Measuring consciousness

Scientists have come up with a tool that could help treat coma patients.

A diagram from the seventeeth century attempts to explain consciousness. – Wikimedia Commons

The French philosopher René Descartes believed consciousness was something that lay outside the material world in a separate realm of its own. Now an international team has shown that, material or not, it is measurable. For the first time, Marcello Massimini at the University of Milan in Italy and his colleagues have come up with an index of consciousness. Not only could that help trauma surgeons assess a patient’s level of consciousness, it offers researchers a tool for developing ways to rouse people from coma.

“It’s a great study; for the first time you can reliably distinguish consciousness and unconsciousness resulting from many different causes,” says Nao Tsuchiya, who researches consciousness at Monash University in Melbourne. The study was published in the August issue of Science Translational Medicine.

The problem of consciousness is not a philosophical one for Massimini. A doctor as well as a neurophysiologist, he knows the anguish of caring for an unresponsive patient. Is the patient truly brain dead or merely unable to respond? A salutary reminder of the difficulties in determining this is the recent case of a comatose patient who showed a response on a brain scan when asked to play tennis. But a test like that only works if the patient can hear and understand words, which is not always the case with brain-injured patients.

As a possible alternative, researchers have tried to identify a direct signature of consciousness using EEGs and fMRIs, which respectively measure brain activity via its electrical signals or its oxygen uptake. But the problem is that the brain of someone who has just passed out in an epileptic fit has a similar synchronised signature to a fully conscious brain.

“We took a step back,” says Massimini. “We tried to come up with a theoretical description of consciousness.” This was based on the Integrated Information Theory put forward by Giulio Tononi, a psychiatrist and neuroscientist at the University of Wisconsin. “It’s the only really promising fundamental theory of consciousness,” says Christof Koch, a neuroscientist who studies consciousness at the Allen Institute for Brain Science in Seattle, US.

In stripped-down form, the theory says that a conscious experience is incredibly rich: colours, shapes, faces, sounds, smells, emotions, all experienced at the one time. It is information-rich, yet overall there is a single experience. So a conscious brain is typified by both richness and unity. By contrast, an unconscious brain may show unity but not richness – for instance the rapid synchronous firing across the epileptic’s brain or the relatively simple pulse emitted by the entire brain of someone in a deep sleep. The difference would be like that between a natural forest and pine plantation. The natural forest is bursting with biodiversity; the pine forest is a repeating motif.

That contrast of rich versus poor information reminded the Italian team of something. Computer algorithms routinely compress information into files small enough to email by “zipping”. Repeating motifs like a pine forest are easy to “zip”. But the information-rich natural forest is much tougher. The team came up with a similar idea to try and measure consciousness: how “zippable” was the brain’s activity?

The idea was to ping the brain of a subject using a magnetic impulse (known as transcranial magnetic stimulation) and then record the electrical echoes using 60 electrodes placed across the scalp. The signals were transformed into a digital value of zeroes and ones. This digital information was zipped and used to give a value called the Perturbation Complexity Index or PCI. The prediction was that a conscious brain would respond with many diverse echoes and score a high PCI. The unconscious brain would either have few echoes or lots of identical echoes. Both would be easy to zip and score a low PCI.

The prediction was tested first in single individuals during their wake-sleep cycle. In deep sleep, they scored a low PCI. In REM sleep or when fully awake they scored a high PCI. Next, working in hospitals, the team tested their instrument on three sets of patients: those undergoing anaesthesia; those in a vegetative state; and those with “locked-in syndrome” who are alert but completely paralysed. The PCI validated the team’s expectation. While patients under anaesthesia or in a vegetative state scored low, those with locked-in syndrome scored as high as alert people.

If the results hold up in bigger studies, the PCI will be a tremendous tool for measuring consciousness in the clinic. But Massimini acknowledges his arguments with philosophers are not going to stop any time soon. “We’re not saying we’re reducing consciousness to a number; our index is actually telling us that consciousness is unzippable.”

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