Electrodes implanted in the brains of people with epilepsy might have resolved an ancient question about consciousness.
Signals from the electrodes seem to show that consciousness arises from the coordinated activity of the entire brain. The signals also take us closer to finding an objective "consciousness signature" that could be used to probe the process in animals and people with brain damage without inserting electrodes.
Previously it wasn't clear whether a dedicated brain area, or "seat of consciousness", was responsible for guiding our subjective view of the world, or whether consciousness was the result of concerted activity across the whole brain.
Probing the process has been a challenge, as non-invasive techniques such as magnetic resonance imaging and EEG give either spatial or temporal information but not both. The best way to get both simultaneously is to implant electrodes deep inside the skull, but it is difficult to justify this in healthy people for ethical reasons.
Now neuroscientist Raphaël Gaillard of INSERM in Gif sur Yvette, France, and colleagues have taken advantage of a unique opportunity. They have probed consciousness in 10 people who had intercranial electrodes implanted for treating drug-resistant epilepsy.
While monitoring signals from these electrodes, Gaillard's team flashed words in front of the volunteers for just 29 milliseconds. The words were either threatening (kill, anger) or emotionally neutral (cousin, see).
The words were preceded and followed by visual "masks", which block the words from being consciously processed, or the masks following the words weren't used, meaning the words could be consciously processed. The volunteers had to press a button to indicate the nature of the word, allowing the researchers to confirm whether the volunteer was conscious of it or not.
Between the 10 volunteers, the researchers received information from a total of 176 electrodes, which covered almost the whole brain. During the first 300 milliseconds of the experiment, brain activity during both the non-conscious and conscious tasks was very similar, indicating that the process of consciousness had not kicked in. But after that, there were several types of brain activity that only occurred in the individuals who were aware of the words.
First, there was an increase in the voltage levels of the signals in their brains. Second, the frequency and phase of neurons firing in different parts of the brain seemed to synchronise. Then some of these synchronised signals appeared to be triggering others. For example, activity in the occipital lobe seemed to cause activity in the frontal lobe.
Because this activity only occurred in volunteers when they were aware of the words, Gaillard's team argue that it constitutes a consciousness signature. As much of this activity was spread across the brain, they say that consciousness has no single "seat". "Consciousness is more a question of dynamics, than of a local activity," says Gaillard.
Bernard Baars of the Neuroscience Institute in San Diego, California, who proposed a "global access" theory of consciousness in 1983 agrees: "I'm thrilled by these results."
He says they provide the "first really solid, direct evidence" for his own theory. He also says that having such a signature will make it easier to look for signs of consciousness in people with brain damage, infants and animals with the help of non-invasive techniques such as EEG.
Journal reference: PLoS Biology, DOI: 10.1371/journal.pbio.1000061
For similar stories, visit the The Human Brain Topic Guide
NEW SCIENTIST: 17 March 2009 by Anil Ananthaswamy