Great Thinkers need apply

Consciousness is back on the scientific agenda, and with a vengeance, with a succession of books, journals and meetings devoted regularly to the subject. This return to the fold of a topic banished from respectable science for decades is obviously a great relief to some, like Thomas Natsoulas, who, in his attack on John Searle in the 1995 volume of Consciousness and Cognition under review, writes that he no longer needs to "seek the approval of a field of science that for many years made it so very difficult . . . to discuss issues pertaining to consciousness" publicly. That muzzle was imposed on psychologists by behaviourism, or so they now like to make out. But it is hard to believe that scientists who are making significant advances in this or any other area would accept to be thus muzzled, and for so long. So does this imply that the return of consciousness to centre stage in current neurobiology signifies that great advances are being made? A reading of the volume under review suggests not. One must therefore seek other explications.

It is often claimed that the current taste for the subject has been generated by eminent men of science who have made their fame and fortune elsewhere and are now attacking the ultimate of problems, the last frontier, and guiding the humbler neurobiologists in this task. They have, it is said, made of consciousness a hard and respectable science. It is unfortunate that many neurobiologists are themselves the willing and even eager victims of this travesty, obsequiously applauding the Great Thinkers and supplying them with the facts demanded of them, in the vain hope that when the grand theories - commonly described in high- sounding but empty terms like "global" and "unifying" - come along, some of the glory will be reflected on them. In fact, of course, consciousness was a central issue for neurologists long before the present generation of Great Thinkers turned their attention to it and the current myth does a disservice to the work of those like Giuseppe Moruzzi, Charles Sherrington, Lord Adrian, Wilder Penfield and Herbert Jasper. That none came within striking distance of even a partial resolution to what constitutes consciousness in neural terms is perhaps more than adequately balanced by the fact that the current books on the subject do not provide answers either and, compared to the earlier work, most have the disadvantage of an arrogant and condescending attitude that alienates the reader without offering him the reward of an insight. I should be surprised if most are not relegated to oblivion.

It is, of course, not the sudden appearance on the scene of the Great Thinkers that has generated the new interest. Rather, the subject has become unavoidable because of developments in brain studies over the past 30 years. No one had much interest in it when recording from single nerve cells in the anaesthetised brain but the question imposed itself forcefully once scientists started recording from the awake, behaving animal and began asking questions about the role of attention. The advent of new imaging techniques to study the human brain has made the question entirely unavoidable. Equally, no visual neurobiologist worried too much about consciousness at a time when most considered that there was a single visual area which analysed the visual image imprinted upon the retina, much like a photographic plate. But once the discovery of many visual areas was made and vision was shown to be a much more active process, the question of how these visual areas interact to provide the integrated visual image, and who then "sees" and understands the finished product - a question that leads directly to a study of consciousness - imposed itself.

Have these studies generated enough reliable facts about consciousness to allow significant new insights? This volume of Consciousness and Cognition well illustrates the uncertainty and confusion that reigns, after 30 years of the new onslaught, even in relation to basic facts. It thus shows also why the Great Thinkers reign supreme, even though they have nothing to offer. A special issue is devoted to the topic of implicit memory, broadly speaking knowledge that a subject has and can put to use or that determines his behaviour in some and possibly all situations but of which he or she is not consciously aware. Much more than that, it delves into issues of localisation and neural pathways, of repression and of perception and is a good source for illuminating us on the present paralysis of the subject, even if the articles are assembled together in a somewhat puzzling way, with those on neurophysiology appearing in separate numbers in the company of other articles that avoid the neural dimension altogether, There is no evident conscious thematic unification.

The problem of implicit knowledge is part of a larger problem that may be phrased thus: Is there a special neural pathway that is critical for consciousness and can it be localised to any given area or region of the brain? If so, what features does it have and how does it lead to that essential quality that we call consciousness. Is it localisable to a single area, to which all other brain areas report, or do all of the many areas in the brain have access to that special circuitry? Is it in the cerebral cortex or elsewhere? Are there conditions in which only part of the hypothetical neural pathway is activated, thus giving implicit knowledge only? Is this what we mean by the unconscious or the preconscious? Neither the Great Thinkers nor the wet neurobiologists have any plausible answers.

The cortex as the source of consciousness was doubted by Penfield among others and a unique cortical area, one that only receives but does not send signals - a sort of cortical terminal station - has not been found to exist. It is therefore little use looking for a pontifical area in the cortex. So perhaps it is all sub-cortical after all, although the subcortex itself is not connected unidirectionally either. Joseph Bogen's illuminating review, rendered somewhat indigestible by the indiscriminate use of abbreviations, does not give any convincing conclusions. His general thesis is that "awareness of content" is the result of an "appropriate interaction", whatever that may mean, between certain subcortical nuclei of the brain (the intralaminar nuclei of the thalamus) and the neural representation of that content (whatever that means) in the cerebral cortex. As an example, he cites area V5, a visual area specialised for the detection of motion. He hypothesises that the conscious awareness of motion must be the result of an interaction between the intralaminar nuclei and area V5, but that the interaction must be mediated through the primary visual cortex, area V1. In the absence of the latter, the undefined "appropriate interaction" cannot occur, he claims, and subjects therefore have blindsight, a syndrome in which patients are reported to be able to "see" though they have no conscious awareness of having seen. But this convenient argument loses much of its validity because of the finding, acknowledged without comment by Bogen in a footnote, that subjects with lesions in V1 can nevertheless have a conscious awareness of motion, which implies that the neural pathways essential for conscious awareness are not immutably routed through given areas. Blindsight is in fact one of the most interesting examples of implicit knowledge, but it remains a highly controversial subject. It is therefore a pity that there is no review devoted to it in this special issue, for it might have warned against the uncritical acceptance of all that is said of it.

Nor will the reader find certainty in other areas. The review of V. S. Ramachandran gives fascinating details, including an account of how implicit knowledge after parietal lobe lesions becomes explicit after irrigating the ear with cold water. One is naturally tempted to speculate about the neural mechanisms underlying so remarkable a phenomenon and much of Ramachandran's article is in fact speculative. But perhaps he pushes things a little too far when he allows himself to develop, from the very few facts at his disposal, a "Darwinian" theory of brain functions. Don Gustafson, in his excellent article entitled "Belief in pain", leaves us in doubt over many of the assumptions that have been made about the phenomenology of pain, but it would have been better if the article had been accompanied by one which defends the traditional belief that pain is cognition independent.

Yet other articles cast doubt on beliefs that are common and popular. Is there such a thing as traumatic memory or even repressed memory, a topic that is also of interest in medicolegal issues? John Kihlstrom tells us that the evidence is "too weak to support global assertions about trauma and memory, amnesia and recovery, and the like" and in their article Helene Hembrooke and Stephen Ceci also leave us in doubt. This general doubt evidently filtered through to some of the other participants in the special issue, for Robyn Fivush tells us that "Reading some of the papers in this issue raises the possibility that memories of past events are complete reconstructions, often bearing no relation to actual events in the world". The latter part of the sentence is hilariously exemplified in the article by Hembrooke and Ceci who misquote "the late physiologist, Bernard Katz" as saying that researchers would rather use each other's toothbrushes than their theories. It was in fact Wilhelm Feldberg, not Katz, who said that and the reference was to nomenclature, not theories. But seriously, rumours of Bernard Katz's death are much exaggerated and he would be as surprised to learn of this as I was. His office is, after all, next to mine and I see him daily. The statement therefore obviously bears no relation to events in the actual world.

There is a strangeness to some of the articles here that is inexplicable. For example, in the article entitled "A rediscovery of Sigmund Freud", the reader is left at a loss to understand why Thomas Natsoulas should need to apologise for Freud and state that Freud "was an accomplished writer as well as a scientist", that he was nominated for the Nobel prize but won the Goethe prize. Equally puzzling is his passionate exhortation to his colleagues to engage in serious debate, though this may be the result of that long period when scientists willingly acquiesced in order to remain stumm on the subject. It seems a pity that an article by John Searle does not appear next to Natsoulas's, for Searle comes in for a good deal of criticism and is accused of not reading Freud properly but interpreting Freud's account of consciousness on the basis of "a single paragraph" from The Unconscious. In spite of this, as well as the accusation that Searle "does not take the time to work out a hard case for his thesis . . . [but] expects others to do the job of showing that Searle, or anyone, cannot actually pull the feat off", Natsoulas's article fails to convince one that Searle has made as fundamental an error as Natsoulas claims, without an accompanying article by Searle himself. It is also difficult to understand why Claparede's 1911 article is reproduced here, especially when the accompanying article by Kihlstrom tells us that Clapar de's article is important not because it anticipates the distinction between implicit and explicit memory but because it "reminds us that consciousness remains the great mystery in psychology and that the self plays a crucial role in consciousness", which is hardly news to anyone. The deadly reminder really serves another, and better, purpose, that in spite of the great hype accompanying the return of consciousness, we have precious little to show for it.

There are many facts of interest covered in this special issue. Many articles are mercifully quite short, and all interested in the topic should consult this volume. Perhaps they will come away with the conclusion that the subject of consciousness does require a Great Thinker after all, though they will be hard put to locate one from these pages.

Semir Zeki is professor of neurobiology and co-head, Wellcome department of cognitive neurology, University College London.