‘I have never seen Francis Crick in a modest mood.’ Thus James Watson opens his notorious account of the discovery of the structure of DNA which won him, Crick and Maurice Wilkins a Nobel Prize in 1962. Whichever other of Watson’s judgments have been controversial – notably his dismissal of Rosalind Franklin, from whom, courtesy of Wilkins, he and Crick were provided with the crucial X-ray photographs of DNA crystals – his assessment of Crick has scarcely been disputed. The subsequent history of the DNA quartet is instructive in this regard. Franklin, miserable in the unfriendly and sexist environment of King’s College, London, switched research topics from DNA to the structure of coal and moved to Birkbeck and the more welcoming lab of Desmond Bernal. Wilkins has remained at King’s for the subsequent forty years, refining the early DNA measurements, working on the tubule-forming proteins of the cell’s internal skeleton and quietly deploying the prestige of the Prize in his concerns over the social responsibility of science. Watson returned from Cambridge to the US and became director of a major research institute. His gift for the outrageously dismissive mauvais mot has never left him: its most recent manifestation a feud with the then director of the National Institutes of Health, Bernadine Healy, which resulted in his abrupt departure from his position as the head of the Human Genome Project.
Crick was a decade older than Watson, in his mid-thirties at the time of their partnership, and had moved from a wartime background in engineering into slightly desultory studies on protein structure at the Cavendish, from which the DNA work began as a diversion. He was then, and has always remained, that rarest of creatures in biology, a theoretician. Theoreticians are common enough in physics, where the pecking order of mental over manual gives them greater power and prestige than mere experimentalists. But the domain of nature which physics studies is relatively constrained: experiments are hard to design and often very expensive. The terrain of biology is broader, its regularities are often historically contingent rather than apparently lawlike. Our experiments are relatively easy and cheap, but the complexities of the universe we study all too readily slip through the theoretical mesh. Perhaps this is why theoretical biologists are often physicists or engineers manqué and are regarded with suspicion by those who spend their lives in the lab or the field.
That Crick has largely – though never entirely – escaped this suspicion is a tribute both to his undisputed brilliance and to his success in choosing a series of superb experimentalists with whom to work. As he and Watson pointed out in their famous 1953 paper, the double helix with its pairs of matched nucleotides (‘bases’) immediately suggested a copying mechanism by which the molecular sequence of DNA could replicate itself in successive generations of cell division. But the working molecules of the cell are chains of amino-acids – proteins – not DNA, and it was clear that the sequence of amino-acids in proteins was somehow defined by the sequence of bases in the DNA molecule. For the next decade, in a series of superbly conceived experiments with Sydney Brenner, Crick was to concentrate on how this process occurred: on cracking the DNA code. In doing so he provided several of the key metaphors which have dominated – haunted, some would say – biological thinking ever since. DNA became an ‘informational macromolecule’, and its sequence of bases the ‘genetic code’ on which life is based. The irreversible nature of the relationship between DNA and protein Crick christened ‘the Central Dogma’ of the new science of molecular biology: ‘DNA makes RNA makes protein’ and ‘once information has got into the protein it cannot get out again.’ In its more modern version, the Central Dogma reappears as the ‘selfish gene’ and for the thirty years since Crick enunciated it, hard-line molecular biologists have struggled to maintain it against a series of increasingly emphatic challenges.
After the Sixties, molecular biology became increasingly the province of an atheoretical technology and the high priests of its early years began to look for other biological worlds to conquer. For a while Crick turned his attention to the origin of life (defined as the origin of the self-replicating molecules of DNA and RNA) and went so far as to suggest ‘directed panspermia’ – the seeding of Earth with self-replicating gemmules of nucleic acids from outer space – to account for it. But more and more his attention has turned to that other great biological challenge, the mechanism of mind, which forms the theme of his new book. In this he has followed a route trodden by a number of other molecular biologists over the past decade, notably Gerald Edelman, with whom The Astonishing Hypothesis is in implicit dialogue. Although one might have expected that those coming to neuroscience from the extreme reductionism of molecular biology would focus their attention on the brain’s unique chemistry, Crick has simply leapt over the biochemical level of analysis to collaborate instead with physiologists and the new generation of computer modellers.
The debate on the neural basis of consciousness has been dominated over the last decade by the growth of such collaborations, especially in the US, where it has been joined by a small community of philosophers of mind. Some of these, like Patricia Churchland and Christine Skarda, have actually relocated themselves within neuroscience laboratories; others, like Daniel Dennett and even John Searle, are content to observe closely from the outside. While this attention to the brain has shifted the centre of gravity of the mind/brain debate, it has hardly reduced the vigour of the polemic that has ensued among both philosophers and neuroscientists. A faintly coded clue to the intensity of the debate is afforded by the tersely annotated ‘further reading’ that Crick provides to The Astonishing Hypothesis, though for the real flavour one needs to turn to his critical rebuttal of Edelman’s so-called Neural Darwinism, which he relabels ‘neural Edelmanism’.
The underlying issues are perhaps best understood by reference to the parallel dispute within the artificial intelligence community about the best way of modelling mind/brain processes. Relatively early on – back in the Fifties – at a time when the brain-computer metaphor was being enunciated with increasing, though misguided, certainty, the way forward seemed to be obvious. One would use the power of the computer to mimic the working of the individual units of the brain: the hundred billion or so neurons of which it is composed. On this basis one could provide a network of simulated neurons which would learn from novel inputs, and change their outputs accordingly. Frank Rosenblatt’s Perceptron was the machine in question, but the ‘bottom-up’ brain modelling approach which it offered was dealt a fatal blow by Marvin Minsky and Seymour Papert’s simple proof of its fundamental limitations. The task next came to be seen as one of modelling outcomes, building machines, for instance, that could behave as if they were intelligent (by recognising faces, or obeying instructions to place green pyramids on top of red cubes, or whatever) irrespective of whether this was how real brains worked. This ‘top-down’ approach was once memorably encapsulated by Margaret Boden as ‘You don’t need brains to be brainy.’ The silicon chemistry of the microchip could effectively incarnate (or better, I suppose, inmachinate) the carbon chemistry of the neurons. The natural allies of such AI theorists are cognitive psychologists, happy to disregard the brain’s innards and dismiss the achievements of the neuroscientists, whom they see as skilled cooks stirring away at the brain’s porridge.
It wasn’t until the mid-Eighties, when a new generation of computer designers sought to overcome the limitations of the essentially linear, one (very fast)-step-at-a-time digital computer by building machines which could perform many operations simultaneously (‘parallel processing’), that it was recognised that such computers were very much more like real brains than were the old digital ones, and a fresh and more promising approach to neural networks (connectionism) was developed. The liaison between modellers and neuroscientists could be renewed. Crick’s account of this revived love-affair makes one of his best chapters: his book, indeed, is dedicated to the modeller Christof Koch, with whom he has worked for a number of years.
Connectionist models have forced both neuroscientists and philosophers to reconsider old positions. It is in part Crick’s commitment to connectionism which separates him from Edelman, though it is hard not to see a clash of egos as well as of philosophies as being involved. (Among the philosophers, Patricia Churchland and, if I read him right, Dennett, are to be numbered among the connectionists; Skarda and Searle among its opponents.) For Edelman, the key to the brain is how it wires up during development; his claim is that neurons, their connections, and the neural pathways between them – and hence behavioural and mental outcomes, including consciousness – are selected by the environment from a prespecified range of possibilities during development, in much the same way as the natural environment ‘selects’ the fittest in Darwinian evolution. He contrasts this selectionism with the ‘instructional’ model provided by connectionism, in which neural networks learn from and adapt to their environment by virtue of their internal organisation. Consciousness, for him, is the ‘remembered present’ which derives from the interactions between these selected neuronal ensembles.
Crick does not share these concerns and the model he presents here is much sketchier than Edelman’s. The ‘astonishing hypothesis’ of the book’s title is little more than the claim that mind, or consciousness – or the soul, as the subtitle would somewhat coat-trailingly have it – is ‘in fact no more than the behaviour of a vast assembly of nerve cells and their associated molecules’. This is as unabashed a statement of reductionism as was the Central Dogma, though because it begs the question of what is meant by ‘nothing but’, only dualists would wish point-blank to deny it and there is nothing in the hypothesis that most scientists or philosophers would find in the least astonishing. The question is whether, to Crick, ‘nothing but’ means that in due course such mind-language phrases as ‘he is depressed’ or ‘she proposes to vote Labour at the next election’ can not merely be translated into brain-language statements about the firing properties of particular neurons and the circulating levels of particular hormones, but can be formally reduced to them in such a way that it will no longer be appropriate to use mind-language terms. Certainly this is what the Times, which sponsored a recent lecture by Crick in London, understood and dissented from in a leading article. Perhaps in deference to his hosts, in the lecture itself Crick carefully backed away from so clearcut a statement, though it is, after all, only what the DNA-centric view of the organism proclaims as the goal of the Human Genome Programme, with its metaphors of DNA as ‘the Book of Life’, or ‘the code of codes’.
In fact, the book scarcely confronts the implications of its subtitle, and even the most resolutely dualist philosopher is unlikely to be troubled by its modesty of content, however irritating the occasional jocular dismissiveness as Crick lapses into the brash obiter dicta which, once the perquisite of physicists, are today too often molecular biology’s stock-in-trade. Indeed, any resemblance this text bears to what readers of Searle, Colin McGinn or Dennett might expect to find in a book about consciousness is almost entirely coincidental, with the exception of a short postscript in which – I hope tongue-in-cheek – Crick solves the problem of Free Will by locating it in the anterior cingulate sulcus, rather as the dualist John Eccles once located the soul in ‘the liaison brain’ of the left hemisphere.
Crick’s approach is to dismiss the traditional problems (of qualia, subjectivity etc) which have beset Western philosophy of mind and to focus on the visual system. His argument is that if we can understand the neural mechanisms of vision and perception, where both the phenomenology and the brain systems that subserve it are relatively well mapped, this will serve as a test case for the elucidation of consciousness. He takes us through the psychology of vision by a route familiar to anyone who has read Richard Gregory. There follows a brief account of neuroanatomy in general and the primate visual system in particular, where he has been somewhat upstaged by the superb new book by the visual physiologist Semir Zeki, A Vision of the Brain.In fact, it is only really in the last fifty pages or so that a specifically Crickian view of consciousness begins to emerge and the contributions of Christof Koch become apparent.
The problem that any neural theory of mental function needs to address is how a unified sense of identity – of consciousness – can emerge from the activity of the brain’s hundred billion neurons. Early assumptions were that there was a steadily ascending hierarchy of neuronal function, so that, for instance, in the visual system the retinal images arriving at the visual cortex were processed by layer on layer of neurons, the lower levels detecting simple properties like edges, movement and colour, the higher ones more integrated functions, until, at the highest level, one arrived at a single neuron, the hypothetical ‘grandmother’ cell, which responded to your grandmother but nothing else. This highest level neuron would then be part of the neuronal Central Committee of consciousness, each individual’s ‘I’. Dennett describes this as the ‘Cartsian theatre’ view of consciousness, in which the homuncular I scans and controls the rest of the brain. These days it is clear that such a centralist model of neural function simply can’t work and that, far from being hierarchical, brains function in a highly distributed manner, so that visual awareness, for instance, depends on somehow integrating the responses of many different regions of the brain each of which analyses the visual universe according to distinct criteria. Dennett, who is not required to find a neural representation of this property, handles it simply by insisting on the distributed nature of his Cartesian theatre. Crick, however, needs a neuronal account, and to develop it he draws on the experimental observations of visual neurophysiologists, especially Wolf Singer, who have found that ensembles of neurons in weakly connected brain regions can nonetheless fire in synchrony, giving coherent, rhythmic bursts of activity. The activity of these neurons is seemingly bound together, resulting in an emergent pattern of activity across the brain. It is in this binding together of many neurons across large regions that Crick finds his solution for the problem of visual awareness and, by extension, of his narrow version of consciousness. Recognition of your grandmother is not achieved by a single cell, or even by a small ensemble of cells, but by the coherent responses of bound-together neurons across the many different visual regions, each ‘recognising’ a specific grandmotherly aspect, and the whole integrating to provide an emergent grandmother representation who is more than simply the sum of her parts.
Crick is not alone in finding this model attractive, and in recognising that it is open to experimental test. It cannot be entirely comfortable for him that even this solution to the mind/brain question inevitably bursts its reductionist bounds, but despite his predilections he is far too good a theorist to abandon it on these grounds. It is hard to see that finding a solution to the binding-together problem is quite the same as finding, or even searching for, the soul, but it is, more modestly, the best current working hypothesis for a major aspect of brain function. Even as an approach to consciousness it remains unsatisfactory, however. Crick myopically limits his definition of consciousness to the almost equally problematic term ‘awareness’, which is simply contrasted with unconsciousness, understood as what happens in the brain when one is asleep or dead. Yet of all the multiple layers of meaning we can give to the sentence ‘he was unconscious of his motivation in concealing his voting intentions from the canvasser’ the least appropriate is that ‘he’ was dead, comatose or sleepwalking. Crickian consciousness is peculiarly static and closed. Locating consciousness entirely in the head ignores the fact that as humans we are open biological systems, in constant interaction with our natural and social environments. And it refuses the developmental, historical and evolutionary perspectives which mean that for each of us our consciousness is a product not merely of the present moment but is shaped by our past life history. Consciousness is thus not simply an emergent property of an ensemble of brain cells located within an individual brain, but the emergent social property of a system within which these brain cells are located. To spring neuroscience’s reductionist trap, an even more astonishing hypothesis may be required.