Dr. Tononi began developing models of the brain and became an expert on one form of altered consciousness we all experience: sleep. In 2000, he and his colleagues found that Drosophila flies go through cycles of sleeping and waking. By studying mutant flies, Dr. Tononi and other researchers have discovered genes that may be important in sleep disorders.

For Dr. Tononi, sleep is a daily reminder of how mysterious consciousness is. Each night we lose it, and each morning it comes back. In recent decades, neuroscientists have built models that describe how consciousness emerges from the brain. Some researchers have proposed that consciousness is caused by the synchronization of neurons across the brain. That harmony allows the brain to bring together different perceptions into a single conscious experience.

Dr. Tononi sees serious problems in these models. When people lose consciousness from epileptic seizures, for instance, their brain waves become more synchronized. If synchronization were the key to consciousness, you would expect the seizures to make people hyperconscious instead of unconscious, he said.

While in medical school, Dr. Tononi began to think of consciousness in a different way, as a particularly rich form of information. He took his inspiration from the American engineer Claude Shannon, who built a scientific theory of information in the mid-1900s. Mr. Shannon measured information in a signal by how much uncertainty it reduced. There is very little information in a photodiode that switches on when it detects light, because it reduces only a little uncertainty. It can distinguish between light and dark, but it cannot distinguish between different kinds of light. It cannot tell the differences between a television screen showing a Charlie Chaplin movie or an ad for potato chips. The question that the photodiode can answer, in other words, is about as simple as a question can get.

Our neurons are basically fancy photodiodes, producing electric bursts in response to incoming signals. But the conscious experiences they produce contain far more information than in a single diode. In other words, they reduce much more uncertainty. While a photodiode can be in one of two states, our brains can be in one of trillions of states. Not only can we tell the difference between a Chaplin movie and a potato chip, but our brains can go into a different state from one frame of the movie to the next.

“One out of two isn’t a lot of information, but if it’s one out of trillions, then there’s a lot,” Dr. Tononi said.

Consciousness is not simply about quantity of information, he says. Simply combining a lot of photodiodes is not enough to create human consciousness. In our brains, neurons talk to one another, merging information into a unified whole. A grid made up of a million photodiodes in a camera can take a picture, but the information in each diode is independent from all the others. You could cut the grid into two pieces and they would still take the same picture.