麻豆传媒

A QUICK glance at the thousands of books that purport to explain consciousness makes the real understanding of it look like a Herculean task. There is, after all, a profound explanatory gap between neural activity of any sort and subjective feelings. The first belongs to the realm of physics, to space and time, energy and mass, the second to experience. And while experiences are ephemeral, they are the very stuff of life. The only way we know about the world, about space and time, about energy and mass, about anything in fact is by seeing, hearing and smelling, by lusting and hating, by remembering and imagining.

That these two realms are closely related is revealed by the effects of a stroke, a strong blow to the head, or by a neurosurgeon stimulating electrically some part of a person鈥檚 brain and evoking a childhood memory. Yet consciousness does not appear in the equations of physics, nor in chemistry鈥檚 periodic table, nor in the A-T-G-C molecular chatter of our genes. Somehow it emerges from the nervous system.

I have spent 25 years 鈥� the first 16 years working with my mentor, colleague and friend Francis Crick 鈥� linking specific aspects of consciousness to the mammalian brain. We popularised the idea of the neuronal correlates of consciousness (NCC): the minimal neuronal mechanisms 鈥� the synapses, neurons and brain regions 鈥� that are jointly sufficient for any one conscious percept.

Since then, much progress has been made. We now know that some sectors of the cerebral cortex making up the bulk of the brain (for its size the most complex organ in the universe) have a privileged relationship to consciousness, that not all of its many regions participate equally in generating the content of a conscious experience. Micro-electrodes and magnetic scanners have also shown us the neocortex can be active without necessarily giving rise to a conscious experience. This is the domain of the non-conscious.

Yet Crick and I looked deeper. Why did a particular NCC give rise to one specific conscious experience? Why should particular vibrations of highly organised matter trigger conscious feelings? It seems as magical as rubbing a lamp and having a genie emerge.

鈥淲hy should vibrations of organised matter trigger conscious feelings?鈥�

What is needed is a fundamental account of how activity in any system can give rise to consciousness. We therefore turned to the ideas of Giulio Tononi at the University of Wisconsin-Madison. He advocates a sophisticated information theory account of consciousness, called integrated information. The theory introduces a precise measure, called phi, which captures the extent of consciousness. Expressed in bits, phi quantifies the extent to which any system of interacting parts is both differentiated and integrated when that system enters a particular state.

This is the heart of phenomenal experience: any one conscious experience is both highly differentiated from any other one but also unitary, holistic. The larger the phi, the richer the conscious experience of that system. Furthermore, the theory assigns any state of any network of causally interacting parts (these neurons are firing, those ones are quiet) to a shape in a high-dimensional space. The shape (think of it as a crystal in a fantastically high-dimensional space) accounts for the peculiar feel of any one conscious experience. If the network switches into a different state 鈥� you fantasise about sex rather than listen to a droning speaker 鈥� the crystalline shape changes as well.

This crystal is the system viewed from within. It is the voice in the head, the light inside the skull. It is everything you will ever know of the world. It is your only reality. It is the quiddity of experience. The dream of the lotus-eater, the mindfulness of the meditating monk, the agony of the cancer patient, all feel as they do because of the shape of the distinct crystals in a space of a trillion dimensions.

Integrated information makes specific predictions about which brain circuits are involved in consciousness and which ones are peripheral players, even though they might contain many more neurons. The theory should let doctors build a consciousness meter to measure the extent to which severely brain-injured patients are in a vegetative state, and which ones are partially conscious but unable to signal their pain and discomfort.

I am now pursuing a different tack at the Allen Institute for Brain Science in Seattle, a few hours north of Pasadena by plane. We have just embarked on an ambitious 10-year project involving hundreds of scientists and technologists. Philanthropist Paul G. Allen, who founded the institute in 2003, has pledged $300 million for the first four years of the project. Our goal is to understand how information is encoded, transformed and represented in the mouse and the human cerebral neocortex and its satellites.

The neocortex is a layered structure: the human neocortex is about twice as thick that of the mouse, and has about 1000 times the surface area. It is a highly versatile, computational tissue that excels at processing sensory information, making and storing associations, and planning and producing complex motor patterns. The neocortex is partitioned into multiple areas, made up of smaller columns with reasonably similar cell types and architectures across species and brain regions.

The institute plans to build a series of brain 鈥渙bservatories鈥� to identify, record and intervene in the cortical networks underlying visually guided behaviours in the mouse, including visual perception, decision-making, and even murine consciousness. The fast-developing technology of optogenetics will allow us to control defined events in defined neurons at defined times in mouse brains. That is, we will move from correlation to causation. Building these observatories is a large-scale effort to synthesise anatomical, physiological and theoretical knowledge into a model of the cerebral cortex, which we think has the potential to revolutionise our understanding of the mammalian brain. The fruits of this cerebroscope will be freely available.

Throughout my quest to understand consciousness, I never lost my sense of living in a magical universe. I do believe some deep and elemental organising principle created the universe and set it in motion for a purpose I cannot comprehend. I grew up calling this god 鈥� but a god much closer to Baruch de Spinoza鈥檚 god than the god of Michelangelo鈥檚 paintings.

A pioneering generation of stars had to die in spectacular supernovae to seed space with the heavier elements needed for the rise of self-replicating bags of chemicals, on a rocky planet orbiting a young star at just the right distance. The competitive pressures of natural selection made possible the accession of creatures with nervous systems. As the complexity of these systems grew to staggering proportions, some of the creatures evolved the ability to reflect on themselves, to contemplate their beautiful but cruel world.

While the rise of sentient life was inevitable, it does not mean Earth had to bear life or that bipedal, big-brained primates had to walk the African grasslands. But I do believe the laws of physics overwhelmingly favoured the emergence of consciousness, and that those laws will lead us to a more or less complete knowledge of it.