The Neuroscience of Dreaming

The purpose of dreaming is learning. While you are sleeping, your brain is digesting the day, deciding which new experiences to consolidate into long-term memory. That's the implication of Matthew Wilson's latest paper, which documented the neural activity in the brains of dreaming rats. Here's the Times:

The finding, reported on the Web site of the journal Nature Neuroscience by Daoyun Ji and Matthew A. Wilson, researchers at the Massachusetts Institute of Technology, showed that during nondreaming sleep, the neurons of both the hippocampus and the neocortex replayed memories -- in repeated simultaneous bursts of electrical activity -- of a task the rat learned the previous day.

The researchers could interpret the memories through electrodes inserted into the rats' brains, including into special neurons in the hippocampus. These neurons are known as "place cells" because each is activated when the rat passes a specific location, as if they were part of a map in the brain. The activation is so reliable that one can tell where a rat is in its cage by seeing which of its place cells is firing.

Earlier this year Dr. Wilson reported that after running a maze, rats would replay their route during idle moments, as if to consolidate the memory, although the replay, surprisingly, was in reverse order of travel. These fast rewinds lasted a small fraction of the actual time spent on the journey.

In the findings reported today, the M.I.T. researchers say they detected the same replays occurring in the neocortex as well as in the hippocampus as the rats slept. The rewinds appeared as components of repeated cycles of neural activity, each of which lasted just under a second. Because the cycles in the hippocampus and neocortex were synchronized, they seemed to be part of a dialogue between the two regions.

This is one of those experiments that just gets more and more interesting as you ponder the data. For one thing, the dreaming simulations occurred first in the neocortex, which suggests that the cortex is starting the conversation with the hippocampus. It decides what to dream about, and then asks the hippocampus to summon up the requisite memories.

"The neocortex is essentially asking the hippocampus to replay events that contain a certain image, place or sound," he said. "The neocortex is trying to make sense of what is going on in the hippocampus and to build models of the world, to understand how and why things happen."

These models are presumably used to direct behavior, Dr. Wilson said. They are able to generate expectations about the world and plausibly fill in blanks in memory.

It's also worth noting that Wilson detected activity in the rat visual cortex as well, which suggests that humans aren't the only animals to dream in visual imagery.

I think Wilson's research is truly groundbreaking - it's the best account of dreaming since Freud - and hasn't gotten nearly enough attention. In my article earlier this year on "learning by doing," I included a brief description of Wilson's seminal 2001 Neuron paper, which also recorded from the dreaming rat brain:

Wilson began his experiment by training rats to run through mazes. While a rat was running through one of them, Wilson measured clusters of neurons in the hippocampus with multiple electrodes surgically implanted in its brain. As he'd hypothesized, Wilson found that each maze produced its own pattern of neural firing. To figure out how dreams relate to experience, Wilson recorded input from these same electrodes while the rats were sleeping. He was astonished by his results. Of the 45 rat dreams recorded by Wilson, 20 contained an exact replica of the maze they had run earlier that day. "During REM sleep, we could literally see these rat brains relive minutes of their previous experience," Wilson says. "It was like they were watching a movie of what they had just done."

But why would rats dream of running through a maze again? What's the advantage of replaying the activities of the day at night? According to Wilson, dreaming is a form of mental cleaning. The brain is figuring out what information it needs to keep. Since Wilson rewarded the successful rats with food, their brains were re-encoding the route, making sure they remembered how to find their way.