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However, Descartes was profoundly wrong, it appears, in his assertion that mind and body are wholly independent. The mind, argues University of Iowa neurologist Antonio Damasio in his book Descartes' Error, is created by the body-specifically by the brain. Utterly contrary to common sense, though, and to the evidence gathered from our own introspection, consciousness may be nothing more than an evanescent by-product of more mundane, wholly physical processes -- much as a rainbow is the result of the interplay of light and raindrops. Input from the senses clearly plays a part; so do body chemicals whose ebb and flow we experience as feelings and emotions. Memory, too, is involved, along with language-the way humans translate concepts into symbolic form.
As neurologists gain deeper insights into each of these processes, they come ever closer to the central mystery of consciousness itself. Among the front-line areas of research:
BUILDING A DATA BASE We think of learning and memory as somehow separate functions; in fact, they're not. Both are processes by which we acquire and store new data in a way that makes them retrievable later on. The storage takes place, says the current theory, as a pattern of connections among neurons, the nerve cells that serve as the brain's basic building blocks. When information-the image of a new acquaintance's face, for example-enters the brain, it arrives in the form of electric impulses streaming from the retina, up the optic nerve and into the cerebral cortex, the so-called gray matter that houses the brain's higher functions.
The impulses die away within milliseconds, but their passage reinforces the particular set of connections between this particular set of neurons, giving them the ability to re-create the image. The more often the pattern is reinforced-by repeated sightings of the person, by the effort to remember him or by connection with some other mental trigger ("This woman is attractive; she's worth getting to know better," or, "This man looks unpleasant; I need to avoid him")-the more likely, says Damasio, the pattern, or image, will not go into short-term memory, lasting weeks or months, but into permanent, long-term memory. And from there, barring brain injury, disease or old age, it can be re-created by inducing the neurons to send up electric impulses in the old, by now familiar pattern.
That's the simple version. In fact, almost every memory is made of many different patterns of neuronal connections, some for sounds, some for sights, some for smells or textures-tens of thousands of neurons firing off minute electric impulses simultaneously. The combination of all these patterns, says Larry Squire, a professor of psychiatry at the University of California at San Diego, "gives you a complete perception. The persistence of the firing patterns over time gives you the transformation from perception to memory of that object." The fact that many overlapping patterns are stored together means that a single stimulus can bring on a flood of remembrance-as Marcel Proust's taste of a cookie triggered intense memories of his childhood, which in turn inspired him to write his monumental Remembrance of Things Past.