Wikimedia, Gordana Adamovic-MladenovicThe neuroscience of unconsciousness just became a little clearer. Recording brainwaves via electrodes embedded in the brains of human patients at the precise moment that a common, general anesthetic knocked them unconscious, researchers found that neuron activity switched from rapid, cross-brain chatter to slow, uncoordinated waves of firing.
The study, published today (November 5) in Proceedings of the National Academy of Sciences, supports a link between consciousness and coordinated communication in the brain, said Mélanie Boly of the Belgian National Fund of Scientific Research, who was not involved in the study. “When you really look at the fine-grain dynamics of the [neural] networks, you can see that the slow oscillation”—which appears at the moment consciousness is lost—“is disrupting brain connectivity,” she said, and those oscillations could one day be used in the clinic to monitor the unconsciousness of patients under anesthetics.
In the study, a team of researchers, measured neuron activity in three epileptic patients, who’d had electrodes implanted in their brains to help doctors understand what was causing their seizures. Each patient had around 30 large electrodes—each measuring the activity of millions of neurons—spaced about a centimeter apart in one area of the outer layer of the brain. The brain of each patient also carried a 4-millimeter square array that contained 96 microelectrodes, which could measure the firing of individual neurons.
As the patients prepared for surgery to have the electrodes removed, the team monitored the brain activity detected by the electrodes as the patients were put under using a common general anesthetic, called propofol. At the same time, the researchers had the patients listen through headphones to a series of words, like chair and telephone. The recordings also played the patients’ own names every few seconds. When the patients heard their names, they pushed a button, signaling that they were still awake and allowing the researchers to pinpoint when the patients lost consciousness.
“We know there’s a lot of differences between an awake and an unconscious brain, but we were wondering which change was the important one,” said lead author Laura Lewis, a postdoctoral researcher at the Massachusetts Institute of Technology. “The slow oscillation was coming on immediately with the loss of consciousness, and it meant that the neurons were starting to fire at a very different pattern: they would fire normally for a few hundred milliseconds, but then they would become completely silent for several more milliseconds.” Usually, she added, neurons fire so quickly that no patterns are distinguishable.
The results also indicated that nearby neurons fired simultaneously in these slow wave patterns, while neurons in disparate regions of the brain fired at different times.
“We observed that slow oscillations at different areas of the brain were sort of out of synch with one another,” said neuroscientist and senior author on the study Patrick Purdon, of Massachusetts General Hospital and Harvard Medical School. “When one area of the brain has neuronal firing, the other one is basically being silenced,” he explained. “[This] implies that the brain as a whole can’t have long-range communication, and that is a state that would basically make consciousness highly unlikely, if not impossible.”
Indeed, conscious experiences—including sensory, visual, and auditory information—are processed as a unified whole, despite the involvement of several different brain areas, said neuroscientist and anesthesiologist George Mashour of the University of Michigan Medical School. “We know that primary sensory processing is relatively unperturbed by anesthetics, which tells us that information is getting to the brain but we’re not experiencing it,” he said. “The current study helps explain why this might be the case and the conditions that might prevent the large scale integration of information in the brain.”
Neuroscientist Giulio Tononi, of the University of Wisconsin, agrees, but he cautions that their experiment only used one anesthetic and in one condition. “To me, this [study] suggests a mechanism by which propofol can block inter-cortical communication.” In fact, rat studies using different anesthetics have also found slow oscillation, but found that some longer-range brain communication is still possible during unconsciousness, noted neuroscientist Nanyin Zhang, of the University of Massachusetts Medical School, who was not involved in the study.
That said, “the slow oscillation correlated well with the loss of consciousness,” Zhang said, suggesting the pattern could be used as a marker of unconsciousness induced by propofol.
L. Lewis et al., “Rapid fragmentation of neuronal networks at the onset of propofol-induced unconsciousness,” Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1210907109, 2012.