In earlier studies on brain implants in rodents and EEG readings from humans, Brown showed that propofol disrupts communication in the cortex. But to push the science further, he and Miller wanted to record different regions simultaneously as an animal slips in and out of consciousness. They wanted to use implanted electrodes to listen to individual neurons changing their tunes to get at how—and where—the brain’s complex communication breaks down under anesthesia. For their new study, they implanted 64-channel microelectrodes into four rhesus macaque monkeys. These were stuck into four sections of their brains: three regions of the cortex and the thalamus. Those three cortical regions are the frontal, temporal, and parietal lobes, which are associated with thinking, auditory processing, and sensory information, respectively. The thalamus is about the size and shape of a quail egg and sits deep in the brain, relaying info all around the cortex.
The scientists hit Record on the electrodes before flowing the first bit of propofol, and then they watched as the monkeys slipped into unconsciousness. “The drug goes everywhere, and it gets there in seconds,” Brown says. Brain waves slowed to a crawl. (Neurons in a healthy, awake brain spike about 10 times per second. Under propofol, that frequency falls to once per second or less.) Brown wasn’t surprised; he’d seen these types of slow oscillations before in other animals, including humans. But the deep electrodes could now answer something more precise: What exactly was going on among the neurons?
Normally, neurons chitchat by pulsing together. “Kind of like an FM radio,” Miller says. “They’re on the same channel, they can speak to one another.” Millions of neurons communicate this way, at many different frequencies. But now, the usual wealth of frequencies morphed into one low rhythm—a strange bit of harmony. Higher frequencies went away, and neurons were left communing on a low-frequency channel. It’s as if the sounds of a lunchroom packed with kids speaking in loud groups, quiet one-on-ones, and everything in between, just collapsed into one deep hum.
According to Brown, the less frequent spikes of neural activity during anesthesia are actually more coordinated than in any other mental state. Whether you’re alert, reading, sleeping, or meditating, your brain waves are chaotic and tough to parse. But no signal is as clear and rhythmic on an EEG as anesthesia. And, critically, he believes, it’s this uniformity that undermines consciousness. That lunchroom chatter from an alert brain seems like noisy chaos, but it’s actually a coherent language of memories, feelings, and sensations. The hum of anesthesia is clear, but it’s an information desert.
“Propofol comes along like a sledgehammer,” Miller says, “and just knocks the brain into this low-frequency mode where none of that is possible anymore.”
Miller and Brown suspected that the thalamus would be especially important for reinstating the rich chaos of being awake. One existing theory suggests that, in order to produce consciousness, this small nub syncs up the various rhythms of the cortex. If the thalamus stops working, the theory goes, cortical waves can’t match their rhythms to communicate cohesive thoughts. “And communication is everything in consciousness,” Miller says.
Once they had observed that anesthesia flattened communication from the thalamus, the researchers wanted to see if stimulating that brain area would bring back signs of conscious activity. Previous work has shown that deep brain stimulation can restore some limb control to a person with a traumatic brain injury, as well as the ability to eat. Still, the idea is new. “It was a bit of a gamble, a long shot,” Miller says.