wider than the sky: the phenomenal gift of …...two subtle but important issues strongly inﬂuence...

G E R A L D M . E D E L M A N

the phenomenal gift of consciousness

y a l e n o t a b e n e

yale university press new haven and london

Contents

Preface xi

Acknowledgments xv

1. The Mind of Man: Completing Darwin’sProgram 1

2. Consciousness: The Remembered Present 4

3. Elements of the Brain 14

4. Neural Darwinism: A Global Brain Theory 32

5. The Mechanisms of Consciousness 48

6. Wider Than the Sky: Qualia, Unity, andComplexity 60

7. Consciousness and Causation: ThePhenomenal Transform 76

8. The Conscious and the Nonconscious:Automaticity and Attention 87

9. Higher-Order Consciousness andRepresentation 97

10. Theory and the Properties of Consciousness 113

11. Identity: The Self, Mortality, and Value 131

12. Mind and Body: Some Consequences 140

Glossary 149

Bibliographic Note 181

Index 187

c o n t e n t sx

The Mind of ManC O M P L E T I N G D A R W I N ’ S P R O G R A M

n 1869, Charles Darwin found himself vexedwith his friend Alfred Wallace, the co-founder of thetheory of evolution. They had differed on several issuesrelated to that theory. But the main reason for Darwin’sdisturbance was a publication by Wallace concerningthe origin of the brain and mind of man. Wallace, whoby that time had spiritualist leanings, concluded thatnatural selection could not account for the human mindand brain.

Darwin wrote to him before publication: “I hopeyou have not murdered too completely your own and

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my child,” meaning, of course, natural selection. Wal-lace, in fact, concluded that natural selection could notexplain the origin of our higher intellectual and moralfaculties. He claimed that savages and prehistoric hu-mans had brains almost as large as those of Englishmenbut, in adapting to an environment that did not requireabstract thought, they had no use for such structuresand therefore their brains could not have resulted fromnatural selection. Unlike Wallace, Darwin understoodthat such an adaptationist view, resting only on naturalselection, was not cogent. He understood that propertiesand attributes not necessarily needed at one time couldnevertheless be incorporated during the selection ofother evolutionary traits. Moreover, he did not believethat mental faculties were independent of one another.As he explained in his book The Descent of Man, forexample, the development of language might have con-tributed to the process of brain development.

This rich work has prevailed, along with Darwin’sother views, but the program he established remains tobe completed. One of the key tasks in completing thatprogram is to develop a view of consciousness as a prod-uct of evolution rather than as a Cartesian substance, orres cogitans, a substance not accessible to scientific analy-sis. A major goal of this book is to develop such a view.

What is required to carry out such a project? Beforeanswering this question, let us consider Darwin’s entryin his notebook of 1838: “Origin of man now proved—metaphysic must flourish—He who understands ba-

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boon will do more towards metaphysics than Locke.”These statements point in the direction we must follow.We must have a biological theory of consciousness andprovide supporting evidence for that theory. The theorymust show how the neural bases for consciousness couldhave arisen during evolution and how consciousness de-velops in certain animals.

Two subtle but important issues strongly influenceour interpretation of these requirements. The first ofthese is the question of the causal status of conscious-ness. Some take the view that consciousness is a mereepiphenomenon with no material consequences. A con-trary view is that consciousness is efficacious—that itcauses things to happen. We will take the position,which we shall explore in detail later, that it suffices toshow that the neural bases of consciousness, not con-sciousness itself, can cause things to happen. The secondmajor challenge to any scientific account of conscious-ness is to show how a neural mechanism entails a subjec-tive conscious state, or quale, as it is called. Before wecan meet these two challenges, it is necessary to providea sketch of the properties of consciousness and considersome matters of brain structure and function.

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ConsciousnessT H E R E M E M B E R E D P R E S E N T

e all know what consciousness is : it is whatyou lose when you fall into a deep dreamless sleep andwhat you regain when you wake up. But this glib state-ment does not leave us in a comfortable position to ex-amine consciousness scientifically. For that we need toexplore the salient properties of consciousness in moredetail, as William James did in his Principles of Psychol-ogy. Before doing so, it will help to clarify the subjectif we first point out that consciousness is utterly depen-dent on the brain. The Greeks and others believed that

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consciousness resided in the heart, an idea that survivesin many of our common metaphors. There is now a vastamount of empirical evidence to support the idea thatconsciousness emerges from the organization and opera-tion of the brain. When brain function is curtailed—in deep anesthesia, after certain forms of brain trauma,after strokes, and in certain limited phases of sleep—consciousness is not present. There is no return of thefunctions of the body and brain after death, and post-mortem experience is simply not possible. Even duringlife there is no scientific evidence for a free-floating spiritor consciousness outside the body: consciousness is em-bodied. The question then becomes: What features ofthe body and brain are necessary and sufficient for con-sciousness to appear? We can best answer that questionby specifying how the properties of conscious experiencecan emerge from properties of the brain.

Before taking up the properties of consciousness inthis chapter, we must address another consequence ofembodiment. This concerns the private or personal na-ture of each person’s conscious experience. Here isJames on the subject :

In this room—this lecture room, say—there area multitude of thoughts, yours and mine, someof which cohere mutually, and some not. Theyare as little each-for-itself and reciprocally inde-pendent as they are all-belonging-together. Theyare neither: no one of them is separate, but each

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belongs with certain others and with none beside.My thought belongs with my other thoughts andyour thought with your other thoughts. Whetheranywhere in the room there be a mere thought,which is nobody’s thought, we have no means ofascertaining, for we have no experience of the like.The only states of consciousness that we naturallydeal with are found in personal consciousness,minds, selves, concrete particular I’s and you’s.

There is no mystery here. Since consciousness ar-rives as a result of each individual’s brain and bodilyfunctions, there can be no direct or collective sharingof that individual’s unique and historical conscious ex-perience. But this does not mean that it is impossibleto isolate the salient features of that experience by obser-vation, experiment, and report.

What is the most important statement one canmake about consciousness from this point of view? It isthat consciousness is a process, not a thing. James madethis point trenchantly in his essay “Does ConsciousnessExist?” To this day, many category errors have beenmade as a result of ignoring this point. For example,there are accounts that attribute consciousness specifi-cally to nerve cells (or “consciousness neurons”) or toparticular layers of the cortical mantle of the brain. Theevidence, as we shall see, reveals that the process of con-sciousness is a dynamic accomplishment of the distrib-uted activities of populations of neurons in many differ-


ent areas of the brain. That an area may be essential ornecessary for consciousness does not mean it is suffi-cient. Furthermore, a given neuron may contribute toconscious activity at one moment and not at the next.

There are a number of other important aspects ofconsciousness as a process that may be called Jamesianproperties. James pointed out that consciousness occursonly in the individual (that is, it is private or subjective),that it appears to be continuous, albeit continuallychanging, that it has intentionality (a term referring tothe fact that, generally, it is about things), and that itdoes not exhaust all aspects of the things or events towhich it refers. This last property has a connection tothe important matter of attention. Attention, particu-larly focal attention, modulates conscious states and di-rects them to some extent, but it is not the same as con-sciousness. I will return to this issue in later chapters.

One outstanding property is that consciousness isunitary or integrated, at least in healthy individuals.When I consider my conscious state at the time of thiswriting, it appears to be all of a piece. While I am payingattention to the act of writing, I am aware of a ray ofsunlight, of a humming sound across the street, of asmall discomfort in my legs at the edge of the chair, andeven of a “fringe,” as James called it, that is of objectsand events barely sensed. It is usually not entirely possi-ble to reduce this integrated scene to just one thing,say my pencil. Yet this unitary scene will change anddifferentiate according to outside stimuli or inner


thoughts to yet another scene. The number of such dif-ferentiated scenes seems endless, yet each is unitary. Thescene is not just wider than the sky, it can contain manydisparate elements—sensations, perceptions, images,memories, thoughts, emotions, aches, pains, vague feel-ings, and so on. Looked at from the inside, conscious-ness seems continually to change, yet at each momentit is all of piece—what I have called “the rememberedpresent”—reflecting the fact that all my past experienceis engaged in forming my integrated awareness of thissingle moment.

This integrated yet differentiated state looks en-tirely different to an outside observer, who possesses hisor her own such states. If an outside observer testswhether I can consciously carry out more than two taskssimultaneously, he will find that my performance deteri-orates. This apparent limitation of conscious capability,which is in contrast to the vast range of different innerconscious states, deserves analysis. I will consider its ori-gins when I discuss the difference between consciousand nonconscious activity.

So far, I have not mentioned a property that is cer-tainly obvious to all humans who are conscious. We areconscious of being conscious. (Indeed, it is just such aform of consciousness that impels the writing of thisbook.) We have scant evidence that other animals pos-sess this ability; only higher primates show signs of it.In the face of this fact, I believe that we need to makea distinction between primary consciousness and higher-


order consciousness. Primary consciousness is the stateof being mentally aware of things in the world, of havingmental images in the present. It is possessed not onlyby humans but also by animals lacking semantic or lin-guistic capabilities whose brain organization is neverthe-less similar to ours. Primary consciousness is not accom-panied by any sense of a socially defined self with aconcept of a past or a future. It exists primarily in theremembered present. In contrast, higher-order con-sciousness involves the ability to be conscious of beingconscious, and it allows the recognition by a thinkingsubject of his or her own acts and affections. It is accom-panied by the ability in the waking state explicitly to re-create past episodes and to form future intentions. At aminimal level, it requires semantic ability, that is, theassignment of meaning to a symbol. In its most devel-oped form, it requires linguistic ability, that is, the mas-tery of a whole system of symbols and a grammar.Higher primates, to some minimal degree, are assumedto have it, and in its most developed form it is distinctiveof humans. Both cases require an internal ability to dealwith tokens or symbols. In any event, an animal withhigher-order consciousness necessarily must also possessprimary consciousness.

There are different levels of consciousness. In rapideye movement (REM) sleep, for example, dreams areconscious states. In contrast with individuals in the wak-ing state, however, the dreaming individual is often gull-ible, is generally not conscious of being conscious, is not


connected to sensory input, and is not capable of motoroutput. In deep or slow-wave sleep, short dreamlike epi-sodes may occur, but for long periods there is no evi-dence of consciousness. In awaking from the uncon-sciousness induced by trauma or anesthesia, there maybe confusion and disorientation. And, of course, theremay be diseases of consciousness, such as schizophrenia,in which hallucinations, delusions, and disorientationcan occur.

In the normal conscious state, individuals experi-ence qualia. The term “quale” refers to the particularexperience of some property—of greenness, for in-stance, or warmth, or painfulness. Much has been madeof the need for providing a theoretical description thatwill allow us directly to comprehend qualia as experi-ences. But given that only a being with an individualbody and brain can experience qualia, this kind of de-scription is not possible. Qualia are high-order discrimi-nations that constitute consciousness. It is essential tounderstand that differences in qualia are based on differ-ences in the wiring and activity of parts of the nervoussystem. It is also valuable to understand that qualia arealways experienced as parts of the unitary and integratedconscious scene. Indeed, all conscious events involve acomplex of qualia. In general, it is not possible to experi-ence only a single quale—“red,” say—in isolation.

I shall elaborate later on the statement that qualiareflect the ability of conscious individuals to make high-order discriminations. How does such an ability reflect


the efficacy of the neural states accompanying consciousexperience? Imagine an animal with primary conscious-ness in the jungle. It hears a low growling noise, and atthe same time the wind shifts and the light begins towane. It quickly runs away, to a safer location. A physi-cist might not be able to detect any necessary causal rela-tion among these events. But to an animal with primaryconsciousness, just such a set of simultaneous eventsmight have accompanied a previous experience, whichincluded the appearance of a tiger. Consciousness al-lowed integration of the present scene with the animal’spast history of conscious experience, and that integra-tion has survival value whether a tiger is present or not.An animal without primary consciousness might havemany of the individual responses that the conscious ani-mal has and might even survive. But, on average, it ismore likely to have lower chances of survival—in thesame environment it is less able than the conscious ani-mal to discriminate and plan in light of previous andpresent events.

In succeeding chapters, I will attempt to explainhow conscious scenes and qualia arise as a result of braindynamics and experience. At the outset, though, it isimportant to understand what a scientific explanationof conscious properties can and cannot do. The issueconcerns the so-called explanatory gap that arises fromthe remarkable differences between brain structure inthe material world and the properties of qualia-ladenexperience. How can the firing of neurons, however


complex, give rise to feelings, qualities, thoughts, andemotions? Some observers consider the two realms sowidely divergent as to be impossible to reconcile. Thekey task of a scientific description of consciousness is togive a causal account of the relationship between thesedomains so that properties in one domain may be un-derstood in terms of events in the other.

What such an explanation cannot and need notdo is offer an explanation that replicates or createsany particular quale or experiential state. Science doesnot do that—indeed, imagine that a gifted scientist,through an understanding of fluid dynamics and meteo-rology, came up with a powerful theory of a complexworld event like a hurricane. Implemented by a sophisti-cated computer model, this theory makes it possible tounderstand how hurricanes arise. Furthermore, with thecomputer model, the scientist could even predict mostof the occurrences and properties of individual hurri-canes. Would a person from a temperate zone withouthurricanes, on hearing and understanding this theory,then expect to experience a hurricane or even get wet?The theory allows one to understand how hurricanesarise or are entailed by certain conditions, but it cannotcreate the experience of hurricanes. In the same way, abrain-based theory of consciousness should give a causalexplanation of its properties but, having done so, itshould not be expected to generate qualia “by descrip-tion.”

To develop an adequate theory of consciousness,


one must comprehend enough of how the brain worksto understand phenomena, such as perception andmemory, that contribute to consciousness. And if thesephenomena can be causally linked, one would hope totest their postulated connections to consciousness by ex-perimental means. This means that one must find theneural correlates of consciousness. Before addressingthese issues, let us turn first to the brain.


Elements of the Brain

he human brain is the most complicated mate-rial object in the known universe. I have already saidthat certain processes within the brain provide the neces-sary mechanisms underlying consciousness. In the pastdecade or so, many of these processes have been identi-fied. Brain scientists have described an extraordinary lay-ering of brain structures at levels ranging from moleculesto neurons (the message-carrying cells of the brain), toentire regions, all affecting behavior. In describing thosefeatures of the brain necessary to our exploration I willnot go into great detail. To provide a foundation for a

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biological theory of consciousness, however, we do needto consider certain basic information on brain structureand dynamics. This excursion will require some patienceon the reader’s part. It will be rewarded when we de-velop a picture of how the brain works.

This short survey on the brain will cover, in order, aglobal description of brain regions, some notion of theirconnectivity, the basics of the activity of neurons andtheir connections—the synapses—and a bit of thechemistry underlying neuronal activity. All this will benecessary to confront a number of critical questions andprinciples: Is the brain a computer? How is it built dur-ing development? How complex are its transactions? Arethere new principles of organization unique to the brainthat were selected during evolution? What parts of thebrain are necessary and sufficient for consciousness toemerge? In addressing these questions, I shall use the hu-man brain as my central reference. There are, of course,many similarities between our brains and those of otheranimal species, and when necessary I shall describe thesesimilarities as well as any significant differences.

The human brain weighs about three pounds. Itsmost prominent feature is the overlying wrinkled andconvoluted structure known as the cerebral cortex,which is plainly visible in pictures of the brain (Figure1). If the cerebral cortex were unfolded (making the gyri,its protrusions, and the sulci, its clefts, disappear) itwould have the size and thickness of a large table napkin.It would contain at least 30 billion neurons, or nerve

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Figure 1. Relative locations of major parts of the humanbrain. The cerebral cortical mantle receives projections

from the thalamus and sends reciprocal projections back;this constitutes the thalamocortical system. Beneath themantle are three major cortical appendages—the basal

ganglia, the hippocampus, and the cerebellum.Below them is the brainstem, evolutionarily the oldest

part of the brain, which contains several diffuselyprojecting value systems.

cells, and 1 million billion connections, or synapses. Ifyou started counting these synapses right now at a rateof one per second, you would just finish counting them32 million years from now.

Neurons are connected to each other locally toform a dense network in portions of the brain calledgray matter; they communicate over longer distances viafiber tracts called white matter. The cortex itself is a six-


layered structure with different connection patterns ineach layer. The cortex is subdivided into regions thatmediate different sensory modalities, such as hearing,touch, and sight. There are other cortical regions dedi-cated to motor functions, the activity of which ulti-mately drives our muscles. Beyond the sensorimotorportions concerned with input and output, there are re-gions such as the frontal, parietal, and temporal corticesthat are connected only to other parts of the brain andnot to the outside world.

Before taking up other portions of the brain, I shallbriefly describe in simplified form the structure andfunction of neurons and synapses. Different neurons canhave a number of shapes, and there may be as many astwo hundred or more different kinds in the brain. Aneuron consists of a cell body with a diameter on theorder of thirty microns, or about one ten-thousandth ofan inch across (Figure 2). Neurons tend to be polar,with a treelike set of extensions called dendrites, and along specialized extension called an axon, which con-nects the neuron to other neurons at synapses. The syn-apse is a specialized region that links the so-called pre-synaptic neuron (the neuron that sends a signal acrossthe synapse) to a postsynaptic neuron (the neuron thatreceives the signal). The presynaptic portion of the syn-apse contains a special set of minute vesicles withinwhich are chemicals known as neurotransmitters. Neu-rons possess an electrical charge as a result of their mem-brane properties, and when a neuron is excited current


Figure 2. A diagram illustrating synaptic connectionsbetween two neurons. An action potential traveling downthe axon of the presynaptic neuron causes the release of aneurotransmitter into the synaptic cleft. The transmitter

molecules bind to receptors in the postsynaptic membrane,changing the probability that the postsynaptic neuronwill fire. (Because of the number of different shapesand kinds of neurons, this drawing is of necessity

a greatly simplified cartoon.)

flows through channels that open across the membrane.As a result, a wave of electrical potential known as anaction potential moves from the cell body down the pre-synaptic axon and causes the release of neurotransmittermolecules from vesicles into the synaptic cleft. These


molecules bind to molecular receptors or channels inthe postsynaptic cell that, acting cumulatively, can causeit to fire an action potential of its own. Thus, neuronalcommunication occurs by a combination of controlledelectrical and chemical events.

Now try to imagine the enormous numbers of neu-rons firing in various areas of the brain. Some firings arecoherent (that is, they are simultaneous), others are not.Different brain regions have different neurotransmittersand chemicals whose properties change the timing, am-plitude, and sequences of neuronal firing. To achieve andmaintain the complex patterns of dynamic activity inhealthy brains, some neurons are inhibitory, suppressingthe firing of others, which are excitatory. Most excitatoryneurons use the substance glutamate as their neurotrans-mitter, while the inhibitory neurons use GABA (gamma-aminobutyric acid). We can ignore the chemical detailsfor now and simply accept that the effects of differentchemical structures are different and that their distribu-tion and occurrence together can have significant effectson neural activity.

I started by describing the cortex. With the pictureof a polar neuron in mind, we can turn briefly to otherkey regions of the brain. One of the most importantanatomical structures for understanding the origin ofconsciousness is the thalamus. This structure, which islocated at the center of the brain, is essential for con-scious function, even though it is only somewhat largerthan the last bone in your own thumb. When nerves


from different sensory receptors serving different modal-ities (located in your eyes, ears, skin, and so on) travelto your brain, they each connect in the thalamus withspecific clusters of neurons called nuclei. Postsynapticneurons in each specific thalamic nucleus then projectaxons that travel and map to particular areas of the cor-tex. A well-studied example is the projection from theneurons of the retina through the optic nerve to the partof the thalamus called the lateral geniculate nucleus andthen to the primary visual cortical area, called V1 (for“visual area 1”).

There is one striking feature of the many connec-tions between the thalamus and the cortex: not onlydoes the cortex receive many axons from thalamic neu-rons but there are also reciprocal axonal fibers goingfrom the cortex back to the thalamus. We speak there-fore of thalamocortical projections and corticothalamicprojections. Reciprocal connections of this type aboundwithin the cortex itself; such reciprocal connections arecalled corticocortical tracts. A striking example of theseis the fiber bundle called the corpus callosum, whichconnects the two cortical hemispheres and consists ofmore than 200 million reciprocal axons. Cutting thecorpus callosum leads to a split-brain syndrome, whichin some cases can lead to the remarkable appearance oftwo separate and very different consciousnesses.

Each specific thalamic nucleus (and there aremany) does not connect directly to any of the others.Surrounding the periphery of the thalamus, however,


there is a layered structure called the reticular nucleus,which connects to the specific nuclei and which can in-hibit their activity. The reticular nucleus, it is suspected,acts to switch or “gate” the activities of the specific tha-lamic nuclei, yielding different patterns of expression ofsuch sensory modalities as sight, hearing, and touch. An-other set of thalamic nuclei called intralaminar nucleireceive connections from certain lower structures in thebrainstem that are concerned with activation of multipleneurons; these then project to many different areas ofthe cortex. The activity of these intralaminar nuclei issuspected to be essential for consciousness in that it setsappropriate thresholds or levels of cortical response—with too high a threshold, consciousness would be lost.

We may now turn to some other brain structuresthat are important to our efforts to track down the neu-ral bases of consciousness. These are large subcorticalregions that include the hippocampus, the basal ganglia,and the cerebellum. The hippocampus is an evolution-arily ancient cortical structure lined up like a pair ofcurled sausages along the inner skirt of the temporal cor-tex, one on the right side and another on the left. Incross section, each sausage looks like a sea horse, hencethe name “hippocampus.” Studies of the neural proper-ties of the hippocampus provide important examples ofsome of the synaptic mechanisms underlying memory.One such mechanism, which should not be equated withmemory itself, is the change in the strength, or efficacy,of hippocampal synapses that occurs with certain pat-


terns of neural stimulation. As a result of this change,which can be either positive for long-term potentiationor negative for long-term depression, certain neuralpathways are dynamically favored over others.

The point to be stressed is that, while synapticchange is essential for the function of memory, memoryis a system property that also depends on specific neuro-anatomical connections.

Increased synaptic strength or efficacy within apathway leads to a higher likelihood of conductionacross that pathway, whereas decreases in synapticstrength diminish that likelihood. Various patterns havebeen found for the so-called synaptic rules governingthese changes, following the initial proposals of DonaldHebb, a psychologist, and Friedrich von Hayek, aneconomist who, as a young man, thought quite a bitabout how the brain works. These scholars suggestedthat an increase in synaptic efficacy would occur whenpre- and postsynaptic neurons fired in close temporalorder. Various modifications of this fundamental rulehave been seen in different parts of the nervous system.What is particularly striking about the hippocampus,where these rules have been studied in detail, is the factthat bilateral removal of this structure leads to a loss ofepisodic memory, the memory of specific episodes orexperiences in life. A very famous patient, H. M., whosehippocampi were removed to cure epileptic seizures,could not, for example, convert his short-term memoryof events into a permanent narrative record, a condition


that was depicted dramatically in the movie Memento.It is believed that such a long-term record results whenparticular synaptic connections between the hippocam-pus and the cortex are strengthened. When these con-nections are severed, the corresponding cortical synapticchanges cannot take place and the ability to rememberepisodes over the long term is lost. Such a patient canremember episodes up to the time of the operation, butloses long-term memory thereafter. It is intriguing thatin some animals, such as rodents, hippocampal functionis necessary for memories of a sense of place. In the ab-sence of hippocampal functions, the animal cannot re-member target places that have been explored.

All of the discussion so far has focused on sensoryor cognitive functioning. The brain’s motor functions,however, are also critically important, not just for theregulation of movement, but also for forming imagesand concepts, as we shall see. A key output area is theprimary motor cortex, which sends signals downthrough the spinal cord to the muscles. There are alsomany other motor areas in the cortex, and there are nu-clei in the thalamus related to motor function as well.Another structure related to motor functions is the cere-bellum, a prominent bulb at the base of the cortex andabove the brainstem (see Figure 1). The cerebellum ap-pears to serve in the coordination and sequencing of mo-tor actions and sensorimotor loops. There is no evi-dence, however, that it participates directly in consciousactivity.


An intriguing set of structures known as the basalganglia is critically important for motor control and se-quencing. Lesions of certain structures in these nucleilead to a loss of the neurotransmitter dopamine, andthus to the symptoms of Parkinson’s syndrome. Patientswith this disease have tremors, difficulty in initiatingmotor activity, rigidity, and even certain mental symp-toms. The basal ganglia, as shown in Figure 1, are lo-cated in the center of the brain and connect to the cortexvia the thalamus. Their neural connectivity, which isradically different from that of the cortex, consists ofcircuits of successive synapses or polysynaptic loops con-necting the various ganglia. For the most part, the recip-rocal connection patterns seen in the cortex itself andbetween the cortex and the thalamus are lacking in thebasal ganglia. Moreover, most of the activity of the basalganglia is through inhibitory neurons using GABA as aneurotransmitter. Nevertheless, since inhibition of inhi-bition (or disinhibition) can occur in these loops, theycan stimulate target neurons as well as suppress theiractivity.

The basal ganglia are believed to be involved in theinitiation and control of motor patterns. It is also likelythat much of what is called procedural memory (remem-bering how to ride a bicycle, for example) and othernonconscious learned activity depends on the functionsof the basal ganglia. As we shall see later, the regulatoryfunctions of basal ganglia are also significant for formingcategories of perceptions during experience.


There is one final set of structures that is criticalin brain activity connected with learning and the main-tenance of consciousness. These are the ascending sys-tems, which my colleagues and I have called value sys-tems because their activity is related to rewards andresponses necessary for survival. They each have a differ-ent neurotransmitter, and from their nuclei of originthey send axons up and down the nervous system in adiffuse spreading pattern. These nuclei include the locuscoeruleus, a relatively small number of neurons in thebrainstem that release noradrenaline; the raphe nucleus,which releases serotonin; the various cholinergic nuclei,so-called because they release acetylcholine; the dopa-minergic nuclei, which release dopamine; and the hista-minergic system, which resides in a subcortical regioncalled the hypothalamus, a region that affects many crit-ical body functions.

The striking feature of such value systems is that,by projecting diffusely, each affects large populations ofneurons simultaneously by releasing its neurotransmit-ter in the fashion of a leaky garden hose. By doing so,these systems affect the probability that neurons in theneighborhood of value-system axons will fire after re-ceiving glutamatergic input. These systems bias neu-ronal responses affecting both learning and memory andcontrolling bodily responses necessary for survival. It isfor this reason that they are termed value systems. Inaddition, there are other loci in the brain with modula-tory functions mediated by substances called neuropep-


tides. An example is enkephalin, an endogenous opioidthat regulates responses to pain. In addition, there areother brain areas, such as the amygdala, which are in-volved in emotional responses, such as fear. For our pur-poses, these areas need not be described in detail.

To summarize our account so far, we may say that,in a gross sense, there are three main neuroanatomicalmotifs in our brains (Figure 3). The first is the thalamo-cortical motif, with tightly connected groups of neuronsconnected both locally and across distances by rich re-ciprocal connections. The second is the polysynapticloop structure of the inhibitory circuits of the basal

Figure 3. Fundamental arrangements of three kinds ofneuroanatomical systems in the brain. The top diagramshows the gross topology of the thalamocortical system,

which is a dense meshwork of reentrant connectivitybetween the cortex and the thalamus and among different

cortical areas. The middle diagram shows the longpolysynaptic loops connecting the cortex with subcortical

structures such as the basal ganglia. In this case, theseloops go from the basal ganglia to the thalamus, thence tothe cortex and back from the target areas of cortex to theganglia. These loops are, in general, not reentrant. Thebottom diagram shows one of the diffusely projecting

value systems, in which the locus coeruleus distributes a“hairnet” of fibers all over the brain. These fibers release

the neuromodulator noradrenaline when the locuscoeruleus is activated.


ganglia. The third consists of the diffuse ascending pro-jections of the different value systems. Of course, thisgeneralization is a gross oversimplification, given the ex-quisite detail and individuation of neural circuitry. Butas we shall see, it provides a useful simplification; wecan dispose of it once we have seen its uses.

So much for simplicity. The picture I have paintedso far only hints at the remarkably complex dynamicsof the neural structures of the brain. After staring at thegross layout of brain regions in Figure 1 and under-standing the synapse pictured in Figure 2, close youreyes and imagine myriad neural firings in millions ofpathways. Some of this neural activity would occur atcertain frequencies while others would show variable fre-quencies. Bodily activity and signals from the environ-ment and the brain itself would modify which of thepathways were favored over others as a result of changesin synaptic strength. Although I hardly expect the readerto be able to visualize precisely the hyperastronomicalnumbers of neural patterns in detail, perhaps this exer-cise will yield a further appreciation of the brain’s com-plexity.

We are now in a position to address some of thequestions posed at the beginning of this chapter. Con-sider the question of whether the brain is a computer. Ifwe examine how neural circuits are built during animaldevelopment, this would seem unlikely. The brain arisesduring development from a region of the embryo calledthe neural tube. Progenitor cells (cells that are precursors


to neurons and support cells called glia) move in certainpatterns to make various layers and patterns. As theydifferentiate into neurons, many also die. From the verybeginning of neuroanatomy, there are rich statisticalvariations in both cell movement and cell death. As aresult, no two individuals, not even identical twins, pos-sess the same anatomical patterns.

In the earliest stages of development, the cellularorganization characteristic of a species is controlled byfamilies of genes among which are the so-called Hoxgenes and Pax genes. But at a certain point, the controlof neural connectivity and fate becomes epigenetic; thatis, it is not prespecified as “hardwiring,” but rather isguided by patterns of neural activity. Neurons that firetogether wire together. While, at earlier stages, patternedcell movement and programmed cell death determineanatomical structure, the movement and death of indi-vidual neurons are nonetheless statistically variable orstochastic. The same holds for which particular neuronsconnect to each other at later stages. The result is a pat-tern of constancy and variation leading to highly indi-vidual networks in each animal. This is no way to builda computer, which must execute input algorithms oreffective procedures according to a precise prearrangedprogram and with no error in wiring.

There are other, even more trenchant reasons forrejecting the idea of digital computation as a basis forbrain action. As we shall see later, what would be lethalnoise for a computer is in fact critical for the operation


of higher-order brain functions. For the moment,though, let us consider some other aspects of brain com-plexity and its relation to brain structure and function.

A review of what I have said about the overall ar-rangement of brain areas might tempt one to concludethat the key to brain function is modularity. Since thereare regions that are functionally segregated for vision(even for color, movement, and orientation), for exam-ple, or similarly for hearing or touch, we might betempted to conclude that specific brain action is mainlythe result of the specialized functioning of these isolatedlocal parts or modules. If pushed to higher levels, thissimple notion results in phrenology, the picture of local-ized separate brain faculties first proposed by Franz Jo-seph Gall. We now know that modularity of this kindis indefensible. The alternative picture, that the brainoperates only as a whole (the holistic view), will also notstand up to scrutiny.

The notion of modularity is based on an overlysimple interpretation of the effects of ablation of partsof the brain, either by animal experiments or as a resultof a stroke, or of surgery for epilepsy. It is clear, forexample, that ablation of cortical area V1 leads to blind-ness. It does not follow, however, that all the propertiesof vision are assured by the functioning of V1, whichis the first cortical area in a series making up the visualpathway. Similarly, although modern imaging tech-niques reveal certain areas of the brain that are active incertain tasks, it does not follow that the activity of such


areas is the sole cause of particular behaviors. Necessityis not sufficiency. But the contrary or holistic argumentis not tenable either—one must account for both inte-gration and differentiation of brain activity. This willbe one of our main tasks in proposing a global braintheory. As we shall see later, the long-standing argumentbetween localizationists and holists dissolves if one con-siders how the functionally segregated regions of thebrain are connected as a complex system in an intricatebut integrated fashion. This integration is essential tothe emergence of consciousness.

This reasoning is critical to understanding the rela-tionship between brain function and consciousness. Ofcourse, there are areas of the brain that if damaged orremoved will lead to permanent unconsciousness. Onesuch area is the midbrain reticular formation. Anotheris the region of the thalamus containing the intralaminarnuclei. These structures are not the site of consciousness,however. As a process, consciousness needs their activ-ity, but to account for the Jamesian properties of con-sciousness requires a much more dynamic picture in-volving integration of the activities of multiple brainregions. We are now in a position to lay the groundworkfor just such a picture by considering a global brain the-ory that accounts for the evolution, development, andfunction of this most complex of organs.


wider than the sky: the phenomenal gift of …...two subtle but important issues strongly inﬂuence...

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