books and arts Clarifying consciousness Work is under way to relate the mind to observable neurobiological events. The Quest for Consciousness: A Neurobiological Approach by Christof Koch Roberts: 2004. 448 pp. $45, £29.99 Wider than the Sky: The .BRIDGEMAN.CO.UK Phenomenal Gift of Consciousness by Gerald M. Edelman Yale University Press: 2004. 185 pp. $24 Jean-Pierre Changeux The relationship between body and soul has been fiercely debated since Ancient Greece, first by philosophers and then by scientists. But until the 1980s it was not considered ‘scientifically correct’ for physiologists to SIMON COLLECTIONS/WWW NORTON mention the word ‘consciousness’ in a refer- eed paper. Christof Koch’s book The Quest for Consciousness shows that this period is definitively over. The many facets of con- sciousness that philosophers have put for- ward are still not yet understood, but Koch’s book represents an exceptionally rich pre- sentation of existing experimental work, accompanied by many cogent, if still infor- mal and debatable, theoretical statements. The ‘exotic landscape’ of our perceptions, reflected in this painting by Henri Rousseau. From the start, the reader is taken on an in-depth exploration of the most recent perception. Koch traces the diverse routes thesia. It is fascinating to follow Koch’s pres- developments in the biology of conscious- from the retina through the visual cortices to entation of the latest clinical observations, ness, guided by Koch and Francis Crick, with the frontal cortex and then to the motor areas, together with abundant psychophysical whom Koch has collaborated for the past 20 defining step by step the necessary conditions experiments, in which reasonable attempts years.The outcome is,in my view,exceptional. for the neuronal correlates of consciousness. have recently been made to uncover neural This book and a few others, in particular This bias towards an input–output view of signatures for conscious and non-conscious Gerald Edelman’s Wider than the Sky, also information processing neglects the role of processing.Examples include the views from reviewed here, bring us to the stage where, in spontaneous states of neural activity and functional magnetic resonance imaging my opinion, objective physiological record- contrasts with the top-down projective style (fMRI) of the masking experiments of ings and measurable physical parameters of the brain, which constantly anticipates Stanislas Dehaene and colleagues,and Nikos can be causally linked with subjective experi- the evolution of the outside world — this Logothetis’ dynamic electrophysiological ence.This is a strong statement.Koch is more should, in my opinion, be an essential part of recordings and fMRI images from binocular- cautious, stating that his quest is for the any theory of consciousness. rivalry experiments. neuronal correlates of consciousness. His Perhaps Koch’s most controversial pro- Koch also discusses the idea that the aim is to establish an “explicit correspon- posal is that “zombies could be living among perception of motion consists of discrete dence”between mental and neuronal events. us”. According to Crick and Koch, these processing epochs, or snapshots. He re- But, in my opinion, causal determination, fictitious creatures would be “devoid of any examines the classical paradox of the con- despite all its difficulties, is required. Koch’s conscious experience” and yet would have tinuity of cinematographic vision, recently position therefore seems less bold than that behaviour identical to that of their normal, refreshed by Oliver Sacks’ clinical obser- set out in Crick’s own book The Astonishing conscious counterparts. This can only be a vations of patients who see movement as a Hypothesis (Simon & Schuster, 1994): “You metaphor, not a serious model, yet it might succession of still images during migraine are in fact no more than the behaviour of a help to illustrate the notion that specialized episodes. This rather simple experimental vast assembly of nerve cells and their associ- sensorimotor reflex processes (referred to as paradigm deserves to be re-explored using ated molecules.” ‘zombie agents’) carry out routine missions the tools available today.Understanding this, The explosive development of new meth- in the absence of any direct conscious sensa- as well as many of the other issues raised by ods of brain exploration, in particular brain tion or control. The Quest for Consciousness, should tell us imaging, as well as imaginative cognitive This concept is reminiscent of Pierre more about the access to, and persistence of, experimentation, has created an important Janet’s “subconscious actions”, as presented visual stimuli in consciousness. experimental literature on consciousness, in his 1889 book L’automatisme psychol- Koch postulates that if perceptual repre- which Koch’s book covers in a vivid and ogique,which are “entirely absent from the sentations are to achieve conscious status, fluent style. He has deliberately not covered vision of the subject, thus forcing ourselves they must be intimately linked to an inten- some important issues, such as the self- to assume their existence outside conscious- tional planning stage. He therefore intro- referential aspect of consciousness and the ness”. Janet, like Koch, based his argument duces a requirement for projection to the role of language and emotions. Instead, the on a clinical framework, including catalepsy, front of the brain. Nevertheless, Crick and focus is on perception, particularly visual somnambulism, hallucinations and anaes- Koch follow Ray Jackendoff in postulating NATURE | VOL 428 | 8 APRIL 2004 | www.nature.com/nature 603 © 2004 Nature Publishing Group books and arts that many aspects of high-level cognitive non-conscious processing of sensory stim- may happen,”Poincaré wrote,“that small dif- functions, such as decision-making, plan- uli? My own answer would be a resounding ferences in the initial conditions produce very ning and creativity, are themselves hidden ‘yes’.I hope that the next generation of neuro- great ones in the final phenomena.” This, in from consciousness: “You don’t know your scientists, inspired by reading The Quest for modern parlance,is chaos,and it implies — in innermost thoughts.” These thoughts are Consciousness, will soon start planning their the more general context of dynamical-sys- carried by “a little person, a homunculus, experiments. ■ tems theory — that scientific prediction over inside my head who perceives the world Jean-Pierre Changeux is at the Molecular long periods of time is generally impossible. through the senses, who thinks, who plans Neurobiology Laboratory, Institut Pasteur, Gribbin tells the story of the modern and carries out voluntary actions”— another 28 rue du Docteur Roux, 75724 Paris, France. rediscovery in the 1960s and 1970s of metaphor.This position implies that “you are Poincaré’s insight. This is an exciting tale aware only of the sensory representations but has been told before, most notably in associated with these mental activities.” James Gleick’s bestseller Chaos(Heinemann, This is reminiscent of a debate in the nine- 1988). On the positive side, the discovery of teenth century about the role of images in Keep it simple chaos reveals that many highly erratic phe- thought. In 1870, Hippolyte Taine compared Deep Simplicity: Chaos, Complexity nomena,ranging from chemical reactions to the mind to “a polype of images”,and recent and the Emergence of Life fluctuations in biological populations, may ingenious experiments by Gordon Shepard by John Gribbin actually arise from very simple underlying and Stephen Kosslyn have confirmed the role Allen Lane: 2004. 272 pp. £18.99 dynamics. This is one way that simplicity of mental imagery as central to the substance Mark Buchanan often lies behind complexity. of conscious thought. Yet it seems highly Gribbin then weaves the story of chaos debatable that all the contents of conscious- The search for simplicity is perhaps the together with more recent developments, ness are sensory. What about mathematical most basic theme of all science. As the late and with a host of topics now gathered concepts and their creation? What about con- social and political scientist Herbert Simon together under the term ‘complexity science’. sciousness of our errors? It seems to me that put it, the purpose of science is “to find The book moves rapidly from spontaneous many abstract non-sensory representations meaningful simplicity in the midst of dis- pattern formation to the mathematics of can also be conscious.But Koch’s speculation orderly complexity”. In his new book Deep fractals and the idea of self-organized criti- does bring forward an important question: Simplicity,John Gribbin explores this theme cality, examining its relation to earthquakes, can we define the particular kind of conscious in the context of two great movements of mass extinctions and a vast range of other and non-conscious representations to which modern science — chaos and complexity — prominently unpredictable phenomena.The the prefrontal cortex contributes? I am con- and argues that the discovery of simplicity book celebrates the contemporary emphasis, vinced that these issues can now be investi- hiding behind surface complexities will especially in physics, on seeking the explana- gated in neurobiological terms. soon explain the origin of life itself. tion of complex phenomena through simple An important quality of The Quest for Gribbin suggests quite plausibly that dynamical models of growth