neisser calls preattentive, parallel processes. In contrast, Posner and Warren note that conscious processes are more variable and that conscious constructions provide the new mnemonic devices needed to store material in long-term representations. It is here that we face the insistence that new encodings for long-term storage depend on a functioning conscious system. LaBerge (1974) has addressed the issue of automatization in a novel wayasking not only about the relation between automatization and attention, but also about the process whereby certain coding systems become automatized. He concludes that during postcriteria performance there occurs a gradual withdrawal of attention from the particular components of a task. Under this process of decreasing attention (consciousness?) the part of the processing involved in coding the particular perceptual material is being made automatic. Eventually much of the perceptual processing can be carried out automatically, that is prior to the focussing of attention on the processing. LaBerge also implies that this postcriterial, overlearning process produces the integration of new, higher order units or chunks. He is obviously addressing the development of preattentive processes, and also the functional unity and autonomy of these units, particularly when he notes that often one cannot prevent the processing once it starts. There is a useful similarity between these propositions about the development of automatic encodings

and my previous discussion on the representation of overlearned action patterns. We have here a possible distinction between two kinds of unconscious (preattentive, primary) processes. Those that are innate or preprogrammed or dependent primarily on some structural characteristics of the organism; those that, though initially conscious, become, by some process such as overlearning, automatic and unconscious. Although the latter are easily brought into consciousness it is also intuitively likely that this might be dicult, if not impossible, for the former. It seems to be agreed that consciousness is a clearing house that enters into the ow of processing under certain speciable, but at present still not specied, conditions. Certain processes operate on conscious information generaged by nonconscious (preattentive) processes. However, it bears repeating that there are important nonconscious postattentive processes that are operative subsequent to information generated by conscious processes. Many of these involve actions (which my older colleagues in psychology have inadequately called behavior) that operate often without conscious attention. These actions and their representation systems are as complex and as nely structured as the preattentive perceptual systems. Unfortunately, partly by accidents of history, cognitive psychologists tend to be somewhat careless about specifying how organisms come to act. However, the hierarchic, structural view of action has an honorable and creative history. I would draw your attention to the most recent and important exposition of this view proposed by Gallistel (1974). In summary, current thought has concentrated on the consciousness of the perceptual or encoding side of information ow. Some attention has been given to its functions in memory storage and retrieval. Much is still to be done at the output side. Current notions have focussed on the functions of consciousness as selecting encoded sensory information and preparing choices among appropriate action or response

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alternatives. Many other functions of consciousness still await detailed analyses or may be incorrect assignments to this particular system. Conscious Contents and Processes My own interest in consciousness arose in part out of a long-term project on the relation between mind and emotion, and more immediately out of some recent considerations of the limits of attention and consciousness (Mandler 1974). In that presentation I argued for a direct translation between focal attention and consciousness. I suggested that some of the so-called short-term memory phenomena are best assigned to the limited capacity mechanism of consciousness and that the limitations of that single conscious system in terms of dimensional analyses may serve as a bridging concept for George A. Millers puzzle about the similar limitations he noted for both short term memory and absolute judgment. The limited capacity of short-term memory, the immediate memory span, as well as the limitation in absolute judgment task to some seven values or categories, can usefully be assigned to the limited capacity of conscious content. The limitation refers to the limited number of values on any single dimension (be it physical, acoustic, semantic, or whatever) that can be kept within the conscious eld. The main points of the argument relevant to the present topic concern certain distinctions among the concepts of attention, consciousness and short-term memory. In the rst instance, I want to restrict the concept of consciousness to events and operation within a limited capacity system, with the limitation referring to number of functional units or chunks that can be kept in consciousness at any one point in time. This concept has much in common with what has been called focal attention. Attentional processes are those mechanisms that deal with the selection of objects or events that occur in conscious-

ness. Second, I want to assert a distinction between short-term memory and consciousness. The limited reach of consciousness has a respectable history, going back at least 200 years (see Mandler 1974). However, it is not a memory systemit does not involve any retrieval. What is in the momentary eld of consciousness is not remembered, it is psychologically present. None of the foregoing denies the utility of the conception of dierent memory systems, whether long term, working, or operational. It is probably most reasonable to consider these dierent systems on a continuum of depth of processing, as proposed by Craik and Lockhart (1972). Dierent types of analyses require dierent processing depths and dierent processing times. But the information that, so to speak, can be read o the contents of consciousness is not memorial as such. Depth of processing determines how and what can be remembered. If processing time is short, or encoding supercial, and if the code decays rapidly, it will be short; if processing is extended or if encoding is complex, information adequate for long term retrieval or reconstruction will be stored. Within certain limits, the storage processesat whatever depthscan only take place on conscious material; conversely, retrieval usually implies retrieval into the conscious eld. However, the memory mechanisms and the contents of consciousness are two very distinct kinds of mental events. Posner, who has contributed much to recent investigations of the structure of the limited capacity system of conscious events, suggested as early as 1967 that operational and shortterm memories should be considered as dier` la ent systems. The operating systems may vary a Craik and Lockhart in their time course and their products, but they should not be confounded with the immediately given content of consciousness. The confounding of these two systems in early investigations and theories is understandable, given the very brief time course of some memory processes and the rapid changes

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in the focus of consciousness (attention). However, within consciousness many dierent kinds of operations may be performedconsciousness is not limited in the complexity of the information it draws on, only in the amount. Consciousness is modality independent, and, depending on the task facing the individual, many involve very complex and abstract operations. In general then, I will prefer to use the term consciousness to focal attention (although they may have to be interchanged) but dierentiate strictly between consciousness and shortterm memory processes, which deal with storage and retrieval (cf. Craik and Lockhart 1972). I do not intend to invoke a separate processing stage or system to accommodate the concept of consciousness. Consciousness, in the rst instance, refers to a state of a structure. Certain operations and processes act on these structures which constitute conscious content. Cognitive structures, or schemas, may, under certain circumstances, become conscious, that is, enter the conscious state; when they do not, they are by denition unconscious. Limited capacity refers to the number of such unitary structures that may be conscious at any one point in time. There is no separate system, however, that contains the conscious contents. Rather conscious structures dier from others in that certain operations, such as storage, retrieval, and choice (see below), may be performed on them. It is beyond the intent of this chapter to discuss the origins of consciousness. It is a characteristic of the organism that certain structures can become conscious, but it is a function of human interactions with the environment that determines which structures do in fact become conscious. Piaget (1953) has discussed extensively these interactions and stressed the transactions among perceptions of the self, the environment, and the development of consciousness. The development of consciousness is not, from my view, some magical burgeoning of internal awareness but rather dependent on specic organism-environment interactions. These

involve, to a large part, the internalizations of actions (see also Mandler 1962). More important, however, the conditions of personal and social development determine what can and what cannot be represented in consciousness. Depending on these conditions, dierent individuals, groups, and cultures will have dierent conscious contentsdierent social and cultural consciousness, dierent realities. However, the primary purpose of the present chapter is to discuss how these conscious contents operate, not what they are or how they might be established. Finally, conscious contents can be spoken about. I shall discuss later the lack of any one-toone correspondence between consciousness and language, but this should not obscure the important relationship between private conscious events and language. It is by the use of the latter that we primarily communicate our own private view of reality, and it is in turn by the use of language that, in the adult at least, many conscious structures are manipulated and changed. With these primarily denitional problems out of the way, I want to address rst some of the ancient and admittedly complex and very special problems that the concept of consciousness poses for any psychological theory. I shall not propose any radical solutions but rather that the problem of the private datum can be approached reasonably and analytically, rather than frantically or mystically. Next, I want to sketch some of the possible uses of consciousness in cognitive theory, followed by some suggestions for the adaptive functions of consciousness. Finally, in the last section, I want to address the broader problem of the ow of consciousness and its relations to limited capacity, with particular attention to special states of consciousness. Consciousness: A Special Problem for Mental Theory The individual experiences feelings, attitudes, thoughts, images, ideas, beliefs, and other contents of consciousness, but these contents are not

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accessible to anyone else. Briey stated, that is the special problem facing psychologists. There are no evasions possible. It is not possible to build a phenomenal psychology that is shared. A theory of phenomena may be shared but the private consciousness, once expressed in words, gestures, or in any way externalized is necessarily a transformation of the private experience. No theory external to the individual, that is, one that treats the organism as the object of observation, description, and explanation, can at the same time be a theory that uses private experiences, feelings, and attitudes as data (see Gray 1971). Events and objects in consciousness can never be available to the observer without having been restructured, reinterpreted, and appropriately modied by structures that are specic to the individual doing the reporting. These structures may even be specic to the kinds of experiences, feelings, and attitudes that are reported. The content of consciousness, as philosophers and psychologists have told us for centuries, is not directly available as a datum in psychology. How then are we to deal with the contents of consciousness? Can the perennial problem of private datum and public inference at least be stated concisely in order to indicate the magnitude of the problem and possible directions for future development? We are faced with a phenomenon that might be called the uncertainty principle of psychology. Adrian (1966) for example, noted: The particular diculty that the questioner may inuence the answer recalls the uncertainty principle in physics, which limits the knowledge we can gain about any individual particle (p. 242). There are two related problems in the study of consciousness. The rst is more fundamental than the question that Adrian addresses. It is not only the case that the nature of the interrogation may aect the reported content of consciousness, but, more basically, the act of examination itself may aect the individually observable conscious contents. This conjecture is reasonable even at the level of processing capacity, since the con-

scious act of interrogating ones conscious content must occupy some part of the limited capacity. As a result the available content is altered by the process of interrogation. Given that the act of interrogation changes the content of consciousness, the source of that inquiry becomes of secondary importance. The second problem to be faced is the fact that the contents of consciousness are not simply reproducible by some one-to-one mapping into verbal report. Even if these contents were always couched in language, which they surely are not, some theory of transmission would be required. As a result we are faced with the individuals observation of the contents of his consciousness on the one hand, and on the other with the psychologists theoretical inference about those contents, based on whatever data, including introspective reports, are available. Both of these knowledges may be used as relevant to the construction of a psychology of cognition, though it may in principle be impossible to determine, in any exact sense, the relation between these two interpretations of consciousness. Private experiences are important aspects of the fully functioning mental system. It is possible to get transformed reports about those events and it should be possible to develop appropriate theories that relate contents of consciousness, their transformations, and their report. However, it is not possible to build a theory that makes direct predictions about private experience since the outcome of those predictions cannot be inspected by the psychologist/observer. This position does admit the development of private theories, by the individualabout himself. To the individual his experience is a datum, and as a consequence his theories about his own structures are, within limits, testable by direct experience. These individual, personal theories of the self are both pervasive and signicant in explaining human action, but they cannot, without peril, be generalized to others or to the race as a whole (see Mandler and Mandler 1974).

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We note, therefore, that peoples reports about their experiences, their behavior, and their actions are very frequently, and may always be, ctions or theories about those events. However, it is only those reports that are available to us. Even the introspecting individual who says that his experience conforms to certain predicted aspects is making statements about derived correspondences resulting from mental transformations. Indirect scientic predictions about experiences are possible, but the test of those predictions is one step removed from the actual experiences, as are all predictions about the values of theoretical entities. If the behaviorist revolution, with all the negative inuences it has had on the development of a fully theoretical psychology, has had one positive eect, it is this realization that even the complete acceptance of the importance of private experience does not thereby make it a possible end point for a scientic theory. Some Uses of Consciousness One of the important processes in which consciousness intervenes is in the testing of potential action choices and the appraisal of the situational givens. The relation between choice and consciousness, as we have seen, has motivated much of the recent research. The analysis of situations and appraisal of the environment, on the other hand, goes on mainly at the nonconscious level, which will be discussed in greater detail later. In any case, the outcomes of these analyses may be available in consciousness and the eect of potential actions on the present situation can be estimated and evaluated. Consciousness is a eld in which potential choices are given the opportunity to be evaluated against potential outcomes. This delay produces ree