Deric Bownds

Chapter 12

Conscious Mind

The two final chapters of this book are included to paint a more comprehensive picture of our modern minds and how they exist in a sometimes uneasy alliance with bodies that were designed for conditions of the Paleolithic. The preceding chapters have elaborated on the theme that our brain is not a general-purpose problem solver, using one common process for all tasks, but rather a collection of semi-independent devices tailored to specific jobs. Large computational problems (such as seeing, hearing, moving, and talking) are split into many parts, all processed by specific brain regions. These regions and their tasks can be revealed when they are damaged by brain lesions or genetic mutations. They can sometimes be seen directly in living brains by using imaging techniques. The specialized modules interact extensively to perform their functions. Some of these modules (for example, the visual areas specializing in form, motion, or color) have been partially dissected not just logically but also literally, because their anatomical locations are fairly precise and well known. The anatomy and physiology---the where, what, and how---of other modules in the brain are generally less clear.

This chapter considers some ideas about how this all comes together to generate consciousness and our subjective experience. We begin by returning briefly to some issues we first raised in Chapter 1, to note some classical debates on the question of consciousness, whether "mindstuff" is in the same category as the rest of the physical reality we know and whether consciousness can ever be explained. Assuming a positive answer to this question, we then return to the arena of cognitive neuroscience to discuss our conscious awareness system more thoroughly, briefly outlining some of the neural underpinnings of the attention and working-memory systems of the frontal lobes. Then we review some experiments in cognitive psychology that show how the players or agents inside our heads, of which we are largely unconscious, render meaningless the "Where does it all come together?" question. The next section considers consciousness by examining its perturbations, as in sleep and other altered states. Finally, after reviewing current work on the neural correlates of consciousness, we outline several models and metaphors for consciousness, ideas of what our "operating system" is like, and how the "I" that you are experiencing right now might emerge from it. Our conscious awareness is a small fraction of the many operations going on in our brains, and the bottom line is that we must be disabused of any notion that "our" thoughts reflect, in any direct sense, the real operations of our minds.

The Mind/Body Problem

Rene Descartes's famous formulation "I think, therefore I am" posited a clear dividing line between the mind and the brain. The accompanying self-experiment illustrates the depth to which this idea has permeated our culture and the way we construct ourselves. If you are a classical dualist, you might assign the "I" to the realm of spirit and the brain to the realm of the physical body, thus taking issue with the materialism of modern science and the points outlined in Chapter 1, which argues that mind is the sum of the physical components of the brain---its ions, molecules, and cells and their activities. However, if the self or "I" is different from the stuff of the brain, what is it? Is it some special kind of mindstuff or essence distinct from the physical matter of which you are composed? Dualists have the problem of explaining how mind, as a nonphysical entity with no mass or energy, could interact with the physical stuff of the brain.



Try the following exercise, which was suggested by the philosopher Daniel Dennett. Look away from this page while you repeat to yourself, "I have a brain." Now look away while you think or speak, "This brain has itself." The first statement probably feels comfortable to you, whereas the second feels a bit more alien, even though, as far as we know, both statements are saying the same thing. Thinking "I have a brain" corresponds to our daily experience of a narrative self, a little person or "I" inside our heads, distinct from our body, always seeing and commenting on what is taking place. 1


There is another way in which you might say that mind is different from brain without having to invent some sort of spiritual or nonmaterial essence. You could believe that mind is ultimately made up atoms and molecules, like everything we know about, but distinguish what mind does---the functions it carries out---from the machinery that carries out those functions. Thus you take mind or consciousness to be more than just the electrical activities of cells in your brain in the same way a movie is different from a description of the moving parts of the movie projector and the path of the light through it, or the way writing is more than small pieces of graphite spread on a paper surface in a continuous line. You could argue that the input-analysis-output functions of our minds (taking in information, analyzing it, then acting on it) could be carried out by different kinds of hardware---perhaps by computers, Martians, or zombies. In this book, we have not been dealing with these more hypothetical options but rather have focused on the biology of mind in humans and other animals as we know them. From this perspective, the analyses of what a mind does and what it is made of begin to converge as we learn more and more from experiments in basic neuroscience and cognitive psychology.

Thinking that you are just the physical stuff of your brain and that a "chemistry of consciousness" specifies your behavior may not feel quite right to you. But consider how profoundly your mood can be altered by just a bit of coffee or alcohol or by the brain chemistry underlying emotions that we discussed in Chapter 10. Consider the numerous cases in the psychiatric literature of people who, with no previous history of mental illness, became psychotic and were miraculously cured when a bacterial infection of their cerebrospinal fluid was diagnosed and antibiotics applied. Where does the body stop and the mind start in these cases? It makes more sense to think of them as a unit. Chemistry can alter our behavior, and our behavior can alter our chemistry. As an example of the latter, remember a time when you have moved quickly to avoid sudden danger---and then remained excited for a while as adrenaline continued to pump into your bloodstream.

Can the Problem of Consciousness Be Solved?

Some modern commentators despair of ever explaining consciousness. They just cannot grant that nerve signals are the kind of stuff that can make up the qualitative feel or content of consciousness. How can understanding nerve signals ever translate to what an apple tastes like? The philosopher Owen Flanagan describes these individuals as the new mysterians, the old mysterians being the Cartesian dualists described in the previous section. The new mysterians don't deny that mind and consciousness exist and operate in accordance with natural principles, but they argue that our very cognitive constitution prevents us from achieving a complete description of them. Asking ourselves to understand consciousness might be like asking a television set to report on its own inner workings. Just as we don't expect a dog's brain to understand quantum mechanics, we can't necessarily expect our own brains to understand everything. 2 However, a useful approach is to act as though we can understand everything, because there is no way of knowing that we can't understand something until after we have tried. 3

It could be that, just as physicists and mathematicians have acknowledged the limits to our understanding that are imposed by Heisenberg's Uncertainty Principle and Godel's Incompleteness Theorem, we are closed off from knowing what sort of natural phenomenon consciousness is. It seems premature, however, to say that our brains can never understand themselves; after all, they can do any number of other unusual things. We should avoid arguments from ignorance, such as "neuroscience doesn't know how to explain X (consciousness, for instance) in terms of the nervous system; therefore it cannot be so explained. Rather it can eventually be explained in terms of Y (pick your favorite thing... quantum wave packets, psychons, ectoplasmic retrovibrations, etc.)." 4 What we can do is pursue descriptions from cognitive psychology of how mental life works---descriptions given by neuroscience research---and also just listen carefully to what individuals have to say about how things seem. 5 In some ways, we are similar to the subject of the familiar joke about the drunk looking for car keys he lost near some dark bushes. He is looking for them under a nearby street light "because the light is better here." We can mimic this drunk when we are looking for an explanation of our minds, and remain infatuated with our ideas of what a mind should be (by insisting, for example, that brains can be modeled on the architecture of modern computers) rather than trying to find out how the conscious human mind actually works. We can become so impressed by the richness of what we know that we lose sight of the much larger area of the unknown.



We are like the map makers of the 16th century who were very pleased with the sophistication of their efforts to depict the globe, especially when compared with Greek and Roman attempts. The large errors and unknowns that we now see in these old maps are revealed only in hindsight. We surely can expect our descendants to be acutely aware of the limitations of our 20th-century efforts to understand the brain.


The Machinery of Awareness

Let's turn now from this brief discussion of what we might or might not be able to know to scientific studies on the nature of our awareness and attention. Our daily life is a constant checking of what is happening right now against what just happened a few moments ago. We hold information, anticipations, or goals in our short-term working memory for a time and then release them if they become irrelevant. Should we wish to retain knowledge of a face or a phone number, the mechanisms of episodic memory described in Chapter 6 are activated. If we later need to remember a name or number, we can fetch it from that long-term memory and place it back in our working memory.

The classical paradigm for engaging the interaction between current awareness and short-term memory is the delayed-choice test, in which a human volunteer or an animal subject is rewarded for recognizing a previously presented image or situation. Electrical recordings from animals and brain imaging experiments on human subjects show activation of the prefrontal cortex during such tests. 6 This activation persists though a delayed-choice test even if distracting and irrelevant auditory and visual stimuli are presented. Prefrontal cortex is linked to all of the perceptual systems, and it apparently orchestrates an emphasis on cues that are relevant to current needs over irrelevant stimuli. This is why we are more likely to notice the smell of food when we are hungry than when we have just eaten.

It has been proposed that working memory has a central controller and a group of "slave systems," the two main slave systems being one for silent speech or verbal memory and the other for visuospatial information. 7 Imaging studies, however, do not reveal an obvious activity corresponding to a central controller. Rather, different kinds of working memory (such as for spatial locations, object identification, facial memory, and verbal memory) may correlate with activation of different prefrontal regions. Moving from simple delayed-choice tests to more complicated tasks (such as articulating a random list of each number from 1 to 10 with no repetition, or saying which words in a list read aloud were vegetable names) causes activation to spread from a limited part of the right frontal cortex to higher dorsolateral areas of both right and left prefrontal cortex.

Phonological memory and subvocal rehearsal activate Broca's area of the left hemisphere as well as several other areas. Our visual working memory recruits areas of the frontal, parietal, and occipital lobes of the right hemisphere. PET scan studies demonstrate that while working memory is performing "what" tasks, different loci are active in our prefrontal cortex than when "where" tasks are being performed. 8 Higher resolution magnetic resonance imaging shows sustained activation of different areas of frontal cortex for face memory and letter memory. Tasks that require working memory for locations activate mainly right brain regions, whereas letter-memory tasks activate primarily left brain areas. 9 When a main goal is being kept in mind while concurrent subgoals are being performed, activation of the most anterior part of the frontal lobes is observed. 10 In both human and monkey brains, the ventral stream of "where" visual information mentioned in Chapter 8 moves into dorsolateral prefrontal cortex; the dorsal "what" stream enters mainly ventrolateral areas. This is shown in Figure 12-1. Interestingly, there appears to have been a displacement of these areas in the human to make room for the expansion of prefrontal areas serving language and higher cognition. Recordings from individual cells in monkey prefrontal cortex show that they monitor expectancy and goal-directed behavior. 11 Some cells fire while "what" information is being held, others fire while "where" information is being held, and still others register a combination of "what" and "where" activity. 12 Working memory is impaired by lowered levels of the neurotransmitter dopamine (this happens in patients with Parkinson's disease) and is enhanced by increasing dopamine levels. 13

Figure 12-1

Projection of spatial perception and object recognition into the frontal lobe working-memory areas of the brain.



The "what" and "where" streams of information sent forward from parietal and temporal lobes project into the working-memory areas of the frontal cortex.


The contents of our awareness and working memory reflect only one of many parallel tracks that deal with perception and action. Many studies have documented this, such as experiments that have demonstrated and mapped brain regions that are responsive to novelty without awareness. 14 Subjects were asked to press a keypad to indicate numbers (1, 2, or 3) appearing on a screen in what appeared to be random order but in fact followed a complex sequence. Improvement in performance indicated that subjects had learned the sequences even though they were unaware of the existence of any order. If the order of the sequence was then changed, changes in right prefrontal and other areas were noted, even though subjects remained unaware of the changes in number presentation.

The Brain's Time and Space: The Disappearance of "I"

What we are holding in our working memory---our current awareness and recall of our immediate past---are central components of our experience of an "I." This "I" is the basis for our common-sense notion that our mind has some central point where sensations and perceptions ultimately are noticed, and where commands and actions ultimately begin. Naturally, this conviction leads us to look for an actual place in the brain where it all comes together. But then, who would be watching that central place? And wouldn't that watcher need yet another to watch it? It is one of the main messages of this book that there is no place where everything comes together and that looking for the place in the brain where the "I" is makes no more sense than looking for the place in a tornado where "the storm" is. The "I" is a construct---an idea that we employ to make sense of what many unconscious processes finally lead us to do---much as "the storm" is the sum of what the turbulent, spinning masses of air in a tornado do.

We have already noted that the distinction between perception and action grows very fuzzy as we work our way further into the brain and shift to the time interval of 50--500 milliseconds after a sensory stimulus or before a movement. This is because there is no single point in the brain through which all information funnels. The reason it seems natural to make the distinction is that most of our conscious experience of sensing or acting unfolds over a period of seconds rather than milliseconds, as when we see a ball tossed and look to where it will land, or when we perform the motions of peeling potatoes. But in the central brain processing that occurs in the interface between our acting and reacting, most of the action is very fast---so fast that auditory input, for example, can influence a visual search on the time scale of 100--300 milliseconds. 15



To give you a feel for the lengths of time we are talking about, it takes about 1000 milliseconds to say "one Mississippi," we can start and stop a stopwatch in roughly 175 milliseconds, one frame of a movie film projects for about 40 milliseconds, most nerve cells work on a time scale of 1--10 milliseconds. If you can drum all the fingers of one of your hands on the desk five times while you are saying "one Mississippi" a finger is hitting the surface every 40 milliseconds.

Try the experiment of holding a 3-x-5 card just above a friend's thumb and forefinger and ask your friend to catch the card when you let it go. He or she can't do it if the card is immediately above the thumb and forefinger. The drop takes 100--150 milliseconds, faster than the nerve signal that it is dropping can go from eye to brain to fingers. If you hold the card 3 inches above the fingers, however, your friend can catch it; 300--400 milliseconds is enough time.


Plastic Representations of Time and Space

Two well-known cognitive psychology experiments show that in these small time intervals, our brain is an assembly of agents with minds of their own (so to speak) that can play with even the most basic representations of time and space in ways to which we are normally oblivious. What happens during these intervals shows the impossibility of the "I" acting as boss or command central. These experiments emphasize perception, but a corresponding list could be drawn up of experiments dealing with the output of the brain, or action. 16

The first experiment involves a phenomenon that has been named the phi effect; it is depicted in Figure 12-2. Two stationary spots of light separated by as much as 4 degrees of visual angle are lit in succession. First one spot is on for 150 milliseconds, then both are dark for 50 milliseconds, and then the other spot is on for 150 milliseconds. They appear to be a single spot moving in a continuous line. If the first spot is red and the second is green, the moving spot appears to have changed its color from red to green abruptly in the middle of its passage, during the period of darkness. Why is it our perception that the color changed to green before the green light actually came on? There is virtually no difference in the response time if a subject is asked to press a button as soon as the red spot comes on, whether or not the green spot follows! It can be shown that button-pressing responses would have to have been initiated before the discrimination of the second stimulus, the green spot. Thus we did not hold up our conscious experience for at least 200 milliseconds and revise history. Rather, processing is continually going on. A revision occurs that leads us to think the spot changed color in mid-flight. At the level of basic brain operations, this could translate into something very straightforward. For example, the V5 region in the cortex (see Chapter 8) responds to motions and to apparent motion. Perhaps some activity in area V5 is the brain concluding that an intervening motion occurred.

Figure 12-2

The phi effect. If a red and a green spot of light are flashed on a screen, as shown, in rapid succession, they appear to be a single moving spot whose color changes in the middle of the motion.

We must distinguish here between the representation and the represented: The brain's representation of time needn't use time-in-the-brain. The objective sequence of events occurring in the brain of an observer can be different from the subjective sequence of events reported. The mis-discrimination of red-turning-to-green occurred in the brain after the discrimination of the green spot (in objective time), but the subjective or narrative sequence is red spot, then red turning to green, then finally green spot. Another way of putting it is to say that a unit of subjective time can incorporate several units of physical time, as shown in Figure 12-3, and, if appropriate, rearrange them. 17

Figure 12-3

Space and time in the brain. The brain can store events that occur during sequential units of physical time (indicated by the Roman numerals) in temporary holding spaces, or buffers (indicated by the Arabic numerals). These buffers can rearrange the events in a way that past experience indicates makes the most "sense" before a subjective report is generated. In the case of the phi effect, the brain appears to be following a rule that "two dots appearing in rapid succession near each other probably represent a single moving spot of light."

A similar situation occurs for the representation of space in the brain, which, as we saw in Chapter 8, does not always use space-in-the-brain to represent space. These discrepancies are no different in principle from shooting scenes in a movie in a different sequence from the one in which they finally appear, or reading "Bill arrived at the party after Sally, but Jane came earlier than both of them." You learn of Bill's arrival before you learn of Jane's earlier arrival. (In fact, the representation of time needn't be temporal at all. Think of the way in which different points of time are represented by a wall calendar.)

A second classical experiment involves what is called metacontrast. A display device that can flash a stimulus for very brief intervals, around 1--500 milliseconds, is used to show the two stimuli represented in Figure 12-4 for 30 milliseconds each, about as long as a single frame of television, the second stimuli immediately following the first. In the figure, the disc appears to the side of the ring, but in the experiment the ring is flashed right on top of the disc, and its inner diameter is exactly the same as the outer diameter of the disc.

Figure 12-4

Metacontrast. If the first and second stimuli are flashed in rapid succession, only the second (the ring) is observed, even though the first (the disc) is observed if it is flashed by itself. (In the experiment, the disc to the left is actually flashed in the center of the ring.) Thus the brain takes a while to decide what is "really" there, and an initial stimulus can be overwritten by another that has features in common with the first.

Individuals report seeing only the second stimulus, the ring, although they can report the disc if it is shown separately. The brain, initially informed that something with a circular contour in a particular place happened, swiftly receives confirmation that there was indeed a ring, with an inner and outer contour. Without further supporting evidence that there was a disc, the brain arrives at the conservative conclusion that there was only a ring. The disc was briefly in a position---functional and spatial---to contribute to a later report, but this state lapsed. There is no reason to insist that the state was inside the charmed circle of consciousness until it got overwritten, or to insist that it never quite achieved this state. Preliminary versions that were composed at particular times and places in the brain were later withdrawn from circulation, replaced by more refined versions. Multiple preliminary versions might be withdrawn, but they also can influence a final version, even if they are not experienced. For example, very brief presentation of a priming word can influence the judged meaning of a target word that follows within 100 milliseconds, even though the priming word is not experienced or reported. 18



Our intuition, what we think is happening, is that information gets moved around the brain with events occurring in sequence, like railroad cars on a track. The order in which the information passes some point, or station, will be the order in which it arrives in consciousness. But that isn't what we observe in the experiments described in this section, and there is a good reason. The brain's job is to use incoming information swiftly to "produce a future," staying one step ahead of disaster, and it has to do this efficiently. The processes that do this are spatially distributed across functional regions of a large brain, and communication between different regions is relatively slow. Why wait for everything to come together if appropriate action could be taken faster without that delay?


The Futility of Asking "Where Does It All Come Together?"

The philosopher Daniel Dennett uses an analogy to drive home the futility of the question "Exactly when do you become conscious of a stimulus?" Imagine your brain to be like the British Empire, and rephrase the question as follows: "Exactly when did the British Empire become aware of the truce in the War of 1812?" News of the truce, signed in Belgium on December 24, 1814, reached different parts of the British Empire (in America, India, Africa, and so on) at different times, and it did not reach the most far-flung outposts until mid-January 1815. There is no way to pin down a day and hour when "the Empire" knew. The battle of New Orleans was fought 15 days after the truce that made it moot. Even if we can give precise times for the various moments at which various officials of the Empire became informed, none of these moments can be singled out as the time the Empire itself was informed. This is entirely analogous to asking exactly when we become conscious of a stimulus. Because cognition and control, and hence consciousness, are distributed throughout the brain, no one moment can count as the precise moment at which each conscious event happens.

In Dennett's "multiple drafts" theory of consciousness, all varieties of perception---of thought or mental activity---are accomplished in the brain by parallel multitrack processes of interpretation and elaboration of sensory inputs. Information entering the nervous system is subject to continuous "editorial revision." The brain adjusting for movements of our head and eyes so that we experience a stable visual world is an example of such revision. Revision lets us watch a French film dubbed in English, not noticing the discrepancy between lip motions and sounds heard. All these editorial processes are occurring over large fractions of a second, during which time various additions, incorporations, emendations, and overwriting of contents can occur.

Spatially and temporally distributed content fixations in the brain (such as visual, auditory, and locomotor) are precisely locatable in both space and time, but their onsets do not mark the onset of consciousness of their content. These content discriminations produce something rather like a narrative stream, or sequence, subject to continuous editing by many processes distributed around the brain. There are ongoing multiple drafts. There is never a final draft, just a current version. This applies not only to input and sensing but also to acting, moving, speaking, and responding to the question "What do I do next?"



The "common-sense" idea has been that a where-it-all-comes-together hierarchy responds to a watcher and then instructs a director who commands output. This view has been decisively replaced by the idea of many competing parallel streams of input and output that are constantly being compared, sorted, and tested for appropriateness. The interpretations and actions that "work" or "are appropriate" rise to the surface to constitute our subjective experience.


The "extended present"---during which multiple drafts can be tested, expanded, contracted, and rearranged to make subjective time---is different from physical time. 19 Different states of vigilance, or mind-altering drugs, appear to change the depth of the conscious present. In his description of being under the influence of mescaline, Aldous Huxley mentions greatly enhanced visual and auditory impressions, as though sensory activity were continuing to reverberate more strongly beyond its normal limits, perhaps changing the duration of the conscious present. Depressed people sometimes describe loss of visual intensity, colors appearing flat and washed out, as though sensory activity had been curtailed and the conscious present shrunk. The ultimate damping down might occur in sleep, when the conscious present effectively shrinks to nothing, and subjective time becomes the same as the stream of physical time.

Sleep and Other Altered States of Consciousness

One route to getting a handle on the experience of consciousness is to contrast it with what might be going on when we either are not conscious or are in another state that feels very different. During a substantial fraction of all our lives, our brain is not computing an "I" but is doing something else: sleeping and dreaming. The two systems that alternate to control waking and sleeping roughly mirror the division of labor between the sympathetic (noradrenergic) and parasympathetic (cholinergic) branches of our autonomic nervous system. In the waking state, aminergic neurons in the brainstem that radiate to the cortex cause alertness by literally "spritzing" the brain with neuromodulators (see Figure 12-5) at the same time that the sympathetic (arousal) part of the peripheral autonomic nervous system is activated. Sleep occurs when this system is suppressed as a cholinergic spritzing system, also shown in Figure 12-5, brings up parasympathetic (resting) nervous activity. Nerve cells in the rostral hypothalamus also play a central role in this sleep generation. 20 During sleep, the brainstem is instructing your spinal cord that you can't feel and you can't move, 21 and the cortex is disconnected from sensory input and motor output.



Waking and sleeping are controlled by two hierarchical systems spanning the brainstem to the cortex, systems so reliable that it is as rare for us to gain awareness that we are dreaming when we are dreaming as it is for us to hallucinate when we are awake.


Figure 12-5

During the waking state, a noradrenergic system is activated to maintain arousal of the cortex (solid lines). During sleep, this system is suppressed as a cholinergic system predominates and induces periods of REM sleep (dashed lines).

Stages of Sleep and Dreaming

A typical night of sleep proceeds in four or five cycles, each usually described as having four stages. A cycle starts with light dreaming (called REM sleep, for rapid eye movement), during which heart and respiration are active and vivid hallucinations and emotions are occurring. It then progresses through two intermediate stages to stage 4, wherein there is no dreaming, and body respiration and temperature are at their lowest point. This non-REM sleep is known to boost immune function. The cycle ends with a return to REM sleep. REM sleep appears to be essential for long-term maintenance of the brain; animal and human subjects deprived of it eventually die. Brain imaging experiments show that REM sleep correlates with the activation of several areas, particularly the amygdala. 22

Why do we dream? No one is sure, but one theory is that dreaming plays some role in the consolidation of memories, experiences, and emotions. 23 One particularly fascinating study shows that in rats, hippocampal cells that show place-specific activity during waking are selectively activated in subsequent sleep, possibly participating in a memory consolidation process. 24 During dreaming, the brain may be making up stories to make sense of emotions. (Human studies show that the emotions are twice as likely to be unpleasant than pleasant.) The sleep researcher Alan Hobson offers an "activation-synthesis hypothesis" that suggests that cholinergic and other activation rising from the brainstem stimulates the cortex relatively nonspecifically, causing it to confabulate the storylines of dreams, which are frequently bizarre and disconnected, to go with the underlying chemistry. Such a model is not unreasonable, given the examples of confabulation exhibited by split-brain and anosognosic patients mentioned in earlier chapters. The brain is essentially insane during dreams, exhibiting bizarre cognitive features such as discontinuity, incongruity, and hallucinations. Anyone who displayed the sort of mental activity during wakefulness that goes on during dreams would be judged psychotic.

What does all of this have to do with understanding consciousness? One interesting suggestion is that wakefulness and REM sleep may be very similar states, subserved by an internal thalamocortical loop that proceeds during both. 25 The idea is that the thalamus and cortex are expending most of their effort talking back and forth with each other. At no time does sensory input constitute more than a fraction of what is going on. The major difference between wakefulness and sleep lies in the weight given to incoming sensory input. In this model, consciousness is essentially a closed loop, driven by intrinsically active cells. Thus we have the image of a central churning processor that can proceed in a manner either dependent on or independent of sensory input---an internal babbler that keeps on going in dreams. Our brains would be continuously spinning a self, automatically generating a web of perceptions, words, and deeds. 26 During waking, this spinning would be guided by the environment.

Mystical Experience

The uncoupling of the brain from sensory input and motor output during sleep is reminiscent of another altered state: mystical experience. Mystical experiences involving a very different sense of "I" have been reported throughout history by individuals in different religious traditions. 27 They frequently are accompanied by feelings of deep conviction, ineffability, and cosmic unity. The science journalist Timothy Ferris suggests possible correlations between these experiences and processes in the brain: The conviction is that something powerful has been learned beyond the world of the senses (an uncoupling of what the brain is doing from sensory input, as in sleep?). Ineffability is the experience being beyond words (reflecting a core symbolic device lying at deeper levels than the modules of language that provide it with input and output?). The cosmic sense of unity is that everything contains the seed of everything else (is this the mechanism that makes a unified mind out of the disparate parts of the human brain?). Might enlightenment correspond to breaking through the barrier of language, to awareness of the mental modules responsible for representing the many functions of the brain as a whole? Or could meditation systematically reduce mental activity to the point of being devoid of content while still aware---a state independent of perceptions and actions?



Perhaps mystical phenomena have more to do with the internal architecture of the brain than with the phenomena of the outer world, corresponding to a mind detached from the thoughts that are its normal objects.


We enter another sort of state in which we are not computing an "I" when we are made unconscious by anesthesia. 28 Interestingly, human subjects show implicit memory of a random word list read while they are unconscious; thus, if presented with another random list, they are more likely to select the words that were read without knowing why. When the brain activity of unconscious and conscious subjects is imaged during reading of the first list, both show increased activity in verbal memory areas, but activity in the mediodorsal thalamic nucleus is much higher in conscious subjects. 29 This suggests that distinctions between conscious and unconscious memory may depend on the presence or absence of correlated thalamic activity, and it reinforces the idea that thalamic activity is a necessary component of the conscious state.

Humor and Laughter

Does laughter give us a window on the mind? Ferris, whom we just mentioned, offers the interesting notion that laughter arises from interaction of two programs in the brain: an Apollo program, which constructs plausible models of reality, and a Pan program, which challenges such models. 30 The Apollo program is sober, responsible, and creative, but the models it creates always have flaws. The Pan program is irreverent, skeptical, and playful. Hence we have a god that builds models and a god that mocks and tears them down. A laugh is the energy released when the Pan program spots a potentially dangerous error in a model crafted by Apollo, but the error turns out to be harmless. The stress first aroused is then quickly dispelled through the physical manifestations of laughter. The idea is that we take pleasure in laughter both because we enjoy the release from anxiety that it provides and because the brain feels pleasure in discovering incongruities between perception and reality.

Possible locations for the Apollo and Pan programs are suggested by observations on anosognosic patients (see Chapter 7). Patients with right hemisphere damage who deny the possession of some parts of their body are much more likely to confabulate rationalizations of their situation. It is as though a right hemisphere locus of emotions involved in detecting and reacting to anomalies and discrepancies has been disabled. The left hemisphere, whose job is to make up plausible stories, is no longer edited or monitored by the right hemisphere. 31 Another dissociation into component selves is observed in split-brain patients who confabulate with their linguistic left hemisphere the reason for a behavior that is initiated by an instruction directed to their nonverbal right hemisphere. Observations such as these remind us again that consciousness is far from being a unitary phenomenon.

Neural Correlates and Models of Consciousness

Much as we earlier asked about the neuronal correlates of our visual experience (see Chapter 8), we would like to know the explicit neuronal correlates of other aspects of our conscious experience. Are there specific neurons whose activity mediates consciousness, whose firing gives rise to the current content of our consciousness? We are looking for mechanisms that support the brain's awareness of a small fraction of all the information available to it---information that includes our experience, identity, sensations, perceptions, action, attention, memory, emotional states, and so on. Conscious awareness is just the tip of a large, unconscious iceberg. Its most common metaphor is that of a spotlight surrounded by a fringe, or halo, of lesser illumination.

You might suppose, given the increasing respectability of efforts to look for neural correlates of consciousness, that a well-defined problem was being pursued. This is far from the truth, because in most cases we lack a thorough or rigorous description of what it is we are trying to explain. Before we can make much headway regarding neural correlates of consciousness we need to specify what varieties of consciousness we're talking about. At a minimum we must distinguish, as outlined in Chapter 1, "consciousness" from "self consciousness" from "reflexive self consciousness." The latter consists of the internal simulation of self-reference---of thinking about thinking---that appears to be unique to humans. Humans have a variety of "meta" levels of cognition, of conscious states that think about each other (such as the ego states mentioned in Chapter 7) or about lower levels (such as the primary and secondary emotions described in Chapter 10). Furthermore, our consciousness is a constant mingling of our present experience with the past memories evoked by that experience. An underlying self-system appears to interpose itself as an editor of our perceptions, as noted in Chapter 8, a system that can alter or delete items that threaten it. 32

Determining what nerve-firing patterns correlate with even the simplest of these varieties of consciousness is a daunting task. Many so-called neuronal codes have yet to be cracked. At a fundamental level we have to know, for example, what the firing of action potentials by single nerve cells in our brain means. It has commonly been supposed that the information is contained just in the firing rate, the number of action potentials sent down its axons in any given period. Recent work has suggested, however, that firing patterns may have clustered substructures, or temporal codes, that communicate several different meanings at once. It is also likely that the meaning of an action potential train is affected by whether it is in synchrony with the action potentials of other cells. 33



We have the technical problem that we can record the detailed activity of only one neuron or a small number of neurons at the same time. This is like trying to decipher a television video image one pixel at a time while the image is constantly changing. 34 Remarkably enough, despite this, the activity of some cells in the visual cortex can be correlated with subjective visual perception. 35 We also can use imaging techniques (such as EEG, PET, and MRI) that sample the activity of many nerve cells without giving much detail about each, but these do not yet provide the resolution required to define functional splotches, blobs, or columns of cells that have dimensions of less than 1 millimeter. What they can give us is gross neuronal correlates such as those showing activation of motion-processing regions of the visual cortex. 36


Brain Structures Required for Conscious Awareness

What, then, is it that binds together, and selects from the many assemblies of neurons, the awareness of the moment? Modern answers to this question of how brain cells might generate behavior or consciousness 37 often starts with a suggestion by the Canadian psychologist Donald Hebb: that nerve cells related to a particular function form a so-called cell assembly, a sort of "three-dimensional fishnet" that can be dispersed widely through the brain. 38 As an analogy at the level of human culture, consider the residents of a large city who organize themselves into different interest groups: families, the Lutheran Church, a motorcycle gang, a university's alumni club, an environmental group, bridge foursomes, and so on. These social assemblies are interconnected by overlapping memberships. Any particular assembly reaches the threshold of being active as a group (in other words, it meets) only a small fraction of the time. And some of these groups may occasionally join together in larger associations to address common interests. Most individuals belong to many such groups.

What evidence is there for such nerve cell assemblies? The closest we come is in observations that neurons activated by the same object in the world discharge in synchrony. 39 Recall that this correlated firing was proposed in Chapter 8 (in the section"A Binding Process Underlies Visual Perception") as a solution to the binding problem of how different aspects of a visual stimulus are organized into a perception. Several observations suggest that the thalamus is needed to bring these bound assemblies to awareness. It sits at the center of the radial organization of the brain, much like the hub of a bicycle wheel, which connects to the rim of the wheel with a large number of spokes (see Figure 12-6). Large areas of cortex can be removed without abolishing awareness (as in patients who have lost most of their frontal, occipital, temporal, or parietal lobes yet remain aware), but bilateral lesions that occur in the relatively small intralaminar nuclei of the thalamus when its blood supply is interrupted do cause loss of conscious awareness. This area makes reciprocal connections with nearly all parts of the cortex. 40



Different cortical areas have different weights in contributing to consciousness and to subjective "what it feels like" experience. The temporal lobes, and in particular the amygdala, appear to be central. The most profound disturbances in consciousness are those that generate temporal lobe seizures. Nothing dramatic happens to consciousness if you damage the frontal lobes, or portions of higher sensory or motor areas. The temporal lobes could be viewed as being an interface between perception and action, where the significance or meaning of things to the organism is being determined.


How might the intralaminar nuclei interact with other cortical areas in generating consciousness? The neurophysiologist Rodolfo Llinas has suggested that the intralaminar nucleus is the seat of a scanning mechanism that operates every 12.5 milliseconds to send out a series of overlapping 40-cycle-per-second (hertz) signals (these are bursts of action potentials occurring every 25 milliseconds that sweep across the brain). These entrain synchronized cells in the cortex that are currently recording sensory information to their own rhythm (see Figure 12-6). The synchronized cells then fire a coherent wave of messages back to the thalamus. The phase of the 40-hertz signal can be reset by new input. Llinas likens the wave of nervous impulses, radiating out from around the intralaminar nucleus to overlying parts of the cortex, to the central arm of an old-fashioned radar screen, illuminating each object in its path as it moves. The responses received by the thalamus within one cycle of its scan are perceived as a single moment of consciousness. A succession of such moments are created so fast that they seem to be continuous. In this model, consciousness is the dialog between the thalamus and the cerebral cortex, as mediated by the senses. 41 The thalamus playing the central role seems logical, because it is the structure through which all of the sensory information coming to the brain must pass.

Figure 12-6

One model for how different aspects of a conscious experience may be bound together. Two systems linking thalamus and cortex are proposed. The first (dashed lines) is a series of closed oscillating loops between specific sensory or motor nuclei of the thalamus and the regions of cortex to which they report via layer 4 of the cortex. Within each modality (visual, auditory, somatosensory, and so on), the firing of cells reflecting the relevant percept are coordinated. Each of these is then bound or recruited into a larger unison by a second, more global assembly of loops between the thalamus and layer 1 of the cortex (solid lines). These sweep as waves of activity from the front to the back of the cortex, as indicated by the arrow at the top, starting every 12.5 millisecond. Each of the waves has a frequency of about 40 hertz, corresponding to bursts of action potentials occurring every 25 millisecond. Each of the sweeps that start every 12.5 millisecond corresponds to one "quantum" of conscious experience.

It is interesting that coherent, 40-hertz electromagnetic activity is strong in people who are awake or in rapid eye movement (REM) dreaming sleep but is very reduced during deep sleep. It is reset by sensory stimuli in the waking state, but not during either REM or deep sleep. 42 This is one basis for the suggestion that REM sleep might be a correlate of mental awareness, reflecting resonance between thalamocortical-specific and nonspecific loops. The specific loops may give the content of cognition, and a nonspecific loop the temporal binding required for the unity of cognitive experience. This system would be modulated by an arousal system, known since the 19th century, that originates in the brainstem reticular core and projects through relays in the thalamus to the cortex. Electrical stimulation of this core can enhance the 40-hertz signal as well as the synchronization of responses in the visual cortex. 43 A word of caution is necessary here: We don't really have evidence yet that these 40-hertz signals are any more significant than the 60-hertz hum of the power transformer on your stereo sound system or personal computer. These signals may just be saying that the system is operational, not telling us much about its specific states. 44

Generating an Apparent "I"

There is a growing consensus on what models for the mind would be appropriate; a view consensus shared by workers in psychology, neurobiology, artificial intelligence, anthropology, and philosophy. All see the mind as a "pandemonium"---a collection of specialized agents acting in parallel, communicating and competing with each other to generate perceptions and behaviors. The unifying idea is that the brain is essentially a Darwin Machine, an array of many modular processors working all at once, without any single center or processor (the "self") running the whole show, 45 from which control and consciousness emerge. Previous chapters have already mentioned Darwin Machine models for skilled action and language generation.

What is being sought is a plausible model for how we generate an apparent "I." Many have remarked on the relatively slow, awkward pace of conscious mental activity, and on how little can be held in awareness at one time, compared with vast amount of processing we know goes on. And some have long suspected that this might be because the brain was not really designed---hardwired---for such activity. Human narrative consciousness in particular might then be the activity of some sort of serial virtual machine implemented on the parallel hardware of the brain. This presumably would have been accomplished by the mythic mind that developed language and thus finally tamed the parallel pandemonium of the present-centered archaic sapiens brain so that very long sequences of verbal meaning could be developed and retained.

In this model, the contents of our awareness at each moment are the current winners from among a horde of competing specialists' reports. Most of these specialists were not designed for specifically human actions such as speaking but for the myriad components of perception, action, and emotions characteristic of higher vertebrates. Perhaps the most accessible metaphor is William Calvin's railroad marshaling yard, described in Chapter 9, in which parallel tracks all connect to one exit track. Each of the parallel tracks is a candidate sequence for perception or action, and a competition determines which one will make it to the exit. The brain produces a virtual center---the illusion of a central viewpoint---just as physicists find it useful to represent the earth's gravity as an attraction to the center of the earth, whereas in fact it is a function of interactions between individual atoms across the whole planet. Our consciousness is composed of many functional subcomponents, but nevertheless we have a unity of conscious experience and the virtual equivalent of a narrator. 46



The apparent unity and continuity of our experience may be generated like the emergent behavior of a mob, much as some flocks of birds maintain cohesion without having any single leader, or like the "hive mind" of bees, who become increasingly activated and then swarm when enough scouts have returned to the nest reporting a favorable destination. 47 We could also invoke the image of a choir of individual voices coalescing into a synchronous chorus, having risen from the cacophony.


Introspection and the Self System

At several intervals throughout this book, we have tried simple exercises on sensing, moving, thinking, and feeling that involve time scales of less than 1 second. They may have permitted you to notice some brain processes that you are normally unaware of. This chapter has reinforced the point that during this period, many normally unconscious agents can massage the self or "I" that emerges as our subjective experience. We are far from being human analogs of a camcorder, using rote neural hardware to link the onset of stimulation and the appearance of a conscious image. A variety of experiments have demonstrated that unconscious processes can bias our perceptions in the interval before they become conscious. After rote processing of elemental chunks of information, as described in Chapter 8 for visual input, we appear to interpose an additional strategic delay that allows for checking and editing the developing sketch or draft, so that elements that might threaten our underlying self system can be altered or deleted.



When incoming information is threatening to the self system, a dissociation can appear between our conscious report, which denies or ignores the potential threat, and behavioral indicators of unconscious processing (such as changes in skin resistance, indicating autonomic arousal). Denial reaches an extreme in anosognosic patients, who, in defense of their self system, might report that nothing is wrong with a paralyzed left arm.


Most of us, after the first 100--200 milliseconds of processing a new input or idea, permit an additional delay of 400--500 milliseconds for strategic editing and revision. This is the time window we were tuning in to during the exercises in Chapter 9 on movement and those in Chapter 10 on dissociating thoughts and feelings. By slowing down enough to notice quietly the flickerings in our thinking and body sensations during this period, we can engage in a sort of "attentional training" in which self operations during the strategic delay can be noted rather than ignored (see Figure 12-7). 48 Such training is a central component of some meditation techniques. The reason why introspective techniques are a core of many cognitive therapies for modifying behaviors is that the observation of feelings associating with thoughts, or the coloring of perceptions by the self-system permits "editorial" modifications of these processes that are not possible if they remain unobserved. Like the Russian dolls that nest inside each other, we can ascend to the level of metacognition, a state of "thinking about thinking."

Figure 12-7

A depiction of time frames of unconscious and conscious experience, suggesting that introspection techniques can expand conscious awareness into the time interval during which unconscious editing normally occurs, making it possible to observe such editing. The time frames shown as very approximate.

Human efforts to think about thinking have a long history, and it is interesting that what modern psychology and cognitive neuroscience say about how the mind works can be taken as a restatement, in modern form, of what Buddhist psychology has been saying about the nature of cognition for the past 1500 years. The labels and lists that are supplied by Buddhists and cognitive neuroscientists differ, but many of the underlying fundamentals appear to be similar. Buddhist texts carefully describe the "co-dependent arising" of observer and object, and of sensing and acting, and emphasize the futility of trying to explain one apart from the other. Points about the relativity of knowledge that we have raised here are explicitly addressed. Meditation techniques best described as "mindfulness" or "awareness" are used to sense the transitory, shifting nature of all phenomena and the various guises of the ego self, "I," that we take to be who we are. An observer is established that is aware of the succession and variety of ego states that we have considered. 49 The attitude of just observing is cultivated: observing the successive graspings or attachments to this or that goal or desire, observing the successions of radically different personalities that can inhabit our bodies from moment to moment. From this state of mindfulness or awareness, one grows to appreciate more and more the relativity of all things, which is called groundlessness or "sunyata" in the Buddhist tradition. This state of non-ego attachment is said to allow energy for compassion, empathy, and service to arise spontaneously.

This position, poise, or process is a logical stance for addressing and dealing with the sorts of questions raised in this book on how the mind works. In the absence of mindfulness we are likely to act from some sort of unconscious ego position as we study and evaluate phenomena. This can include having a set of unconscious starting assumptions or positions that bias how we look at things without our being aware of it (such as assuming the clean separateness of sensing and acting or of subject and object). We don't have to look far for examples of this. Even though many neuroscientists know Cartesian dualism to be discredited, they still are practicing dualists. Their research does not fully integrate an appreciation of the constructive interchange that is involved in cognition, the constant co-development and co-shaping of a nervous system and its environment. Being aware of the many selves, egos, or biases that we generate is a better basis for action in the world than being unconsciously wrapped up in one of those selves and unaware of the processes that generated it. Put more simply, it can be more useful to be conscious, than unconscious, of the relativity of the self that is driving you---the mind in place at any one time.

Computer Metaphors

No discussion of models of the brain would be complete without mention of what has become a most pervasive model: the metaphor of the brain as a giant computer, with neurons being the hardware circuit elements. It has been postulated that the way computers carry out functions might be relevant to how the brain does similar things and that conversely, what brains do might serve as a model for designing more intelligent machines. A recent approach has been to ask how what we know about the workings of a brain and the organism it serves might inform the construction of computers or robots that have similar properties. In particular, because we know that the development and behavior of an organism are shaped through an evolutionary process of replicating what trial and error has shown to work, why not build computers that "try" to behave in a complicated way using these same Darwinian mechanisms? 50

The study of artificial neural nets modeled on computers is one example of this approach. An army of neuron-like elements are linked together in a parallel array with many interconnections. Such nets can be taught to recognize words, generate speech, distinguish male and female faces and their emotions, diagnose heart conditions from EKG data, or drive a truck down a freeway. During a training phase, intermediate results are fed back into the system to let it know how it i doing. 51 Such neural nets have even been used to simulate the effect of brain damage on reading and to model the development of language capabilities. 52 Because there is a lot of redundancy in the network, many of its elements can be removed with only a small effect on overall performance.

Another approach has been to play with the design of "creatures," small mobile robots attempting to mimic the behavior of a simple animal such as a cockroach. These are constructed from a number of activity-producing subsystems, each of which can individually connect sensation to action. The creatures are tested in the real world. (One example is a simulated cockroach with a neural net of 78 neurons and 156 connections 53 ). Learning rules are applied that select and reinforce the connections that work best in guiding the insect to wander, follow edges, and find and consume patches of food. This approach to trying to understand whole embodied agents interacting with a world, whether these agents are ourselves or simpler machines that we build, is an antidote to placing undue emphasis on the brain by itself. The generation of action and the problem solving done by the brain simply can't be separated from the environment it must continually exploit, interact with, and be transformed by. Mind includes the interactive loops an organism makes with its environment---a point raised in several previous chapters and developed further in the final chapter.


We have taken several different trajectories in this chapter, trying to frame the central issue of explaining our self consciousness. Even to engage the issue requires that we sidestep a number of arguments-in-principle that the problem of consciousness can never be solved and that we assert, instead, that it doesn't hurt to keep trying until a more convincing case for the impossibility of our quest has been stated. We can't help but feel that we are getting closer as we observe the activity of our prefrontal cortexes while they carry out tasks involving attention and working memory, or when we devise clever experiments that show how our brains can rearrange time and space. We can see, through studies of thalamus-cortical interactions during sleep and waking, as well as through brain lesion and brain imaging studies, some plausible candidates for neural correlates of consciousness.

Where does all this leave us, in trying to think about the "I" of our self consciousness? It seems perfectly natural for us to think that our brains see and feel the world as "we" do---that they chunk concepts and perceptions according to the linguistic conventions "we" use. We encourage this inclination when we play with introspective exercises of the sort that have been sprinkled throughout this book. What we have to keep in mind is that such exercises may be dealing with only a very limited kind of access, for our brains basically are survival machines whose sophisticated perceptual and motor modules were shaped by evolution long before the recent add-on of linguistic abilities. The awareness we try to explain with these abilities is probably just that small fraction of our total brain/body activities that has proved useful for survival and reproduction. There would be no point in our consciously attending to the vast number of subterranean activities that massage and process our perception and action, for example, to directly sense knowledge of animals, persons, or tools being stored in different parts of our temporal lobes. What our brains display to our awareness is like the display panel on our car's dashboard: indicators that inform us and can make a difference in how we drive the car. 54 Our linguistic habits for "chunking" reality bear no deep resemblance to the parallel and distributed brain mechanisms used in information storage and retrieval. Our main hope is that through new techniques such as more sophisticated monitoring of brain activity, we can gain better insight into the relationship among the local environment, brain activity, and the patchwork construction of our sense of self---between our "I" and the array of background processes going on. We have to accept, for the present, the jarring idea that the "I" inside our heads is a virtual "I" constructed anew in each moment from a cacophonous interaction among brain, body, and world.

At this point, you may feel you have been left with something like the proverbial Chinese meal that doesn't stick to your ribs. There is no clear "So that's how it works!" The chapters on perceiving, acting, emotional, linguistic, and now conscious minds have left us with bits and pieces waiting to be assembled into a whole. Many of us take it as an article of faith that it will someday be possible to do this; others think not. We do have some models for our consciousness that seem worth further pursuit and testing. More immediately, the information we have assembled thus far has consequences for how we view and comport ourselves in the modern world, and this is the subject of the next chapter.

Questions for Thought

1. Our prefrontal lobes play a central role in awareness, attention, and working memory. Are awareness and attention the same thing? If not, how would you distinguish between them? (Think about keeping your eyes directed at the middle of a clock face while you are being asked to describe one of the specific numbers for a time of day.)

2. Cite some of the evidence that supports the following contention: Our subjective experience of being a central observer in a constant world actually reflects the construction of a "best guess" of what that world is like from a series of fleeting "snapshots" (of visual, auditory, proprioceptive, and other sensory inputs)---snapshots that sample very narrow windows of time and are constantly renewed. What are some of the ways in which this process can be misled?

3. Consciousness can be abolished by lesions in a very small region of the thalamus. Does this mean that these regions are the seat or locus of consciousness?

4. This chapter outlines a "Darwin Machine" model for generating the apparent unity and continuity of our experience. In this model, our current thread of awareness is the winner that has emerged from a competition among many alternative options. Can you think of any other models---models that do not have the flaw of ultimately suggesting a little human, or homunculus, inside our heads that in turn has to be explained?

Suggestions for further general reading

Dennett, D.C. 1991. Consciousness Explained. Boston: Little, Brown. This chapter's discussion of time and space in the brain (noting the phi and metacontrast effects), and its account of the futility of asking "Where does it all come together?" was drawn largely from Chapters 5 and 6 of Dennett's book.

Flanagan, O. 1992. Consciousness Reconsidered. Cambridge, MA: M.I.T. Press. This book is a broad introduction to philosophy of mind and general models of brain mechanisms of consciousness. It discusses the idea of consciousness as arising from Darwin Machine--like mechanisms.

Hobson, J.A. 1994. The Chemistry of Conscious States: How the Brain Changes Its Mind. Boston: Little, Brown. This account of Hobson's sleep research also gives his more general views on how both "bottom-up" (from brainstem to cortex) and "top-down" (from cortex to brainstem) processes shape our cognition.

Ferris, T. 1992. The Mind's Sky---Human Intelligence in a Cosmic Context. New York: Bantam. This chapter notes some interesting speculations on altered states of consciousness that are offered in Part 2 of Ferris's book.

Churchland, P.M. 1995. The Engine of Reason, the Seat of the Soul: A Philosophical Journey into the Brain. Cambridge, MA: M.I.T. Press. This book presents the brain as a neurocomputational device and gives an accessible account of using artificial neural networks, simulated by computers, to model brain processes.

Reading on more advanced or specialized topics

Gazzaniga, M.S., Ivry, R.B., & Mangun, G.R. 1998. Cognitive Neuroscience. New York: Norton. Chapter 11 of this book deals with the role of the frontal lobes in awareness, executive functions, and complex behaviors.

Ungerleider, L.G., Courtney, S.M., & Haxby, J.V. 1998. A neural system for human visual working memory. Proceedings of the National Academy of Sciences USA 95:883--890. A comparison of pathways in the monkey and human brains that regulate visual working memory, with different areas of prefrontal cortex serving spatial and object memory.

Llinas, R., & Ribary, U. 1993. Coherent 40-Hz oscillation characterizes dream state in humans. Proceedings of the National Academy of Sciences USA 90:2078--2081. A proposal that an oscillation in cortical activity that occurs in the awake and rapid eye movement (REM) sleep states is a correlate of cognition.

Newman, J. 1997. Putting the puzzle together. Towards a general theory of the neural correlates of consciousness. Journal of Consciousness Studies 4:47--66. This article expands on a model that makes the thalamus a central part of an extended activating system that links cortex and lower brain centers to generate the conscious state.

Ramachandran, V.S., & Hirstein, W. 1998. Three laws of Qualia: what neurology tells us about the biological functions of consciousness. Journal of Consciousness Studies 4:429--457. This is a fascinating account of neurological syndromes in which consciousness seems to malfunction, suggesting regions of the brain that are central to maintaining a conscious state.

1. This exercise is taken from Dennett, 1991, Ch.2

2. Searle ,1992.

3. A succinct summary of the basic positions in contemporary philosophy of mind is given in Epstein and Hatfield, 1994.

4. Churchland and Sejnowski, 1992, pg. 2

5. These points are made by Flanagan, 1992, Ch. 1., in arguing for a "constructive naturalism."

6. Beardsley, 1997b.

7. Raichle, 1993; Baddeley, 1992; Baddeley, 1993

8. Goldman-Rakic, 1992; Wilson et al., 1993; Horgan, 1993b; Tovee and Cohen-Tovee, 1996. More rapid magnetic resonance imaging shows sustained activation of different areas of frontal cortex for face memory and letter memory, see Goldman-Rakic, 1997. A recent account of working memory systems and their locations is given by Smith and Jonides, 1999: Storage for verbal materials activates Broca's area and left-hemisphere supplementary and premotor areas; storage of spatial information activates the right-hemisphere premotor cortex; and storage of object information activates other areas of the prefrontal cortex. Selective attention and task management active the anterior cingulate and dorsolateral prefrontal cortex. See also Levy and Goldman-Rakic, 1999.

9. Wickelgren, 1997a.

10. Koechlin et al., 1999.

11. Watanabe, 1996.

12. Rao et al, 1997.

13. Williams and Goldman-Rakic, 1995.

14. Berns et al., 1997. These experiments are reminiscent of the work of Bechara et al., 1997, mentioned in Chapter 9.

15. Tanenhaus et al., 1995.

16. Large portions of the material in this and the following sections were taken from Dennett,1991, chapters 5 and 6.

17. Humphrey, 1992, pg. 184

18. Greenwald et al., 1996.

19. Humphrey,1992, Ch 24

20. Sherin et al., 1996.

21. Many of the details in these paragraphs are taken from the book by Hobson, 1994, an engaging account of sleep research.

22. Maquet et al, 1996.

23. Hobson and Stickgold, 1995.

24. Wilson and McNaughton, 1994.

25. Llinas and Pare, 1991; see also Steriade et al, 1993

26. Dennett, 1991, Ch. 13.

27. Most of the material on enlightenment and humor is taken from Ferris, 1992.

28. Fiset et al., 1999, have described brain regions whose activities are changed by anesthesia. The largest relative decreases in activity are observed in regions previously implicated in the regulation of arousal, performance of associative function, and autonomic control.

29. Alkire et al., 1996.

30. Ferris, 1992, pg. 117 ff.

31. Ramachandran, 1995.

32. Claxton, 1996.

33. Stevens and Zador, 1995; Ferster and Spruston, 1995.

34. Deadwyler and Hampson, 1995.

35. Leopold and Logothetis, 1996

36. Kolb and Braun, 1995.

37. See the book by this title edited by Koch and Davis., 1994. The Handbook of Brain Theory and Neural Networks (Ed. M.A. Arbib, M.I.T. Press, 1995) is a massive tome devoted to this area.

38. Hebb, 1949

39. Singer, 1995; Roelfsema et al, 1997. See Gray and McCormick, 1996, and Traub et al., 1996 for proposed mechanisms of synchrony.

40. Bogen, 1995

41. Blakeslee, 1995a.

42. Llinas and Ribary, 1993.

43. Steriade, 1996; Munk et al., 1996.

44. Koch, 1993. However, see Tallon-Baudry and Bertrand, 1999, for review of converging evidence that induced gamma activity in the range of 40 Hz correlates with the construction of object representation.

45. Flanagan, 1992, pg 191

46. Calvin, 1990, pg. 86.

47. Kelly, 1994.

48. See Claxton, 1999, on "moving the cursor of consciousness" to shorter times, allowing more processes to become visible.

49. For a discussion of mysticism, mind, and consciousness, see Forman, 1999.

50. see Varela et al (1992, Chapters 3, 5 and 9), and Dennett, 1991.

51. The books by Kosslyn and Koenig, 1992, and Churchland and Sejnowski, 1992, give examples of the use of nerve networks to model nerve information processing. Churchland, 1995 has a very accessible popular account.

52. Hinton et al., 1993; Elman et al, 1993.

53. Beer et al, 1991

54. Clark, 1995. This essay, "I am John's brain" makes the points in this paragraph as if they were being spoken by a brain trying to explain the "I" that normally takes itself to be much more than it really is. You might try going through the points listed as an exercise on yourself, as in "I am a survival machine whose spohisticated perceptural and motor modules were shaped by evolution long before the recent add-on of linguistic abilities." For me, this was a jarring and interesting experience.

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