Gerald M.Edelmann
A UNIVERSE OF CONSCIOUSNESS
Basic 2000
pg 35
P A R T T W O
Consciousness and the Brain
It is a reflection of human arrogance that entire philosophical systems have been constructed on the basis of subjective phenomenologythe conscious experience of a single, philosophically inclined individual. As Descartes recognized and took as his point of departure, such arrogance is partly justified, since our conscious experience is the only ontology of which we have direct evidence. As Schopenhauer noted,' this statement generates a curious paradox.The immense richness of the phenomenological world that we experienceconscious experience as suchappears to be dependent on what seems a mere trifle in the furniture of that world, a gelatinous piece of tissue contained in the skull.
Our brain, presenting itself as a fleeting and minor actor on the stage of consciousness that most of us have never seen, seems to hold the key to the entire performance. As we are all too painfully aware when we enter a hospital, any insult to our brain may permanently modify our entire world. Indeed, we can be annihilated by a simple chemical, anesthetic, or toxin, acting on our brain.
In order to give readers a better understanding about this remarkable organ, we first provide some basic information on the structure and dynamics of the brain and of its cells or neurons.
With this picture of the brain in place, we then consider a number of neurophysiological and neuropsychological facts that shed significant light on the neural mechanisms of consciousness. In organizing these facts, we present evidence that the neural processes underlying conscious experience share common general characteristics. Our analysis leads to several conclusions. First, conscious experience appears to be associated with neural activity that is distributed simultaneously across neuronal groups in many different regions of the brain. Consciousness is therefore not the prerogative of any one brain area; instead, its neural substrates are widely dispersed throughout the so-called thalamocortical system and associated regions.
Second, to support conscious experience, a large number of groups of neurons must interact rapidly and reciprocally through the process called reentry. If these reentrant interactions are blocked, entire sectors of consciousness disappear, and consciousness itself may shrink or split. Finally, we show that the activity patterns of the groups of neurons that support conscious experience must be constantly changing and suffficiently differentiated from one other. If a large number of neurons in the brain start firing in the same way, reducing the diversity of the brain's neuronal repertoires, as is the case in deep sleep and epilepsy, consciousness disappears.
Building a Picture of the Brain
Chapter Four
pg 37
To understand consciousness as a process, we must understand how the brain works; we must know about its architecture, its development, and its dynamic functions.
This chapter presents a usable but by no means exhaustive picture of the brain, highlighting the brain's most important features: its anatomical organization and the remarkable dynamics that it generates. Althongh it is painted with a broad brush, this picture is necessary for understanding how consciousness emerges.
The brain is among the most complicated objects in the universe and is certainly one of the most remarkable structures to have emerged during evolution. Even before the advent of modern neuroscience, it was well known that the brain is necessary for perception, feelings, and thoughts. Less obvious is how consciousness is causally associated with certain brain processes and not others. As an object and a system, the human brain is specialits connectivity, dynamics, mode of functioning, and relation to the body and the world is like nothing else science has yet encountered. This uniqueness makes building a picture of the brain an extraordinary challenge. Although we are far from a complete view of that picture, a partial view is better than none, especially if it gives us enough information to generate a successful theory of consciousness.
FIGURE 4.1 GROSS ANATOMY OF THE BRAIN. The figure shows:
1. the cerebral cortical mantle connected to the thalamus (the white oval in the middle, together constituting the thalamocortical system;
2. (2) the three great cortical appendages (basal ganglia, cerebellum, and hippocampus; and
3. 3) the brain stem, the oldest part of the brain, which contains the source of several diffusely projecting value systems.
The adult human brain weighs about 3 pounds and contains about 100 billion nerve cells, or neurons. The most recently evolved outer corrugated mantle of the human brain, the cerebral cortex, contains about 30 billion neurons and 1 million billion connections, or synapses. If we counted one synapse per second, we would not finish counting for 32 million years. If we considered the number of possible neural circuits, we would be dealing with hvper- astronomical numbers: 10 followed by at least a million zeros. (There are 10 followed by 79 zeros, give or take a few, of particles in the known universe.)Neurons, which come in a variety of shapes, have treelike projections called dendrites that receive synaptic connections. They also have a single longer projection, called an axon, that makes synaptic connections either at the dendrites, or at the cell bodies, of other neurons. No one has made an exact connt of different types of neurons in the brain, but a crude estimate of fifty would not be excessive. The lengths and branching patterns of dendrites and axons from a given type of neuron fall within certain ranges of variation, but even within a given type, no two cells are alike.
A key characteristic of neuronal patterns at the microscopic level is their density and spread. The body of a single neuron measures up to about 50 microns (thousandths of a millimeter) in diameter, although its axon can range from microns to meters in length. In a tissue like the cerebral cortex neurons are packed together at an extraordinary density; if all of them wer stained with silver in the so-called Golgi stain used to visualize them in the microscope, the stained microscopic section would be pitch-black. (Actually, this classical stain is useful because it affects only a small fraction of cells in a given area, so the cells can be individually discerned, as figure 4.2 shows.Interspersed among the neurons are nonneuronal cells, called glia, that support and nourish nerve cells without being directlv involved in signaling. In some places, glia even outnumber neurons. .Annother important feature is th extraordinary bloodsupply that nourishes this jungle; trough large arteries emptying into a dense network of capillaries, the brain receives the oxygen and glucose it needs as the most metabolically active organ in the body. The regulation of blood flow is exquisite down almost to single neurons, and synaptic activity is tightly linked tO blood flow and oxygenation. Indeed, modern techniques to image brain activity in living people rely on changes in blood flow and oxygenation.
In the dense networks of the brain, it is the spread and overlap of neuronal arborsof dendritic trees and axonal projectionsthat are the most striking features. In some places, the spatial spread of an axon forming an arbor can be over a cubic millimeter. Overlapping that arbor, with all its intricate branchings, are arbors from countless other neurons. The overlap can be as great as 70 percent in three-dimensional space. (No self-respecting forest, made of trees and root structures, would permit such a large overlap.) Moreover, as the axonal arbors overlap, they can form an enormous variety of synapses, or connections (see figure 4.3), with cells in the paths of their branches, resulting in a pattern that is unique for each small volume of brain tissue. To this day, though we can trace the full arborization of a single nerve cell, we have no clear picture of the microanatomy of the interspersed arbors of the many neighboring cells at the scale of their synapses.
THE SYNAPSE
While the general cellular functions of neurons, such as respiration, genetic inheritance, and protein synthesis, are like those of other cells in the body, the special features of these cells related to the functioning of neurons mainly concern their ability to communicate through connections called synapses.Neurons come in two flavors, excitatory and inhibitory, and at the microscopic level, their synapses have different and characteristic structures. But, for each, the basic principles are similar and involve both electrical and chemical signaling. Although in certain species some synapses can be completely electrical, the vast majority of the synapses in human brains are chemical. In most cases, the so-called presynaptic neuron and postsynaptic neuron are separated by a cleft forming a single synapse (see figure 4.3).
The inside of a neuron is negatively charged with respect to the outside. After a cell is stimulated as a result of the flow of ions, such as sodium and potassium, across a particular portion of the cell membrane, it becomes less negative. The resulting electrical signal, called an action potential, spreads down an axon, and when it reaches the region of the synapse, it causes the release of neurotransmitters from a series of vesicles in the presynaptic neuron. If the neuron is excitatory, the released neurotransmitters then cross the synaptic cleft, bind to specific receptors on the postsynaptic neuron, and cause the postsynaptic neuron to become less negative. These processes occur over periods of tens to hundreds of rnilliseconds. If the postsynaptic neuron becomes sufficiently less negative after several such events, it will fire, (generate an action potential of its own), relaying the signal, in turn, to other neurons to which it is connected. This is the action of an excitatory neuron. Inhibitory neurons act similarly but change the electrical charge of the postsynaptic neuron in such a fashion as to prevent firing.
As intricate as the microstructure of neuronal connections may be, this intricacy is magnified by the number of different interactions, in space and time, that can affect synaptic transmission. The brain contains a variety of chemicals called neurotransmitters and neuromodulators that bind to a variety of receptors and act on various biochemical pathways. The chemical identity of these neurotransmitters and of their receptors, the statistics of their release, and the time and place of electrical and biochemical interactions all govern the thresholds of response of neurons in an extraordinarily intricate and variable manner. Furthermore, as a result of the release of the neurotransmitters, electrical signaling not only takes place, but leads to changes in the biochemistry and even in gene expression of the target neurons. This molecular intricacv and the resulting dynamics superimpose several more layers of variability on that of the neuroanatomical picture. However, anatomical segregation is only half the story. The other half is anatomical integration: Most of these groups of neurons are reciprocally connected in certain patterns. Neurons within the same groups in a given location are tightly linked, so that many of them respond simultaneously when an appropriate stimulus is presented.'
Neuronal groups with different locations but similar specificities are preferentially connected to each other; for example, neuronal groups that respond to vertical edges are linked by reciprocal connections much more tightly than neuronal groups that respond to edges in different orientations.
Furthermore, neuronal groups that respond to nearby positions in the visual field are more strongly connected than those that respond to distant positions. In this way, when a long contour or line is presented to the eye, these linked groups fire simultaneously. Similar rules seem to apply to other areas of the cortex, whether these areas are devoted to perception or to action.
At a still larger scale, cortical areas containing a large number of neuronal groups are themselves linked by reciprocal, convergent, and divergent connection pathspaths that connect disperse areas to a local area, and vice versa.
Such paths from one area to another are sometimes called projections. There are, for example, at least three dozen visual areas in the visual system of the monkey (and probably more in humans). These areas are linked by more than 305 connection paths (some with millions of axonal fibers), over 80 percent of which have fibers running in both directions. In other words, the different functionally segregated areas are, for the most part, reciprocally connected.
These reciprocal pathways are among the main means that allow for the integration of distributed brain functions. They provide a major structural basis for reentry, a process of signaling back and forth along reciprocal connections, that, as we describe later, offers the key to resolving the problem of integrating the various functionally segregated properties of brain areas despite the lack of a central coordinative area.
With some effort of imagination, we can therefore form the following picture of the thalamocortical mode of organization. There are hundreds of functionally specialized thalamocortical areas, each containing tens of thousands of neuronal groups, some dealing with responses to stimuli and others with planning and execution of action, some dealing with visual and others with acoustic stimuli, some dealing with details of the input and others with its invariant or abstract properties. These millions of neuronal groups are linked by a huge set of convergent or divergent, reciprocally organized connections that make them all hang together in a single, tight meshwork while they still maintain their local functional specificity. The result is a three-dimensional tangle that appears to warrant at least the following statement: Any perturbation in one part of the meshwork may be felt rapidly everywhere else.
FIGURE 4.4 THREE MAIN TOPOLOGICAL ARRANGEMENTS OF FUNDAMENTAL NEUROANATOMY IN THE BRAIN.
(A}The top diagram shows the tbalamacortical systema dense meshwork of reentrant connectivity between the thalamus and the cortex and between different cortical regions thraugh so-called corticocortical fibers.
(B)The middle diagram depicts long, polysynaptic loops that are arranged in parallel and that leave the cortex, enter the so-called cortical appendages (indicated here are the basal ganglia and the cerebellum), and return to the cortex.
(C) The bottom diagram indicates one of the diffusely projecting value systems (the noradrenergic locus coeruleus), which distributes a "hairnet" of fibers all over the brain and can release the neuromodulator noradrenaline.
Altogether, the organization of the thalamocortical meshwork .
seems remarkably suited to integrating a large number of specialists into a unified response. The second topological arrangement is organized not at all like a meshwork but, rather, like a set of parallel, unidirectional chains that link the cortex to a set of its appendages,
each with a special structurethe cerebellum, the basal ganglia, and the hippocampus.
The cerebellum is a beautiful structure, appended to the back of the brain, that is organized in thin, parallel microzones, many of which receive connections from the cortex, and after a number of synaptic steps project back to the thalamus and through it back to the cortex. Traditionallv, the cerebellum is considered to be concerned with the
coordination and synchrony of motion, although its involvement in certain aspects of thought and language appears to be substantial.
Another cortical appendage, called collectively the basal ganglia, consists of a set of large nuclei deep in the brain that receive connec
tions from much of the cortex, go through a series of successive synaptic steps, and then project to the thalamus and from there back
to the cortex. These nuclei are involved in the planning and execution of complex motor and cognitive acts and are dysfunctional in Parkinson's and Huntington's diseases.
Still another structural motif appears in a third cortical appendage, the
hippocampus, an elongated structure that runs along the lower edge of the temporal cortex of the brain. Inputs from many different cortical areas are funneled into the hippocampus, which deals with these inputs in a series of synaptic steps and sends projections back to many of the same cortical areas. The hippocampus probably subserves manv functions, but it certainly plays a major role in consolidating short-term memory into long-term memory in the cerebral cortex.
Although the specific ways in which these different cortical appendages interact with the cortex are of central importance, the appendages all seem to share a fundamental mode of organization (especially the cerebellum and basal ganglia): Long, parallel paths involving multiple synapses leave the cerebral cortex and reach successive synaptic stations within these cortical appendages and, eventually, whether they pass through the thalamus or not, thev go back to the cortex (see figure 4.4B). This serial polysynaptic architecture differs radically from that of the thalamocortical system: The connections are generally unidirectional, rather tban reciprocal, and form long loops, and there are relatively few horizontal interactions among different circuits except for, possibly, those locally responsible for reciprocal inhibition.
In short, these systems seem admirably suited to the execution of a variety of complicated motor and cognitive routines, most of which are as functionallv insulated as possible from each other, a feature that guarantees speed and precision in their execution.
The third kind of topological arrangement resembles neither a meshwork nor a set of parallel chains, but, rather, a diffuse set of connections resembling a large fan (see figure 4.4C). The origin of the fan is in a relatively small number of neurons that are concentrated in specific nuclei in the brainstem and hypothalamus that have beguiling and imposing technical names connected with the substance they release: the noradrenergic locus coeruleus, the serotonergic raphe nucleus, the dopaminergic nuclei, the cholinergic nuclei, and the histaminergic nuclei.All these nuclei project diffusely to huge portions of the brain, if not to all of it. The locus coeruleus, for example, consists of only thousands of neurons in the brainstem, but sends a diffuse sweeping "hairnet" of fibers to cover the entire cortex, hippocampus, basal ganglia, cerebellum, and spinal cord, and by this means, has the potential to influence up to billions of synapses. Neurons belonging to these nuclei appear to fire whenever something important or salient occurs, such as a loud noise, a flash of light, or a sudden pain. The firing of these neurons brin~s about the diffuse release in the brain of chemicals called neuromodulators that are capable of influencing not only neural activity but neural plasticitya change in the strength of synapses in neural circuits yielding adaptive responses. Given their unique anatomical properties, their discharge characteristics, their effects on target neurons and synapses, and their evolutionary origins, we have designated them collectively as value systems.
Although these systems have long captured the attention of neurobiologists and pharmacologists, there has been little historical agreement on their function. What is certain is their extreme importance as targets of pharmacological intervention in mental illness and dysfunction. The major sites of action of most of the modern drugs used to treat mental illness include cells from these groups. Small alterations in the pharmacology of these cells can have drastic effects on global mental function. As we discuss in chapter 7, such value sytems appear perfectly suited to signalling the occurence of salient events to the entire brain, leading to changes in the strength of billions of synapses.
THE BRAIN IS NOT A COMPUTER
Our quick review of neuroanatomy and neural dynamics indicates that the brain has special features of organization and functioning that do not seem consistent with the idea that it follows a set of precise instructions or performs computations. We know that the brain is interconnected in a fashion no man-made device yet equals.
First, the billions and billions of connections that make up a brain's connections are not exact: If we ask whether the connections are identical in any two brains of the same size, as they would be in computers of the same make, the answer is no. At the finest scale, no two brains are identical, not even those of identical twins. Although the overall pattern of connections of a given brain area is describable in general terms, the microscopic variability of the brain at the finest ramifications of its neurons is enormous, and this variability makes each brain significantly unique. These observations present a fundamental challenge to models of the brain that are based on instruction or computation. As we shall see, the data provide strong grounds for so-called selectional theories of the braintheories that actually depend upon variation to explain brain function.
Another organizing principle that emerges from the picture we are building is that in each brain, the consequences of both a developmental history and an experiential history are uniquely marked. For example, from one day to the next, some synaptic connections in the same brain are likely not to remain exactly the same; certain cells will have retracted their processes, others will have extended new ones, and certain others will have died, all depending on the particular history of that brain. The individual variability that ensues is not just noise or error, but can affect the way we remember things and events. As we shall see, it is also an essential element governing the ability of the brain to respond to and match the countless unforeseeable scenes that may occur in the future. No present-day machine incorporates such individual diversity as a central feature of its design, although the day will certainly come when we shall build devices that are truly brainlike.
If we compare the signals a brain receives with those of computers, we uncover a number of other features that are special to brains. First, the world certainly is not presented to the brain like a piece of computer tape containing an unambiguous series of signals.
Nonetheless, the brain enables an animal to sense the environment, categorize patterns out of a multiplicity of variable signals, and initiate movement. It mediates learning and memory and simultaneously regulates a host of bodily functions.
The ability of the nervous system to carry out perceptual categorization of different signals for sight, sound, and so forth, dividing them into coherent classes without a prearranged code, is certainly special and is still unmatched by computers.
We do not presently understand fully how this categorization is done but, as we discuss later, we believe it arises throngh the selection of certain distributed patterns of neural activity as the brain interacts with the body and the environment.
We have also shown that the brain contains a special set of nuclei with diffuse projections the value systemswhich signal to the entire nervous system the occurrence of a salient event and influence changes in the strength of synapses. Systems with these crucial properties are typically not found in man-made devices, y et their importance for learning and adaptive behavior is well documented. Together with the morphological peculiarities of the brain and its neural connections with a specific bodily phenotype, these systems provide an animal with a large set of constraints whose role in fostering species-specific perceptual categorization and adaptive learning cannot be underestimated.
Finally, if we consider neural dynamics (the way patterns of activity in the brain change with time), the most striking special feature of the brains of higher vertebrates is the occurrence of a process we have called reentry. Reentry, which we discuss in detail in chapters 9 and 10, depends on the possibility of cycles of signaling in the thalamocortical meshwork and other networks mentioned earlier. It is the ongoing, recursive interchange of parallel signals between reciprocally connected areas of the brain, an interchange that continually coordinates the activities of these areas' maps to each other in space and time.
This interchange, unlike feedback, involves many parallel paths and has no specific instructive error function associated with it. Instead, it alters selective events and correlations of signals among areas and is essential for the synchronization and coordination of the areas' mutual functions.
One striking consequence of reentry is the widespread synchronization of the activity of different groups of active neurons distributed across many different functionally specialized areas of the brain. This synchronous firing of widely dispersed neurons that are connected by reentry is the basis for the integration of perceptual and motor processes. This integration ultimately gives rise to perceptual categorization, the ability to discriminate an object or event from a background for adaptive purposes. If the reentrant paths connecting cortical areas are disconnected, these integrative processes are disrupted.
As we discuss in detail in chapter 10, reentry allows for a unity of perception and behavior that would othenvise be impossible, given the absence in the brain of a unique, computerlike central processor with detailed instructions or of algorithmic calculations for the coordination of functionally segregated areas.
Indeed, if we were asked to go beyond what is merely special and name the unique feature of higher brains, we would say it is reentry. There is no other object in the universe so completely distinguished by reentrant circuitry as the human brain. Although a brain has similarities to a large ecological entity like a jungle, nothing remotely like reentry appears in any jungle. Nor in human communication systems: Reentrant systems in the brain are massively parallel to a degree unheard of in our communication nets. In any event, communication nets are unlike brains, in that they deal with previously coded and, for the most part, unambiguous signals. Because of the dynamic and parallel nature of reentry and because it is a process of higher-order selection, it is not easy to provide a metaphor that captures all the properties of reentry. Try this: Imagine a peculiar (and even weird) string quartet, in which each player responds by improvisation to ideas and cues of his or her own, as well as to all kinds of sensory cues in the environment. Since there is no score, each player would provide his or her own characteristic tunes, but initially these various tunes would not be coordinated with those of the other players. Now imagine that the bodies of the players are connected to each other by mvriad fine threads so that their actions and movements are rapidly conveyed back and forth through signals of changing thread tensions that act simultaneously to time each player's actions. Signals that instant-aneously connect the four players would lead to a correlation of their sounds; thus, new, more cohesive, and more integrated sounds would emerge out of the othervvise independent efforts of each player. This correlative process would also alter the next action of each player, and by these means the process would be repeated but with new emergent tunes that were even more correlated. Although no conductor would instruct or coordinate the group and each plaver would still maintain his or her style and role, the players' overall productions would tend to be more integrated and more coordinated, and such integration would lead to a kind of mutually coherent music that each one acting alone could not produce.
All these special features of the brain connectivity, variability, plasticity, ability to categorize, dependence on value, and the dynamics of reentry operate heterogeneously to yield coordinated behavior. As we hinted earlier, the nonlinear aspects of the interaction among the brain, the body, and various parallel signals from the environment must be considered together to understand the process of perceptual categorisation, movement, and memory that underlie consciousness.
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