A Framework for Consciousness Francis Crick and Christof Koch

commentary A framework for consciousness Francis Crick and Christof Koch Here we summarize our present approach to the problem of consciousness. After an introduction outlining our general strategy, we describe what is meant by the term ‘framework’ and set it out under ten headings. This framework offers a coherent scheme for explaining the neural correlates of (visual) consciousness in terms of competing cellular assemblies. Most of the ideas we favor have been suggested before, but their combination is original. We also outline some general experimental approaches to the problem and, finally, acknowledge some relevant aspects of the brain that have been left out of the proposed framework. General strategy framework may well apply, however, to see, for example, either introns or RNA The most difficult aspect of consciousness is other sensory modalities. We have been editing. And who would have guessed the so-called ‘hard problem’ of qualia1,2— especially interested in the alert macaque that DNA replication usually starts with the redness of red, the painfulness of pain, monkey because to find the NCC, not the synthesis of a short stretch of RNA, and so on. No one has produced any plau- only widespread neural activities but also which is then removed and replaced by sible explanation as to how the experience the detailed behavior of single neurons (or DNA? The broad framework acted as a of the redness of red could arise from the small groups of neurons) must be inves- guide, but careful experimentation was http://www.nature.com/natureneuroscience actions of the brain. It appears fruitless to tigated on very fast time scales. This is dif- needed for the true details to be discov- approach this problem head-on. Instead, ficult to do systematically in humans. ered. This lesson is broadly applicable we are attempting to find the neural corre- Methods such as fMRI are too coarse in throughout biology. late(s) of consciousness (NCC), in the hope both space and time to be of much use for that when we can explain the NCC in causal our problem. On the other hand, experi- A preamble on the cerebral cortex terms, this will make the problem of qualia ments in visual psychology are much eas- By the ‘cortical system’, we mean the cere- clearer3. In round terms, the NCC is the ier to do on humans than on monkeys. bral cortex plus other regions closely asso- minimal set of neuronal events that gives Moreover, humans can report what they ciated with it, such as the thalamus and rise to a specific aspect of a conscious per- are conscious of and thus convey the ‘con- the claustrum, and probably the basal cept. We discuss elsewhere4 why we think tents’ of their consciousness. For these ganglia, the cerebellum and the many consciousness has to be largely private. By reasons, experiments on monkeys and widespread brainstem projection systems. ‘private’, we mean that it is accessible exclu- humans should be pursued in parallel. We shall refer to the ‘front’ of the cor- sively to the owner of the brain; it is impos- tex and the ‘back’ of the cortex because © Group 2003 Nature Publishing sible for me to convey to you the exact Framework the terms ‘frontal’ and ‘prefrontal’ can be nature of my conscious percept of the color A framework is not a detailed hypothesis ambiguous. For example, is the anterior red, though I can convey information about or set of hypotheses; rather, it is a sug- cingulate prefrontal? it, such as whether two shades of red appear gested point of view for an attack on a sci- The dividing line between front and to me to be the same or different. entific problem, often suggesting testable back is somewhat arbitrary. It roughly Our main interest is not the enabling hypotheses. Biological frameworks differ coincides with the central sulcus. It may factors needed for all forms of conscious- from frameworks in physics and chem- turn out that a good operational defini- ness, such as the activity of the ascending istry because of the nature of evolution. tion is that the front is all those parts that reticular systems in the brainstem. Rather, Biological systems do not have rigid laws, receive a significant input, via the thala- we are interested in the general nature of as physics has. Evolution produces mech- mus, from the basal ganglia. (This simple the neural activities that produce each anisms, and often sub-mechanisms, so division is probably not useful for olfac- particular aspect of consciousness, such that there are few ‘rules’ in biology which tion, however.) as perceiving the specific color, shape or do not have occasional exceptions. There is an absolutely astonishing vari- movement of an object. A good framework is one that sounds ety and specificity of actions performed As a matter of tactics, we have con- reasonably plausible relative to available by the cortical system. The visual system centrated on the visual system of primates, scientific data and that turns out to be of the higher mammals handles an almost leaving on one side some of the more dif- largely correct. It is unlikely to be correct infinite variety of visual inputs and reacts ficult aspects of consciousness, such as in all the details. A framework often con- to them in detail and with remarkable emotion and self-consciousness. Our tains unstated (and often unrecognized) accuracy. It is clear that the system is high- assumptions, but this is unavoidable. ly evolved, is likely to be specified epige- Francis Crick is at the Salk Institute for An example from molecular biology netically in considerable detail and can Biological Studies, 10010 N. Torrey Pines Road, might be helpful. The double-helical learn a large amount from experience. La Jolla, California 92037, USA. structure of DNA immediately suggest- The main function of the sensory cor- Christof Koch is at the California Institute of ed, in a novel way, the general nature of tex is to construct and use highly specific Technology, 1200 East California Boulevard, gene composition, gene replication and feature detectors, such as those for orien- Pasadena, California 91125, USA. gene action. This framework turned out tation, motion or faces. The features to e-mail: [email protected] to be broadly correct, but it did not fore- which any cortical neuron responds nature neuroscience • volume 6 no 2 • february 2003 119 commentary are usually highly specific but multi- rather than following just one method (Further discussion of this idea comes at dimensional. That is, one neuron does not very accurately. This appears to be how, the end of subhead #3 below.) respond to a single feature but to a family for example, people see in depth. We have discussed elsewhere17 whether of related features. Such features are some- Incoming visual information is often the neural activity in the front of the brain times called the ‘receptive field’ of that incomplete or ambiguous. If two similar is largely unconscious. One proposal18,19, neuron—the ‘non-classical receptive stimuli are presented in rapid succession, for example, is that humans are not direct- field’5 expresses the relevant context of the the brain blends them together into one ly conscious of their thoughts, but only of ‘classical receptive field’. The visual fields percept. If they are different but in con- sensory representations of them in their of neurons higher in the visual hierarchy tradiction, such as a face and a house, the imagination. At the moment, there is no are larger and respond to more complex brain selects one at a time (as in binocular consensus about this20. features than those that are lower down. rivalry) instead of blending them togeth- The hypothesis of the homunculus An important but neglected aspect of er. In cases where there is not enough is very much out of fashion these days, the firing of a neuron (or a small group of information to lead to an unambiguous but this is, after all, how everyone associated neurons) is its ‘projective interpretation of one’s environment9, the thinks of themselves. It would be sur- field’6. This term describes the perceptual cortical networks ‘fill in’—that is, they prising if this overwhelming illusion and behavioral consequences of stimulat- make their best guess, given the incom- did not reflect in some way the general ing such a neuron in an appropriate man- plete information. Such filling-in is like- organization of the brain. ner (for further discussion of motor and ly to happen in many places in the brain. premotor cortex, see ref. 7). Both the This general principle is an important 2. Zombie modes and consciousness receptive field and the projective field are guide to much of human behavior (as in Many actions in response to sensory dynamic, not merely static, and both can ‘jumping to conclusions’). inputs are rapid, transient, stereotyped be modified by experience. and unconscious14,21. They could be How are feature detectors formed? A Our present framework thought of as cortical reflexes. Con- http://www.nature.com/natureneuroscience broad answer is that neurons do this by Having outlined a few general points sciousness deals more slowly with broad- detecting common and significant corre- about the cortical system, let us now con- er, less stereotyped aspects of the sensory lations in their inputs and by altering their sider specifically the NCC and its atten- inputs (or a reflection of these in imagery) synapses (and perhaps other properties) dant properties. We are mainly interested and takes time to decide on appropriate so that they can more easily respond to in time periods on the order of a few hun- thoughts and responses.

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