In previous studies on brain implants in the Rodents The EEG readings from Humans, Brown showed that propofol disrupts communication in the cortex. But to push the science even further, he and Miller wanted to simultaneously record different regions when an animal slipped in and out of consciousness. They wanted to use the implanted electrodes to listen to individual neurons changing their tone to see how and where the complex connections in the brain break down under anesthesia. In their new study, they implanted 64-channel electrodes into four rhesus macaques. These were stuck in four sections of their brains: three regions of the cortex and thalamus. These three cortical regions are the anterior, temporal, and parietal lobes, which are related to 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, transmitting information throughout the cortex.
The scientists punched the recording onto the electrodes before the first portion of propofol flowed, then watched the monkeys slide into a state of unconsciousness. “Medication goes everywhere, and it gets there in seconds,” Brown says. Brain waves slowed to creep. (Neurons in a healthy, awake brain rise about 10 times per second. Under propofol, this frequency drops to 1 time per second or less.) Brown was not surprised; He’s seen these types of slow oscillations before in other animals, including humans. But deep electrodes can now answer something more subtle: What exactly happens between neurons?
Usually, neurons speak through a pulse together. “It’s kind of like FM radio,” says Miller. “They are on the same channel, and they can talk to each other.” Millions of neurons communicate in this way, at various frequencies. But now, the usual wealth of frequencies has turned into one low beat – a strange part of harmony. The high frequencies disappeared, and the neurons were left to communicate on the low-frequency channel. It’s as if the sounds of a dining room full of children talking in noisy groups, one-on-one quiet, and everything in between, just collapsed into one deep humming.
According to Brown, the less frequent elevations of nerve activity during anesthesia are actually more coordinated than any other mental condition. Whether you are awake, reading, sleeping, or meditating, your brain waves are messy and difficult to analyze. But there is no clear and regular signal on the EEG like anesthesia. Crucially, he believes that it is this symmetry that undermines consciousness. Lunch room chatter from an alert brain sounds like a noisy mess, but is actually a coherent language of memories, feelings, and sensations. The anesthetic wheeze is obvious, but it’s a desert of information.
“Propofol comes like a hammer, and hits the brain in a low-frequency setting where none of that is possible anymore,” says Miller.
Miller and Brown suspected that the hypothalamus would be especially important to recreate the rich, chaotic state of wakefulness. One current theory proposes that in order to produce consciousness, this tiny fraction synchronizes the different rhythms of the cortex. The theory says that if the thalamus stops working, the cortical waves cannot match their rhythms to convey coherent thoughts. And communicate everything Miller says.
Once they noticed that anesthesia flattened communication from the thalamus, the researchers wanted to see if stimulating this brain region would restore signs of conscious activity. previous job It showed that deep brain stimulation can restore some control of a person’s limbs with a traumatic brain injury, as well as the ability to eat. The idea is still new. “It was kind of a gamble, and long shots,” says Miller.