Sizing up consciousness by its bits

After he began developing models of the brain,Tononi 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,Tononi and other researchers have discovered genes that may be important in sleep disorders.

For 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 synchronisation of neurons across the brain. That harmony allows the brain to bring together different perceptions into a single conscious experience. Tononi sees serious problems in these models. When people lose consciousness from epileptic seizures,for instance,their brain waves become more synchronised. If synchronisation were the key to consciousness,you would expect the seizures to make people hyperconscious instead of unconscious,he said.

While in medical school,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. 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 isnt a lot of information,but if its one out of trillions,then theres a lot, 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.

Consciousness,Tononi says,is nothing more than integrated information. Information theorists measure the amount of information in a computer file or a cellphone call in bits,and Dr Tononi argues that we could,in theory,measure consciousness in bits as well. When we are wide awake,our consciousness contains more bits than when we are asleep.

For the past decade Tononi and his colleagues have been expanding traditional information theory in order to analyse integrated information. It is possible,they have shown,to calculate how much integrated information there is in a network. Tononi has dubbed this quantity phi,and he has studied it in simple networks made up of just a few interconnected parts. How the parts of a network are wired together has a big effect on phi. If a network is made up of isolated parts,phi is low,because the parts cannot share information.

But simply linking all the parts in every possible way does not raise phi much. Its either all on,or all off, Tononi said. In effect,the network becomes one giant photodiode. Networks gain the highest phi possible if their parts are organised into separate clusters,which are then joined. What you need are specialists who talk to each other,so they can behave as a whole, Tononi said. He does not think it is a coincidence that the brains organisation obeys this phi-raising principle.

Tononi argues that his Integrated Information Theory sidesteps a lot of the problems that previous models of consciousness have faced. It neatly explains,for example,why epileptic seizures cause unconsciousness. A seizure forces many neurons to turn on and off together. Their synchrony reduces the number of possible states the brain can be in,lowering its phi.

Koch considers Tononis theory to be still in its infancy. It is impossible,for example,to calculate phi for the human brain because its billions of neurons and trillions of connections can be arranged in so many ways. Koch and Tononi recently started a collaboration to determine phi for a much more modest nervous system,that of a worm known as Caenorhabditis elegans. Despite the fact that it has only 302 neurons in its entire body, Koch and Tononi will be able make only a rough approximation of phi,rather than a precise calculation.

Tononi is also testing his theory in other ways. In a study published this year,he and his colleagues placed a small magnetic coil on the heads of volunteers. The coil delivered a pulse of magnetism lasting a tenth of a second. The burst causes neurons in a small patch of the brain to fire,and they in turn send signals to other neurons,making them fire as well. To track these reverberations,Tononi and his colleagues recorded brain activity with a mesh of scalp electrodes. They found that the brain reverberated like a ringing bell,with neurons firing in a complex pattern across large areas of the brain for 295 milliseconds. Then the scientists gave the subjects a sedative called midazolam and delivered another pulse. In the anesthetised brain,the reverberations produced a much simpler response in a much smaller region,lasting just 110 milliseconds. In this months issue of the journal Cognitive Neuroscience,he and his colleagues reported that dreaming brains respond more like wakeful ones. Tononi is now collaborating with Steven Laureys of the University of Liège in Belgium to test his theory on people in persistent vegetative states.