Projected on a color monitor, the scans yield rows of wrinkled raisin-shaped images of my brain. Daubs of bright color highlight the regions that became active when I physically moved my fingers and those regions that became active when I imagined the same action.
Both tasks--real and imagined--lighted up the same areas of the brain responsible for movement.
At some fundamental level, thinking about an action and performing it appear to be almost the same.
"To me that is a fascinating response," said Cohen. It runs "counter to everything I was expecting."
The images his machine creates appear to draw a direct connection between the inner life of the mind and the physical mechanisms of the brain.
"In a sense, this kind of functional imager . . . allows me to study covert mental activity--in essence, thought," he said.
Other recent imaging studies of brain function reinforce the relationship between conscious mental states and the physical structures of the brain:
* When mental patients hallucinate and hear imaginary voices, those portions of their brain responsible for hearing respond as if the voices were real.
* When people are asked to picture a map of the United States in their mind, their brains respond as if they were looking at an actual map, activating that part of the cortex responsible for vision.
* When people are asked to picture an object and then rotate it mentally, their brains act as if the object were turning in front of them.
One surprising thing about these images of perception is that they do not show where or how the mind's eye is focused in the brain.
Subjectively, perceptions always appear seamlessly, presented in the mind as an integrated whole. But these scans showed no sign of an active anatomical area where sensory stimuli were coordinated and collated.
So how is consciousness organized in the physical structures of the brain?
To Cohen, the images suggest that consciousness is itself the act of attention.
As the brain evolved, it must have developed a way to focus itself selectively; otherwise, the constant burble of brain activity and sensory perceptions would overwhelm it. In this theory, consciousness may arise from the brain's need to concentrate, momentarily highlighting some neural activities at the expense of others.
Consciousness therefore may not be continuous, Llinas of New York University suggested.
It may flicker on and off as needed, like a spotlight flashing in the darkened theater of the mind.
An Elusive Quarry
Some scientists contend that consciousness emerges from the union of all the brain's physical properties, the way a rainbow arises from the interplay of light, suspended water droplets and air.
If that is the case, they argue, scientists will never find the human mind no matter how hard they scrutinize the brain's physical structure, any more than someone will find a rainbow in any one of its scattered parts.
Indeed, Chalmers believes scientists will discover eventually that human consciousness is an irreducible quality of the universe, like space, mass or time.
"Instead of trying to explain consciousness purely in terms of its physical processes, you should take it as a fundamental entity in its own right," he said.
"We would like a unified theory of consciousness in the same way that physicists are searching for a unified theory of matter," he said.
Some worry that by reducing human consciousness to its biological components, scientists will tarnish the worth of the human spirit or undercut ideas of personal responsibility and free will. But other researchers believe that a more detailed understanding of the brain will only enhance individual self-respect by affirming the uniqueness of each human mind.
"You will understand," said Nobel laureate Gerald Edelman at the Neurosciences Institute in La Jolla, "why your individuality is important."
In some ways, the search for the roots of human consciousness stands much of traditional scholarly inquiry on its head.
Scientists who, by temperament and training, devote their lives to the collection of objective facts about the universe around them are forced to grapple with the imponderables of the world within.
Researchers who are trained to exclude the human element from experiments discover that the human element is the experiment.
Philosophers, more accustomed to purely metaphysical speculation, are now expected to buttress their ideas with hard data.
All of them feel caught up in a uniquely human endeavor that--for better or worse--promises to alter forever humanity's sense of itself.
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THE BRAIN: A WORK IN PROGRESS
A growing number of scientists are trying to study the one thing that many believe cannot be labeled, scrutinized or defined--human consciousness.
THE SEARCH FOR UNDERSTANDING
'There is something very mysterious about consciousness. Why can objective physical systems have subjective states? It is baffling. It gets at the central idea of the soul.'