Illustration by Dave Casey
What is consciousness? This really breaks down into two questions: The first is the nature of qualia--how does the awareness of sensations like bitter, or painful, or red arise from the activity of neurons? The second: How does the sense of self—the person who experiences qualia—arise?
It has been suggested that the first problem is more tractable and should be tackled before going on to the issue of the self, which has elements of unity, continuity, a sense of agency and less obviously, the attachment of meaning to mere sensations. I disagree. I suggest that qualia (e.g., visual awareness) and self are two sides of a coin; you cannot solve the qualia problem without understanding the self. The reason is obvious: You cannot have "free floating" qualia without a self to experience them and to give them meaning.
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We know that awareness is not a property of the whole brain, so the problem can be reduced to, "What particular neural circuits are involved in consciousness? And what's so special about these circuits that they can explain consciousness?"
I suggest that a new set of brain structures evolved during hominid evolution, turning the output from more primitive sensory areas of the brain into what I call a "metarepresentation." I think they edited, enhanced and packaged sensory information into more manageable chunks, used for juggling symbols and language. And most important, they made the link to meaning—whereby the sensory objects we perceive evoke multiple parallel implications in our minds. For example, an apple has potentially infinite nuances of meaning for humans, such as baking, keeping the doctor away, tempting Eve. But for a lemur, apple has no "meaning"; it's simply identifiable as food.
I believe the anatomical structures involved in creating this metarepresentation include the inferior parietal lobule, Wernicke's language comprehension area and the anterior cingulate cortex. Find out how these structures perform their job and we will have figured out what it means to be a conscious human being.
—V.S. Ramachandran, MD, PhD, is director of the Center for Brain and Cognition at UC, San Diego, and was the BBC Reith lecturer for 2003.