Anesthesia steals consciousness in stages

The brain doesn’t really go out like a light when anesthesia kicks in. Nor does neural activity gradually dim, a new study in monkeys reveals. Rather, intermittent flickers of brain activity appear as the effects of an anesthetic take hold.

Some synchronized networks of brain activity fall out of step as the monkeys gradually drift from wakefulness, the study showed. But those networks resynchronized when deep unconsciousness set in, researchers reported in the July 20 Journal of Neuroscience.

That the two networks behave so differently during the drifting-off stage is surprising, says study coauthor Yumiko Ishizawa of Harvard Medical School and Massachusetts General Hospital. It isn’t clear what exactly is going on, she says, except that the anesthetic’s effects are a lot more complex than previously thought.

Most studies examining the how anesthesia works use electroencephalograms, or EEGs, which record brain activity using electrodes on the scalp. The new study offers unprecedented surveillance by eavesdropping via electrodes implanted inside macaque monkeys’ brains. This new view provides clues to how the brain loses and gains consciousness.   

“It’s a very detailed description of something we know very little about,” says cognitive neuroscientist Tristan Bekinschtein of the University of Cambridge, who was not involved with the work. Although the study is elegant, it isn’t clear what to make of the findings, he says. “These are early days.”

Researchers from Massachusetts General, Harvard and MIT recorded the activity of small populations of nerve cells in two interconnected brain networks: one that deals with incoming sensory information and one involved with some kinds of movement, and with merging different kinds of information. Before the anesthetic propofol kicked in, brain activity in the two regions was similar and synchronized. But as the monkeys drifted off, the networks dropped out of sync, even though each networks’ own nerve cells kept working together.

Around the moment when the monkeys went unconscious, there was a surge in a particular kind of nerve cell activity in the movement network, followed by a different surge in the sensory network about two minutes later. The two networks then began to synchronize again, becoming more in lockstep as the anesthetic state deepened.