What is consciousness? People have been trying to understand it for thousands of years. One University of Wisconsin professor has made developing a theory of consciousness his career, and he may be closing in on an answer.
According Professor Tononi in this article from the New York Times, consciousness is integrated information. What this means is that the neurons in our brains (or I should say, within a single brain) are sharing information. They're not doing this generally; they're organized into structures which then interact with each other. According to information theory, this is the best way to maximize shared information, referred to as phi in the article. Phi cannot be calculated because there are too many possible connections between neurons. However, Tononi is testing his theory by studying how brain activity changes after a sedative is used. Pulses of brain activity are longer, more complex, and more widespread in conscious people than unconscious ones. Someday, doctors may use these differences in brain activity to determine if a person is conscious or not. This test would work even if the patient is unable to respond.
Of course, as a science fiction writer, I'd like to look for ways to test consciousness in animals, robots, and even aliens. Would a test of this type work on these subjects? It probably would on animals, would need to be modified for robots and computers, and unfortunately be unlikely to work in aliens, given that their physiology would be so much different from ours. We'd need to find more universal tests. If we are able to find a universal test for consciousness, it would enlighten us to the general natural of consciousness and perhaps answer some of our basic questions.