HOLD that thought. When it comes to consciousness, the brain may be doing just that. It now seems that conscious perception requires brain activity to hold steady for hundreds of milliseconds. This signature in the pattern of brainwaves can be used to distinguish between levels of impaired consciousness in people with brain injury.
The new study by Aaron Schurger at the Swiss Federal Institute of Technology in Lausanne doesn't explain the so-called "hard problem of consciousness" – how roughly a kilogram of nerve cells is responsible for the miasma of sensations, thoughts and emotions that make up our mental experience. However, it does chip away at it, and support the idea that it may one day be explained in terms of how the brain processes information.
Neuroscientists think that consciousness requires neurons to fire in such a way that they produce a stable pattern of brain activity. The exact pattern will depend on what the sensory information is, but once information has been processed, the idea is that the brain should hold a pattern steady for a short period of time – almost as if it needs a moment to read out the information.
tested this theory by scanning 12 people's brains with fMRI machines. The volunteers were shown two images simultaneously, one for each eye. One eye saw a red-on-green line drawing and the other eye saw green-on-red. This confusion caused the volunteers to sometimes consciously perceive the drawing and sometimes not.
When people reported seeing the drawing, the scans, on average, showed their brain activity was stable. When they said they didn't see anything, it was more variable. Now, Schurger and colleagues have repeated the experiment – using electroencephalography and magnetoencephalography, which measure the electrical and magnetic fields generated by brain activity. These techniques provide greater temporal resolution than fMRI, allowing the team to see how the pattern of activity changes over milliseconds within a single brain.
Based on their earlier work, the team expected the volunteers' brain activity to stabilise and stay that way for hundreds of milliseconds when they reported having seen the drawing, but become highly variable otherwise. "This was borne out really, really nicely in the data," says Schurger.
The team then tested their technique in 116 people with disorders of consciousness. The patients, who were either minimally conscious, in a vegetative state or had just recovered from coma, were played a tone while their brain activity was recorded. The ability of their brains to converge on a stable state correlated with their clinical diagnosis: the more conscious the patient, the greater the stability. "The stability of their brain's response to an auditory beep is an indicator of their state of consciousness," says Schurger.
Neurologist Steven Laureys of the University of Liège in Belgium thinks the work is significant. "The challenge is to translate this into real clinical practice, by cross-validating this new test with other measures. This is terribly important in a field where we have no gold standard of consciousness," he says.
The work augments the global neuronal workspace theory of consciousness – which argues that a number of brain regions erupt in synchronous activity when people report being conscious of something. The findings support the idea that this synchronous activity should remain stable for a few hundred milliseconds.
Michael Graziano of Princeton University is also impressed. "Somehow, processes in the brain lead us to report that we have conscious experiences. What are those processes? How do they occur? This study is a beautiful example of pinning down the speculation with data," he says.
However, it doesn't directly address the hard problem of consciousness. Many neuroscientists, including Graziano, think that the hard problem is an illusion, one that will go away as we better understand the brain. "There is no hard problem," says Graziano. "There is only the question of how the brain, an information-processing device, concludes and insists it has consciousness. And that is a problem of information processing. To understand that process fully will require the kinds of stepwise experiments you see here."