This is your brain under anesthesia

In research on brain implants rodents and EEG readings humans, Brown showed that propofol disrupts cortical communication. But to encourage more science, he and Miller wanted to record different regions at the same time as animals entered and left consciousness. They wanted to use implanted electrodes to hear neurons that change their melodies, to find out how and where the complex communication of the brain breaks down under anesthesia. For the new study, 64-channel microelectrodes were installed in rhesus macaque monkeys. These were stuck in four parts of the brain: the three regions of the cortex and the thalamus. These three cortical regions are the frontal, temporal, and parietal lobes, which are associated with thought, auditory processing, and sensory information, respectively. The thalamus is the size and shape of a quail egg and is deep in the brain, transmitting information throughout the cortex.

Scientists hit the Record at the electrodes before first spilling a bit of propofol, and then saw that the monkeys were slipping unconscious. “The drug goes everywhere, and it arrives in seconds,” Brown says. Brain waves slowed as they crawled. (Neurons in a awake and healthy brain are about 10 times per second. According to Propofol, this frequency drops to a second or less.) Brown was not surprised; he had previously seen these types of slow oscillations in other animals, including humans. But deep electrodes could answer more precisely: what exactly was going on between the neurons?

Typically, neurons speak by pulsing together. “It’s like an FM radio,” Miller says. “They’re on the same channel, they can talk to each other.” Millions of neurons communicate in this way, at many different frequencies. But now the richness of the usual frequencies turned into a low rhythm — a strange harmony. Higher frequencies disappeared, and neurons became common in a low-frequency channel. It seems as if the sounds of a dining room talking in large groups of children, one at a time quietly talking to everything in between, fall into a deep murmur.

According to Brown, the points with the lowest frequency of neuronal activity during anesthesia are more coordinated than in any other mental state. Be attentive, read, sleep, or meditate, your brain waves are chaotic and difficult to study. But not a single signal in an EEG is as clear and rhythmic as anesthesia. And, critically, he believes that this uniformity is what weakens consciousness. This chatter of an alert brain in the dining room may seem like noisy chaos, but it is a consistent language of memories, feelings, and sensations. The rumor of anesthesia is clear, but the information is desert.

“Propofol comes in like a stick,” Miller says, “and it hits the brain in this low-frequency way, because nothing like that is possible.”

Miller and Brown suspected that the thalamus would be especially important in restoring the rich chaos of staying awake. An existing theory suggests that in order to create consciousness, this small lump synchronizes various rhythms in the cortex. If the thalamus stops working, theory goes, cortical waves cannot match their rhythms to communicate cohesive thoughts. “And it’s communication everything in consciousness, ”says Miller.

Once they saw that anesthesia equalized thalamic communication, the researchers wanted to see if stimulating the brain area would lead to signs of conscious activity. Previous work deep brain stimulation has been shown to control a person with a traumatic brain injury and the ability to eat. It’s still a new idea. “It was a bit of a bet, a long shot,” Miller says.