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Altered States of consciousness

Altered States of consciousness

Susan Greenfield

EVERYONE would claim to be an individual; everyone would claim they were different from everyone else. And yet the macabre idea is that if we all took our brains out, they would look pretty much the same. Just looking at the brain is not very helpful. If you were to look at the heart, you would see it pumping, and if you saw the lungs, they would be inflating like bellows, and that would inspire some idea as to how both function. But if you look at the brain, it does not move. The brain has no intrinsic moving parts, so you cannot actually guess by looking at it how it is working, and, even worse, you certainly cannot tell how one person might be individual from another.

What you can see clearly, if you do look at the brain, are the different bits to it. So you have that cauliflower thing on the back, like a little brain, that is called the cerebellum; you have the stalky part, continuous with the spinal cord, and the “cortex,” so named after the Latin for bark because it wraps around the brain like its arboreal namesake around a tree. But what do these structures actually do? That is the first question if we are to understand how brains work or how an individual’s brain works: How are the functions that we have, the awesome range of mental activities that we are capable of performing, embedded and related to this structural variation?

One idea is that each brain region has its own function–that the brain is really a set of minibrains. But with this idea we would simply enter an infinite regress that miniaturizes the problem but does not solve it.

Instead, we know that the brain functions like an orchestra, where different instruments each play a different part. Or it is like a stew or some complex food, where each ingredient plays its own part. With the advent of scanning techniques, this holistic organization has become clear. In a test a subject was asked to passively view, listen to, and speak words, or to generate verbs–all fairly subtle. But even a single “function” like language is, in terms of the brain, subtly different. Scans show that the brain divides different aspects of the task according to constellations of different brain regions. But the important take-home message is that there is no single brain area lighting up for language, and there is certainly, no single brain area lighting up even for aspects of language. The regions are working like instruments in an orchestra or like ingredients of some complex dish.

How is this organized? Let us look at how the brain is put together, because there is far more to your brain than mere gross brain regions. We start with consciousness. That is the ultimate and blanket function of the brain. What is consciousness? We will come to that later, because it is a very hard question. It is the first-person world as it seems to you. It is the world that no one can hack into directly. No one knows what you are experiencing or feeling at this moment.

Of course you can have aspects of consciousness that go wrong, dysfunctions such as depression, schizophrenia, or anxiety that would be described as a constellation of different features that would then be described as forms of consciousness. And language, or your memory, or the way you think, or even sometimes your senses, which can become distorted, are indeed functions of the brain. These can be divided even further; for example, vision has 30 different brain regions in the brain. And even within those regions one has so-called parallel processing, where the brain processes vision in parallel, dealing with form, color, and motion simultaneously. We have yet to discover how those different aspects of vision are brought back together again but we know that it is shared around.

We come then to the brain regions themselves that we have looked at. If you were to open up a brain region, you would arrive at large-scale assemblies of brain cells or neurons. These could be broken down further to isolated circuits. And then, beyond the circuits, you could look at one connection between one brain cell and another, across a narrow gap, which is called a synapse. And across the synapse you could start looking at the machinery that enables one brain cell to communicate with another. And then, finally–and only finally–do you come to the subject of much biomedical science, which is causing such delirium at the moment, and that is the gene. The genes will express a protein that is part of the biochemical machinery for working across the synapse.

So the brain is organized in a nested hierarchy. It is not just a brain region functioning as some final unit; you can break it down further and further and further. Let us look at how far you can break it down and try to discover where the secret of your individuality might lie, because it does not lie at the level of the gross brain regions. Instead, let us look at the networks of neurons that make up a brain region. You have 100 billion neurons, as many trees as there are in the Amazon rain forest. But even more awesome than that are the connections between your brain cells.

You have up to 100,000 connections onto any one brain cell. If you were to count them at one a second in the outer layer of the brain, in the cortex alone, it would take 32 million years to accomplish the task. If you wanted to work out the permutations and combinations, it would exceed the particles in the universe.

Why are they so important? Let us go back again to see where they fit in. I will remind you they are midway between the genes and the gross brain regions. The number of genes you have is about [10.sup.5]. Even if one makes the assumption–which is completely wrong–that every single gene in your body was accounting for a brain connection, you can see that you have about [10.sup.15] brain connections, so you would be out by [10.sup.10]. You just do not have enough genes to determine your brain connectivity. People who hope one day to manipulate their genes so that they are good at housekeeping or cooking or being witty or not being shy–all these other things that people fondly hope they can start targeting with molecular biology–should bear this number in mind. There is far more to your brain than your genes.

I am not saying that genes are not important and I am not saying that if a gene goes wrong you will not have some kind of terrible malfunction. What I am saying is that there is far more above and beyond the single genes that is really important. And I am talking, of course, about nurture, not just nature, because the most marvelous thing about being born human is that as you grow your brain connections grow with you. So, although you are born with pretty much all the brain cells you will ever have, it is the growth of the connections after birth that accounts for the growth of your brain.

The reason this is so exciting, and the reason I have been emphasizing the connections, is because this means, if the connections are growing as you are growing, then they will mirror what happens to you. This means that even if you are a clone (that is, even if you are an identical twin), you will have a unique configuration of brain cell connections that will trigger your reactions to events and will mirror your experiences such that you will see the whole world in terms of your brain cell connections.

Let us look at some evidence for that. London taxi drivers, as you may know, are masters at remembering. Every working day they must remember how to get from one place to another–and not only the configurations of the streets but also the one-way systems and how best to navigate round the streets of London. In a fascinating study, scientists scanned the brains of London taxi drivers and compared them with scans of other people of a similar age. Surprisingly, they found that the hippocampus was enlarged in taxi drivers compared to people of a similar age. Now, could it be that people with an enlarged hippocampus are disposed to become taxi drivers? No, because it was found that the longer they had been plying their trade the more marked this structural difference was. It was a result of what they were doing–what they were physically doing–that their brains had physically changed.

Your brain will relate to whatever you do. Here is another simple example. Human subjects were asked to practice five-finger piano exercises. The study found that with physical practice of even only five days, an enormous enhancement occurred in areas of the brain relating to the digits–just by engaging in five-finger piano exercises. But more remarkable still is a comparable change in brain territory when people were not practicing the piano but were imagining they were practicing. In real terms you can see that brain territory reflects every mental processes, and it is physically measurable, which is why we now wish to shoot down the myth between mental activity and brain activity, as if airy fairy thoughts were something that floated free, beamed in from Planet Zog or somewhere. Everything that happens to you, everything you are thinking, has a physical basis rooted in your physical brain. What we are realizing now is how exquisitely sensitive the brain is to your experiences and what you do, and therefore how it makes you the individual you are.

Even into old age, one’s brain remains continuously “plastic”; that is, it is constantly dynamic, constantly evolving and changing, mirroring whatever happens to you. Sadly, sometimes things can go wrong. For example, dementia (which is a name for confusion and memory loss that characterizes Alzheimer’s disease) is not a natural consequence of aging; when it strikes a brain cell in the vulnerable region, the main part of the cell (the cell body) remains, but the branches (dendrites) with which it makes contact with other cells have atrophy. Therefore, as one becomes senile–and I stress this is not going to happen to everyone, it is a particular illness–it is almost like becoming a child again. As the brain connections are dismantled, you retrace back. Where, just as in childhood the world means more and more, so this time the world means less and less. It means less because you cannot see things in terms of other things anymore because the connections are no longer there.

This prompts me to ask the question–which is not a scientific question at all, but is one I think puzzles many people: What is a mind? We are clear what brains are, but why do people now talk about minds as opposed to brains? I myself do not subscribe to the idea that it is some alternative to the biological squalor that scientists work in. I would like to suggest that mind has a very clear physical basis in the brain. We have seen that you are born with almost all your brain cells. The growth of the connections between cells accounts for the growth of the brain after birth. These connections reflect your experience, and in turn they will influence your further perception so that you see the world in terms of what has happened to you. You are born, in the words of the great William James, into a world that is a “booming, buzzing confusion,” where you judge the world in terms of how sweet, how fast, how cold, how hot, how loud, how bright. You judge it in terms of its pure sensory qualities. But as you get older, the sweet, the bright, the noisy, the loud, the fast, the cold, the hot acquire labels; they become objects or people or processes or phenomena. They have labels, then they have memories and associations attached to them, and gradually you can no longer deconstruct the world (unless you are some brilliant artist) in terms of colors and noises and abstract shapes; instead, you see it with a meaning, a meaning that is special to you. That is how it continues to occur and, as we have seen, the brain connections remain highly dynamic. I think this is what the mind is: the personalization of the brain.

As you go through life the world acquires a highly personalized significance, built up by “hardwired circuits” in the brain. But although we have this mind rooted in personalized circuits in the brain and we therefore see the world in a certain way, this organization is not always accessed.

Let us consider “blowing the mind.” Here people are not using their mind: they are engaged in a mindless pursuit. Sadly, with dementia you are losing your mind on a permanent basis, but, amazingly, some people pay money to lose their minds or “let themselves go.” The very word ecstasy means “to stand outside of yourself.” I think phrases like “lose your mind,” “blow your mind,” “out of your mind,” “let yourself go” are exactly what we are talking about.

But what about accessing the mind or otherwise? I think this is the real challenge to anyone with aspirations to model the brain on a computer and, indeed, to neuroscience in general. Because what I am talking about is of course “consciousness.” Consciousness and the mind are very different things, although of course one will be related to the other. You can lose your mind but still be conscious. Tonight, when you lose your consciousness, I imagine you are not expecting to lose your mind. So the two concepts are separate and can be differentiated. What I want to turn to now–having looked at how the brain is hardwired and how it is reflecting experience–is the much harder question: Why do we sometimes access those connections but sometimes not? For people at a rave, for example, what is actually changing in the brain? That really is the much harder question.

Some people, like MIT AI expert Marvin Minsky, believe that it is going to be possible to build computers that are conscious. My own view is that at the moment biological brains are not like computers–and I am using the word “computer” to mean in general artificial intelligence and not just the laptop. I am not saying that in the future it is not going to be possible, since that would be an arrogant assertion; you cannot say something is not going to happen. But I cannot see how it is going to happen, especially in light of the way people are dealing with the problem at the moment.

First, the brain is based on chemicals. Chemical transmission is the absolute cornerstone of brain function; it is the most basic principle there is, and yet it is not allowed for in modeling. This is important because it gives a qualitative dimension, because you have many chemical messengers that are not found in computer modeling, and, more important, we have access to manipulating those chemicals with drugs. If you are to start manipulating at this level, which is a basic level, imagine how you would be manipulating the mind quite dramatically, and hence perhaps even consciousness, by doing that. Every psychoactive drug works on the principle of modifying chemical transmission in the brain in different ways to different extents. We thus have chemically based events in the brain and we know that if you take a drug like morphine, it will give you a dreamlike euphoria; it is manipulating your emotions. We know it works at the chemical level, and therefore emotions must be chemically based, which is something as yet not modeled in computers. As Stuart Sutherland, the psychologist, once said, he would believe a computer was conscious when it ran off with his wife. It is not good enough to have learning and memory machines. We already have that–a Cray Computer makes a very good learning and memory machine–but it is not conscious. Are we not missing the point here that it is not all about learning a memory and building up the mind? That it is about something that you had before you have a mind even–when you are a one-day-old baby–and that is an emotion?

The final important point about biological brains and computers is the issue of responses. I am sure you may be familiar with the so-called Turing test devised by the great computer pioneer Alan Turing. He said a computer would be conscious if it satisfied the following test: if a person given impartial access to a computer or a person and allowed to ask any question he or she liked could not tell from the answers whether it was a computer or a person, then the computer would have succeeded in being conscious. At the moment there are modified versions of the Turing test because the test is still too hard for a computer to do, but the modified versions make it easy by having limited subject matter to ask the computer. But even then there is no computer yet that has passed the Turing test (although, rather amusingly, there is a human being who has failed it). My own worry is that you do not even have to point to the Turing test because responses are not key. Therefore, you can be conscious without making responses, and, indeed, as we know from speak-your-weight machines, responses can occur without consciousness. The fact that you have a computer that does things is not a testament to the fact that there is a consciousness inside the computer. Instead, we have this wonderful personal world that only you can experience firsthand.

What unique property is in the brain that generates consciousness? It is not the hardwired connections; it is not our learning ability. It might have something to do with chemicals; my own contention is that it has a lot to do with feelings–but then, of course, feelings and consciousness are very similar, if not often synonymous.

So what is the basis for that? Why is it unique to the brain and why is it not possible in machines? We have seen that there is no such thing as a center for this or a center for that, and certainly, therefore, there is no such thing as a center for consciousness. We have seen already that autonomous brains within brains make no sense. Now I can show you, scientifically, additional evidence against this idea. If you look at brain imaging again and give subjects anesthesia, thus removing their consciousness, you can see that no single area of the brain shuts down. There is no one brain area that has just stopped. There is no center for consciousness.

If there is no center for consciousness, where is it? Let me compound this by introducing another concept. Is a dog conscious? If so, what is the difference between dogs and us? And how does that give us a clue as to what is special about the brain for generating consciousness?

More controversially, let us extend the riddle to that of the fetus. It is still a commonly held belief that the fetus is not conscious. But if it is not conscious, when does it become conscious? When does a baby become conscious? At the time of birth? Fine, but when is a baby born? Some babies are born prematurely, and they are conscious. You would not for two months just ignore the baby in the incubator in the hospital and say, “Ah, 40 weeks are up, it is going to be conscious today. Now we can go and visit.” That would not be likely. Even less likely would you do it after the birth and say, “Ah, a few weeks have gone now, it is coming up to six months from birth, it might be conscious.” It does not make sense.

Or is it the manner of birth, squeezing down the birth canal? That is tough on babies born by Caesarean section, if this is the case, because they will never be conscious. Clearly the manner and the timing of birth, because it is so variable, cannot determine, cannot be the trigger, for consciousness. To my mind, it is a much more logical deduction that the fetus must be conscious.

But, for our purposes, we say, “OK, so this is a bit like the problem with cats and dogs or, indeed, rats or other animals: How is it different from consciousness?” When would the fetus become conscious? What would it be conscious of? I think the big problem here, and one that stops us from developing the idea, is that we normally think of consciousness as all or none. I myself defined it as “the thing you are going to lose tonight.” But what if I was wrong? What if instead of consciousness being all or none, on or off, what if consciousness grew as brains grew? What if, therefore, it gradually developed? And so a fetus was conscious, but not as conscious as a child, and a baby was conscious but not as conscious as an adult, and a cat was conscious but not as conscious as a primate, and a monkey was conscious but not as conscious as a human.

If consciousness grows as brains grow, two interesting issues are raised. One is that you as an adult human being are more conscious at some times than at other times. If you think about it, we talk about raising our consciousness or deepening our consciousness–it does not matter which way you go, you can go up or down–consciousness is something that is variable. If that is the case, science finally has purchase on the problem, because, instead of looking for some magic brain region or some magic gene or some magic chemical, we can look for something that varies in degree, something we can measure. We can look for something conceivably that ebbs and flows within your brain, something that changes in size within your brain. Now, what could that be?

Let us look at the properties of consciousness. We have seen there is no special brain region; consciousness is spatially multiple, many brain regions must be contributing to it, but you have only one consciousness at any time. I would like to think that you see only one thing at one time. Although it is a complex pattern, you will see it as a pattern. Think of those famous vases and profiles, where either you see the profiles or you see the vase; which is true? Both are valid, but looking at it one way negates for that moment looking at it the other. You have only one at a time. I have just suggested to you that it is continuously variable.

Finally, we are always conscious of something. When we become very sophisticated, of course, we have an inner hope, fear, dream, thought, or fantasy: you can close your eyes and have consciousness triggered internally. But in the simplest form we have momentary states triggered by the changing input from the sensory world.

Let me suggest a metaphor to capture how these properties might be accommodated in the brain. Imagine a stone falling in a puddle. The stone is fixed; if the “stone” in the brain is a hub of brain circuits, then it could be, if you like, hardwired. You can see where this is leading. It could be a fixed thing, but, when a stone is thrown, it generates, just for a moment, ripples that are highly transient, that are vastly bigger in their extent than the size of the stone itself; and those ripples can vary enormously according to the size of the stone, the height from which it is thrown, the force with which it is thrown, and the degree of competition from other stones coming in. All these factors will determine the extent of the ripples. What I am suggesting is that in the brain you do have the equivalent of stones: hardwired little circuits, as we have seen, riddling your adult brain, which are sometimes accessed, sometimes not. The equivalent of throwing the stone would be, for example, me seeing my husband. That would then go through certain parts of my brain and start activating the circuitry that is related to my husband. Still, I would not be conscious of him. What would happen then? How could we now get ripples occurring in the brain? Amazingly enough, you have something very special in your brain: you have chemical fountains, not just circuits, and these chemical fountains actually emanate from those primitive parts of your brain and access the cortex and other areas. These are the chemicals that are targeted, for example, by Ecstasy or by Prozac, and these are the chemicals that vary during sleep-wakefulness and during high arousal, and these chemicals fulfill a very special function: they put brain cells on red alert.

Imagine I see my husband. That is the equivalent of throwing the stone. It activates the hub of hardwired circuitry, established over long experience of married life. If that is coincidental with a group of brain cells being sprayed upon by a fountain of chemicals related to arousal, then that would predispose those adjacent cells to be corralled just for that moment, and just for that moment this very active hub will activate a much larger group of cells, and that larger group of cells will determine the extent of my consciousness at that particular moment. That is the model.

How do we know the brain works like that? How do we know that you can get ripples in the brain. I was very fortunate to visit Israel and meet Arnivam Grinvald, who works at the Weizmann Institute. He showed me experiments–not on humans because it is invasive–but using optical dyes that register the voltage of brain cells. Arnivam showed that even to a flash of light there is indeed the neuron equivalent of ripples. These ripples (in this particular experiment to a simple flash of light) are extending over 10 million neurons, and they are extending very quickly, in less than a quarter of a second, or about 230 milliseconds. So this means that in your brain you can have tens, even hundreds of millions of brain cells corralled into transient working assembly in less than a quarter of a second and then it is all gone again, just like a ripple. That, in my view, is the best place to look if we are trying to find out about consciousness.

As a kind of interim thought on the physical basis of consciousness, I would like to suggest that there is no magic ingredient. The critical factor is not qualitative but quantitative: the larger the assembly of brain cells, the greater the depth of your consciousness.

Let us play around with that idea. What would happen if you had an abnormally small assembly of brain cells? Let us just think about what kind of consciousness you might have, because the advantage of this model is that there are different reasons for which you could have an abnormally small assembly of brain cells. For example, if the connectivity was modest, or if the epicenter was weak or only weakly activated (as a tiny pebble laid very gently on the surface of the water), or if the fountains of chemicals malfunctioned, or, indeed, if there was competition from new, rapidly forming assemblies–all these factors could give you different types of consciousness. Modest connectivity occurs in childhood, as we have seen. What do we know about children? Very young children are living in the moment. They are not doing vast learning and memory tasks, the world does not “mean” much to them; they judge the world literally at face value, on how fast and cold and sweet and so on. They have a literally sensational time. They are judging the world not in terms of associations but on the impact of their senses at that very moment. Let’s look at an example of when the center is weakly activated. A particular example is one that we are most likely to experience tonight, and that is a dream. A dream could be a small assembly because in your sleeping state your senses are not working heavily, so they are therefore not able to recruit a very large assembly and you are thus dependent on the small spontaneous activity of brain cells. That is why in dreaming the world seems to be highly emotional, not very logical; you have ruptures in your logic like with schizophrenic states.

We know also that children have a much greater predisposition to dreaming than people who are older. It seems that these different states all have one element in common. They are caused for different reasons, but dreaming, along with fast-paced sports and childhood, are all characterized by living in the moment, by having a strong emphasis on the senses, and by not putting a great premium on anything you have learned or remembered; that is, not using your mind. The mind is not accessed in any of these states.

Let me summarize: childhood, dreaming, schizophrenia, fast-paced sports, and, dare I say it, raves are all examples of where the mind is not being accessed. It is not being accessed for different reasons: lack of connectivity (childhood), lack of strong sensory stimulation (dreams), an imbalance with those fountains of chemicals (schizophrenia), or a degree of competition from other stimulations (fast-paced sports and also raves, where, for good measure, people are flooding their brains with a drug that deliberately confuses those fountaining chemicals).

This brings us to pleasure–indeed, it brings us to the opposite of that–to what would occur if you had a large neuronal assembly. It would be where the world seems gray and remote, where you feel cutoff from other people, where, instead of the senses imploding in on you, as they do in dreaming or fast-paced sports or childhood or in schizophrenia, you feel numb and remote, and your emotions are turned down–you perhaps feel nothing at all or you do not think you feel anything. That, of course, you might recognize as the features of clinical depression: the outside world is remote, the senses are understimulated, and a continuity of thought exists, even a persistent thought. In such conditions people do not feel pleasure. It is not that they feel desperately sad; they just feel nothing. They suffer from something clinically called anhedonia, meaning literally “not enjoying yourself.” A depressed person may have the sun on her face or the grass between her bare toes but she does not feel that sensual pleasure that I would like to think we all feel to some measure and certainly children feel considerably. They are completely cutoff from it. In clinical depression, then, we have an imbalance of the fountains of the modulating chemicals and therefore a lack of pleasure and a lack of emotion. I would like to suggest, because of that, the greater the neuronal assembly, the fewer emotions you have. High emotional states, like childhood, dreaming, or schizophrenia, are associated with small assemblies and the emotions therefore must be the basic form of consciousness, not learning and memory.

This has all been a rather long-winded way to get round to the question: How we are going to model this on a computer if, indeed, we wish to? Let us go the other way round and say: What kind of consciousness could we have? How would we interpret that in terms of this model? Sadly, something everyone has experienced is pain. Most people think of pain as rather boring, as something that surely we would all feel the same if, for example, we put our hands over a flame. But nothing could be further from the truth.

First, we know pain is expressed as other associations: pricking, stabbing, burning, chilling. We know that it can vary, interestingly enough, throughout the day. There are some particularly sadistic experiments where volunteers had electrical shocks through their teeth and had to report when they felt the pain. Amazingly, if that happens, you find that throughout the day your so-called pain threshold (when you report that the pain is particularly intense) varies. But the conduction velocity of your pain fibers has not changed, so something in the brain is changing, something in its chemical landscape; something transient is changing if you as the same individual do not experience the same pain depending on what time of day it is. We know that the more people anticipate pain, the more they will perceive it as painful, and I would suggest that this is because there is a buildup of the connections. If you are anticipating pain that is because more and more assemblies are being recruited before you feel it. And, incidentally, the diurnal threshold occurs because throughout the day those chemical fountains are changing. We know that phantom limb pain is felt by people who do not have the limb but feel as though they do. That is because a so-called neuronal matrix–which I would call “assembly”–corresponds to the severed limb that would be stimulated by the lack of input from signals resulting from it. Pain is absent in dreams–which, we have seen, would be a small assembly state. Similarly, morphine, which is a strong analgesic or painkiller, creates a dreamlike euphoria that works through a natural opiate that makes the brain cell assemblies less efficient at being corralled. People will therefore often say they feel the pain but it no longer “matters,” it is no longer significant to them. With schizophrenia, which I have suggested is a small assembly state, people have a higher threshold for pain. For people who are depressed, it is the opposite: they feel pain more. Finally, anesthetics–which have always proved a puzzle to understand how they work because there are many different types of anesthetics chemically–could work by depressing the activity, so that in the end gradually it reduced the size of your assembly to such a small one that you did not have appreciable consciousness. If that were the case, you would expect, as the assembly was shrinking, you would go through the small assembly state and have some form of delirium, some madness, or some kind of euphoria or pleasure–a rather odd idea if you are going under an anesthetic. Nowadays it is not possible because anesthetics are so efficient that they work very quickly, but in the old days people would actually have “ether frolics” or inhale nitrous oxide at fun fairs to have the pleasure of taking the anesthetic. (Even Ketamine, which is an anesthetic in high doses, is a drug of abuse in low doses.) So, paradoxically, a link exists just as you are going under: because it is less efficient, you go through that period of euphoria where suddenly you are experiencing a sensual time.

One can actually work out the different factors that will determine the size of a brain cell assembly, and they can be expressed bilingually, in neuroscientific or phenomenological terms, so we can talk about the “intensity”‘ of our senses, and that is the degree to which your neurons are active. We can talk about “significance,” and that can be the existence of preexisting associations. We can talk about how “aroused” you are, and that is the availability of those chemical fountains that are called amines. I have not talked about predisposition or mood but they can be modified by other chemicals that also put cells on red alert, such as hormones. And, finally, we can talk about “distraction,” which bilingually one could refer to as the formation of competing assemblies.

What I have done in this paper is relate that which you feel to that which could be happening in the brain so you can build up some kind of match. I think in the future we could image these brain cell assemblies and manipulate those factors differentially and make predictions as to the type of consciousness someone might have according to how big their assembly was or vice versa. I am not suggesting this model is right, but its strength is that it can be tested one day.

Finally, we must remember–and this is another problem for computer modeling–that the brain is an integral part of the body, that your central nervous system, your hormones, and your immune systems are all interlinked; otherwise you would have biological anarchy. So these brain cell assemblies that I am suggesting are related to consciousness are merely an index of consciousness. If you put one in a dish, you would not have consciousness. Somehow that causes a readout to the rest of the body and somehow the rest of the body signals back through chemicals that will influence the size of the assembly and hence the consciousness. One candidate group of chemicals is the peptides, which can also function as hormones and could coordinate the immune, endocrine, and nervous systems.

In conclusion, we will never be able to get inside someone else’s brain. Yet science is starting to make a contribution by being a little more modest, by actually saying, “Well, let’s match up physical brain states with what people are feeling and then perhaps we will have some insight into why people take drugs, what happiness is, and, perhaps most important of all, why people go bungee jumping or to raves.”

Susan Greenfield is Professor of Pharmacology at the University of Oxford and Director of the Royal Institution of Great Britain. She is also cofounder of a spin-off company specializing in novel approaches to neurodegeneration, Synaptica Ltd. Her books include Journey to the Centres of the Mind (1995) and The Private Life of the Brain (2000).

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