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Crick and the claustrum

Francis Crick believed that, in biology, structure is the natural path to understanding function. In his later career, he applied this dictum to the study of consciousness.

Pretty much everyone is interested in the big questions about the brain, and the biggest big question is: what is consciousness? Just as historically the vitalists could not imagine how life can be explained by just physics and chemistry — they believed that a non-physical ‘life force’ had to be involved — the dualists of today cannot believe our experience of the feeling of love or the redness of red could arise just through nerve impulses in a bunch of brain cells. Although everyone who enters the field of neuroscience starts with an interest in the big questions, we soon settle into much smaller questions that we can see how to answer with the tools of modern biology. Questions about consciousness were therefore mostly left to philosophers and kooks, and no respectable neuroscientist would even have considered working on such a problem — until Francis Crick, that is.

After he and James Watson solved one of biology's really big problems, the mechanism of inheritance, Crick moved to neuroscience and set himself the task of answering that field's biggest question. Working closely with Christof Koch, Crick made the study of consciousness respectable and, directly and indirectly, had a profound influence on all of neuroscience and on the types of questions that are considered acceptable to study. Crick's final paper, written with Koch, has just been published in Philosophical Transactions of the Royal Society of London (doi: 10.1098/rstb.2005.1661) and it proposes that an obscure part of the brain, the claustrum, may be involved in consciousness. Crick was working on this paper literally on his deathbed, and Koch has put the finishing touches on it for publication.

How can a scientist think about consciousness? Crick's approach had two parts. The first was to identify what properties of consciousness had to be explained, and the second was to find brain structures that might account for those properties. Crick and Koch note that a key feature of our conscious experiences is that all of the components are integrated into a unified whole: how a rose looks, smells and feels are bound together with our emotional experience of it. Because these different aspects of experience are related to neuronal processing in distinct and often widely separated brain circuits (those responsible for vision, olfaction, somatic sensation, together with the amygdala and other centres involved in emotion), this unification of experiential components implies some sort of coordination between different brain areas. In their survey of various notions about consciousness, Crick and Koch observe that a common thread through all of the thinking about consciousness is the recognition of a need to bind together information from many separate parts of the brain.

In the paper Crick writes, “In biology, if seeking to understand function, it is usually a good idea to study structure”. And thus he takes a fundamentally structural approach to consciousness: what brain regions, he asks, have properties that would suit them for the information gathering and analysis that is at the heart of the conscious experience? I know from conversations with Crick that he had a very strong hunch, one that bordered on a conviction, that the structure underlying consciousness is the claustrum.

What is the claustrum, and why pick on it as a key for understanding consciousness? The claustrum is a thin sheet of grey matter that resides parallel to and below part of the cortex (the cortex is the grey matter covering of the brain that carries out the computations involved in feeling, seeing, hearing, language and deciding what to do). The claustrum is present in all mammals, but it has been little studied and its function is not known. What is known, however, is that there are two-way connections between the claustrum and most, if not all, parts of the cortex as well as subcortical structures involved in emotion.

So the claustrum is not just a sort of shadow of the cortex, but rather a neural circuit with overlapping inputs from various cortical regions and outputs back to cortex. Because of its widespread connections, Crick and Koch liken the claustrum to the conductor of an orchestra, who is responsible for binding the performances by individual musicians into an integrated whole that can be much more than the sum of the parts. The neuroanatomical connections of the claustrum, then, just match with the ‘conductor’ required to bind together the various disparate components of the conscious experience represented in many different brain regions.

Crick told me that one of his main purposes in this paper was to encourage new studies of the claustrum, and had he lived longer, he would have liked to start a centre for investigating the claustrum, where neuroanatomical, electrophysiological and novel molecular biological approaches to the claustrum could be combined. Some of these ideas for studying the claustrum, like using molecular biological methods to specifically disrupt claustral function, are sketched in this paper.

Not everyone will buy the Crick and Koch idea that the claustrum is the seat of consciousness. For example, the fact that all mammals have a claustrum could be an argument against the proposal for those who cannot imagine consciousness without language and high-level symbolic reasoning. And I expect others will be sceptical on other grounds. Nevertheless, the proposal is an interesting and challenging one, from a scientific giant, and I believe every scientist will be fascinated to see how one of the greatest biologists attacked such a difficult problem.

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Stevens, C. Crick and the claustrum. Nature 435, 1040–1041 (2005).

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