Perhaps the most surprising insight that has come out of the past 20 years of scholarly investigation into the nature of consciousness is that it might be far more widely shared among all of nature's children than most of us think. By consciousness I mean the ability to feel something, anything -- whether it's the sensation of an azure-blue sky, a tooth ache, being sad, or worrying about the deadline two weeks from now. Indeed, it may be possible that all animals share some minimal amount of sentience with people, that all animals have some feelings, however primitive.
Pet owners vigorously assert that their dogs and cats experience the pains and pleasure of life. Anyone who has observed a chimpanzee grimace at its own face in a mirror and then inspect its teeth and its backside will grant it at least some limited form of self recognition and feelings of self. Nature documentaries bring us closer to the suffering and joys of animals in the wild. Indeed, in the U.S., the Animal Welfare Act of 1966 encodes this general public understanding by granting mammals (with the notable exception of rats, mice, and farm animals) special protection against needless suffering not afforded to birds or to cold-blooded animals.
Detailed investigations by behavioral biologists have demonstrated that many other species have complex cognitive abilities such that were they to occur in humans, would be associated with consciousness. Octopuses can learn from each other; and ravens, magpies, parrots, and other birds can perform feats of problem solving, insight, and memorization, and even bees (with under one million nerve cells) can recognize individual faces, learn to navigate by landmarks, and chose a new hive site by deliberations and several days of dancing.
The two principal features that distinguish people from other animals is our hypertrophied ability to reflect upon ourselves (self-consciousness) and language. Yet there is little reason to deny consciousness to animals simply because they are mute or, for that matter, to premature infants because their brains are not fully developed. There is even less reason to deny it to people with severe aphasia who, upon recovery, can clearly describe their experiences while they were incapable of speaking. The perennial habit of introspection has led many intellectuals to devalue the unreflective, nonverbal character of much of life. The belief in human exceptionalism, so strongly rooted in the Judeo-Christian view of the world, flies in the face of all evidence for the structural and behavioral continuity between animals and people.
While the ancient Egyptians and Hebrews placed the psyche in the heart and the Mayans located it in the liver, we now know that the conscious mind is a product of the brain. That the world of the mind is closely related to the physical structure of the brain is dramatically demonstrated by a stroke or a strong blow to the head that extinguishes conscious experiences.
However, exactly how organized brain matter gives rise to images and sounds, lust and hate, memories, dreams, and plans, remains unclear. I have spent the last quarter of a century linking specific aspects of consciousness to the brain. Sixteen of those years I worked closely with my mentor, colleague, and friend, Francis Crick, introducing the idea of the neuronal correlates of consciousness (NCC), the minimal neuronal mechanisms that give rise to any one conscious experience.
We now know that some regions of the cerebral cortex, which make up the bulk of the brain, have a more privileged relationship to consciousness. Not all of its many regions participate equally in generating the content of any one conscious experience. We also know that the neocortex can be active, as measured by microelectrodes or by magnetic scanners, without necessarily giving rise to a conscious experience. This is the domain of the non- or un-conscious -- those bodily functions and instincts that are automatic, that occur without thought -- moving our bodies in complex ways, responding instantaneously to perceived threats, some emotion welling up.
The cerebral cortex is remarkably constant across different species. Indeed, it takes an expert neuroanatomist to distinguish between a pea -sized chunk of cerebral cortex taken from a mouse, a monkey, and a person. Our brains are big, but other creatures -- elephants, dolphins, and whales -- have bigger ones. There are no qualitative differences between mice, monkeys, or people at the genomic, synaptic, cellular, or connectional levels. The differences are quantitative -- the human brain has about 86 billion neurons, a thousand times more than the brain of a mouse.
But knowing that the number of neurons contributes to the level of consciousness is only the beginning. The challenge that remains is to understand how the whispering of nerve cells, interconnected by thousands of gossamer threads (their axons), give rise to any one conscious sensation. This is a problem that is being vigorously being tackled by neuroscientists. Indeed, it will be essential to "crack the neural code" to finally understand the ancient mind-body riddle.
No matter what the NCC will prove to be, a skeptic can always ask why does this particular NCC give rise to a conscious experience but not another one? The cause and effect between neuronal activity in the brain and conscious thought can seem as magical as rubbing a brass lamp and having a genie emerge. It is here that the ideas of Giulio Tononi, a psychiatrist and neuroscientist, prove crucial. He advocates for a sophisticated theory that links information to consciousness. His integrated information theory introduces a precise measure capturing the extent of consciousness called Φ (phi). 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. 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 are quiet) to a shape in a high-dimensional space.
Integrated information makes specific predictions about which brain circuits are involved in consciousness and which one are peripheral players, even though they might contain many more neurons. The theory should allow clinicians to build a consciousness-meter to assess, in a quantitative manner, the extent to which severely brain injured patients are truly in a vegetative state, versus those who are partially conscious, but simply unable to signal their pain or discomfort. Most of us will remember Terri Schiavo, the woman who came to be at the heart of such a debate.
I've been careful to stress that any network possesses integrated information. The theory is very explicit on this point: Any system whose functional connectivity and architecture yield a phi value greater than zero has at least a trifle of experience. This would certainly include the brains of bees. Just because bees are small and fuzzy does not mean that they cannot have subjective states. So, the next time a bee hovers above your breakfast, attracted by the golden nectar on your toast, gently shoo her away. She might be a fellow sentient being, experiencing her brief interlude in the light.