Hume's internal bundle grows a few extra knots

Thinking about consciousness is no longer the preserve of philosophers; it is part of science too. Experts from a range of disciplines give their individual views

What is the relationship between our internal world of perceptions, thoughts and memories and the activity of nerve cells that must somehow be responsible for all this rich, subjective experience?

Francis Crick and I are pursuing our investigations on the basis of the tentative hypothesis that there are specific neurons in the brain whose activity mediates awareness that might include, of course, an event occurring in my own body. These neurons must expresses the fact that I am looking at the face of my daughter and that I "see" her blonde hair and "hear" her voice and that I can report on this awareness.

If - by some yet to be invented technical means - one could directly stimulate an appropriate set of such neurons in an awake human, the subject should have the experience associated with the features encoded by these nerve cells. It is quite likely that such neurons are distributed in a specific layer throughout parts of the cerebral cortex, that they have a specific shape and specific cellular properties and that they make specific connections with other neurons, most likely in the planning stages of cortex, that is in the frontal lobes. On-going experiments suggest that such neurons are absent from the earliest part of visual cortex.

But finding such neurons is only the beginning. Much will be learnt from studying areas of the brain that these neurons project to. And what about the crucial relationship between awareness and short-term memory? Is it not likely that these neurons will express the substrate of this type of memory? And what about the many types of mental diseases that affect awareness: can they be related to the specific loss of the awareness neuron? Finding the neurons that express the neural correlate of awareness is just another step, albeit a critical one, in understanding consciousness.

We now have the experimental tools in hand to attack these issues on an experimental basis. It is an open question as to whether all the puzzling aspects of consciousness will be explained once science has run its course or whether it will always defy any testable explanations.

Christof Koch is professor of computation and neural systems, California Institute of Technology.