Sizing Up Consciousness by Its BitsBy CARL ZIMMER
One day in 2007, Dr. Giulio Tononi lay on a hospital stretcher as an
anesthesiologist prepared him for surgery. For Dr. Tononi, it was a
moment of intellectual exhilaration. He is a distinguished chair in
consciousness science at the University of Wisconsin, and for much of
his life he has been developing a theory of consciousness. Lying in
the hospital, Dr. Tononi finally had a chance to become his own

The anesthesiologist was preparing to give Dr. Tononi one drug to
render him unconscious, and another one to block muscle movements. Dr.
Tononi suggested the anesthesiologist first tie a band around his arm
to keep out the muscle-blocking drug. The anesthesiologist could then
ask Dr. Tononi to lift his finger from time to time, so they could
mark the moment he lost awareness.

The anesthesiologist did not share Dr. Tononi’s excitement. “He could
not have been less interested,” Dr. Tononi recalled. “He just said,
‘Yes, yes, yes,’ and put me to sleep. He was thinking, ‘This guy must
be out of his mind.’ ”

Dr. Tononi was not offended. Consciousness has long been the province
of philosophers, and most doctors steer clear of their abstract
speculations. After all, debating the finer points of what it is like
to be a brain floating in a vat does not tell you how much anesthetic
to give a patient.

But Dr. Tononi’s theory is, potentially, very different. He and his
colleagues are translating the poetry of our conscious experiences
into the precise language of mathematics. To do so, they are adapting
information theory, a branch of science originally applied to
computers and telecommunications. If Dr. Tononi is right, he and his
colleagues may be able to build a “consciousness meter” that doctors
can use to measure consciousness as easily as they measure blood
pressure and body temperature. Perhaps then his anesthesiologist will
become interested.

“I love his ideas,” said Christof Koch, an expert on consciousness at
Caltech. “It’s the only really promising fundamental theory of

Dr. Tononi’s obsession with consciousness started in his teens. He was
initially interested in ethics, but he decided that questions of
personal responsibility depended on our consciousness of our own
actions. So he would have to figure out consciousness first. “I’ve
been stuck with this thing for most of my life,” he said.

Eventually he decided to study consciousness by becoming a
psychiatrist. An early encounter with a patient in a vegetative state
convinced Dr. Tononi that understanding consciousness was not just a
matter of philosophy.

“There are very practical things involved,” Dr. Tononi said. “Are
these patients feeling pain or not? You look at science, and basically
science is telling you nothing.”

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.

Consciousness, Dr. 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, Dr. Tononi and his colleagues have been expanding
traditional information theory in order to analyze integrated
information. It is possible, they have shown, to calculate how much
integrated information there is in a network. Dr. 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. “It’s either all on, or all off,” Dr. Tononi said. In
effect, the network becomes one giant photodiode.

Networks gain the highest phi possible if their parts are organized
into separate clusters, which are then joined. “What you need are
specialists who talk to each other, so they can behave as a whole,”
Dr. Tononi said. He does not think it is a coincidence that the
brain’s organization obeys this phi-raising principle.

Dr. 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.

Dr. Koch considers Dr. Tononi’s 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. Dr. Koch and Dr. 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, Dr. Koch and Dr. Tononi
will be able make only a rough approximation of phi, rather than a
precise calculation.

“The lifetime of the universe isn’t long enough for that,” Dr. Koch
said. “There are immense practical problems with the theory, but that
was also true for the theory of general relativity early on.”

Dr. 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, Dr. 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 anesthetized brain, the reverberations
produced a much simpler response in a much smaller region, lasting
just 110 milliseconds. As the midazolam started to wear off, the
pulses began to produce richer, longer echoes.

These are the kinds of results Dr. Tononi expected. According to his
theory, a fragmented brain loses some of its integrated information
and thus some of its consciousness. Dr. Tononi has gotten similar
results when he has delivered pulses to sleeping people — or at least
people in dream-free stages of sleep.

In this month’s issue of the journal Cognitive Neuroscience, he and
his colleagues reported that dreaming brains respond more like wakeful
ones. Dr. Tononi is now collaborating with Dr. Steven Laureys of the
University of Liège in Belgium to test his theory on people in
persistent vegetative states. Although he and his colleagues have
tested only a small group of subjects, the results are so far falling
in line with previous experiments.

If Dr. Tononi and his colleagues can get reliable results from such
experiments, it will mean more than just support for his theory. It
could also lead to a new way to measure consciousness. “That would
give us a consciousness index,” Dr. Laureys said.

Traditionally, doctors have measured consciousness simply by getting
responses from patients. In many cases, it comes down to questions
like, “Can you hear me?” This approach fails with people who are
conscious but unable to respond. In recent years scientists have been
developing ways of detecting consciousness directly from the activity
of the brain.

In one series of experiments, researchers put people in vegetative or
minimally conscious states into fMRI scanners and asked them to think
about playing tennis. In some patients, regions of the brain became
active in a pattern that was a lot like that in healthy subjects.

Dr. Tononi thinks these experiments identify consciousness in some
patients, but they have serious limitations. “It’s complicated to put
someone in a scanner,” he said. He also notes that thinking about
tennis for 30 seconds can demand a lot from people with brain
injuries. “If you get a response I think it’s proof that’s someone’s
there, but if you don’t get it, it’s not proof of anything,” Dr.
Tononi said.

Measuring the integrated information in people’s brains could
potentially be both easier and more reliable. An anesthesiologist, for
example, could apply magnetic pulses to a patient’s brain every few
seconds and instantly see whether it responded with the rich
complexity of consciousness or the meager patterns of

Other researchers view Dr. Tononi’s theory with a respectful

“It’s the sort of proposal that I think people should be generating at
this point: a simple and powerful hypothesis about the relationship
between brain processing and conscious experience,” said David
Chalmers, a philosopher at Australian National University. “As with
most simple and powerful hypotheses, reality will probably turn out to
be more complicated, but we’ll learn something from the attempt. I’d
say that it doesn’t solve the problem of consciousness, but it’s a
useful starting point.”

Dr. Tononi acknowledged, “The theory has to be developed a bit more
before I worry about what’s the best consciousness meter you could
develop.” But once he has one, he would not limit himself to humans.
As long as people have puzzled over consciousness, they have wondered
whether animals are conscious as well. Dr. Tononi suspects that it is
not a simple yes-or-no answer. Rather, animals will prove to have
different levels of consciousness, depending on their integrated
information. Even C. elegans might have a little consciousness.

“Unless one has a theory of what consciousness is, one will never be
able to address these difficult cases and say anything meaningful,”
Dr. Tononi said.

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