Many scientists have speculated that dreams help us process memory or learn new things. But there’s a lot of work between that and figuring out what our bodies are actually doing when we dream. Now, a group of researchers at UCBL in Lyon, France say they’ve found a link between REM sleep and a few specific areas of the brain — at least in rats. It hasn’t cracked the puzzle of dreams, but it might point the way towards some valuable clues.
Humans (and rats) cycle through several stages of sleep every night, and dreaming is possible across multiple stages. But the vivid, elaborate dreams we tend to remember most clearly occur during REM (rapid eye movement) sleep. There's a lot of similarity between imagining doing something in this state and deciding to do it in real life — if our muscles weren't usually paralyzed, we'd start to act out our dreams. The brain activity that produces them, though, is quite different. This latest research found that in rats, REM sleep is associated with only a handful of substructures, which might ultimately be controlled by an even smaller set of structures.
Past research has found that when humans enter REM sleep, there's unusually high activity in areas of the brain related to memory and learning. But Dr. Pierre-Hervé Luppi, a co-author on the study that was published today in Science Advances, says it’s been hard to pinpoint exactly how different this is to normal patterns. "The previous theory was that waking and REM sleep were more similar states," he says. Now, he believes that this activity is limited to a select few areas, compared to our waking brains.
"The previous theory was that waking and REM sleep were more similar states."
Unlike humans, rats can be sacrificed in order to examine their brains in closer detail. At UCBL, the team focused on an indirect measurement of mental activity, checking the expression of specific genes in various parts of the brain. To figure out what expression was specific to REM sleep, they divided the rats into three groups: a control group that slept normally, a second group that had REM sleep disrupted for three days, and a third group that was also deprived of REM sleep but given time to recover. These hypersomniac rats, as they were called, spent significantly more time in REM sleep before being euthanized, and they showed significantly more gene expression in five particular structures — indicating that they play a role in dreams. The structures were part of the limbic system, which is linked with both emotion and memory.
Human studies have already suggested that the limbic system is important to dreaming. Luppi says that his team’s research narrows that down further, to particular groups of neurons. "In humans they knew, okay, it's around there, but now we have a good definition," he says. But the work goes further than that. As a next step, researchers looked for what might be "turning on" these neurons. By injecting tracers around them, they followed a path back to two structures. In theory, this path would work both ways: if you disabled these structures, known as the supramammillary nucleus and the claustrum, the activity would stop. This wasn't tested completely, but researchers did manage to find more evidence of a direct link. In a followup experiment, they created lesions on some rats’ supramammillary nuclei, then checked their brains against a control group. Activity, it turned out, had dropped in one key REM-related area.
Two substructures might play a big role in how we dream
The problem is that Luppi and his team don’t really know what any of these groups or pathways are doing during REM sleep. "Really, what is the function of the activation of these neurons?" he asks. Like other researchers, he believes that memory and emotion are involved, but that’s a fairly general statement, and more specific tests are needed. Fortunately, he says, these results will give them a place to start. Specifically, it’s possible to stop one of these structures from functioning during REM sleep and study the effect this has on an animal. "Do they have problems with emotional behavior?" he says. "Do they have problems with learning? What type of learning?"
Researchers, of course, have been working for a long time on the toll that dream deprivation can take on humans and animals. Luppi points to research demonstrating that animals become less adept at learning if they’re denied REM sleep. "There is already a link," he says. "But we don't know exactly where in the brain, [or] what is happening exactly in the brain."
Luppi says that the team started applying its newly published research long ago. "This article took us 11 years, to put everything together. So it's a huge work. But it means that also during the last two or three years, we already started the next step," he says. That said, don’t bank on anyone finding the true meaning of your dreams — or the precise role of anything in our extraordinarily complex brains — soon. A moment later, he's joking that it might take 10 more years to publish the team’s next big leap.