ELECTRODES implanted in the brains of people with epilepsy might just have resolved a question about the mysterious process of 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 鈥渃onsciousness signature鈥� that could be used to probe the process in animals and people with brain damage without inserting electrodes.
Previously it wasn鈥檛 clear whether a dedicated brain area, or 鈥渟eat of consciousness鈥�, is responsible for guiding our subjective view of the world, or whether consciousness is so complicated that it must arise from 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.
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Now neuroscientist Rapha毛l Gaillard of 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鈥檚 team flashed words in front of the volunteers for just 29 milliseconds. The words were either threatening (kill, anger) or emotionally neutral (cousin, see). Sometimes the words were followed by visual 鈥渕asks鈥�, which block the words from being consciously processed. Other times the masks weren鈥檛 used, allowing the words to be consciously processed. The volunteers had 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 almost covered the whole brain. During the first 30 milliseconds of the experiment, brain activity during 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 (PLoS Biology, ).
Because this activity only occurred in volunteers when they were aware of the words, Gaillard鈥檚 team argues that it constitutes a consciousness signature. As much of this activity was spread across the brain, they say that consciousness probably has no single 鈥渟eat鈥�. 鈥淐onsciousness is more a question of dynamics than of a local activity,鈥� says Gaillard.
鈥淐onsciousness has no single seat. It is more a question of dynamics than of local activity鈥�
of the Neuroscience Institute in San Diego, California, who had proposed a 鈥済lobal access鈥� theory of consciousness, agrees: 鈥淚鈥檓 thrilled by these results.鈥� He says they provide the 鈥渇irst really solid, direct evidence鈥� 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.