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Perchance to Dream…

Do you ever wake up and feel like you’ve spent the whole night replaying a tape of the stresses of the day before? Well, at least you didn’t spend you didn’t spend that day running through mazes. Oh, you did? In that case, you might want to grab a chunk of cheese and sit down for some comfort eating with a friendly rat who can commiserate with you.

Matthew Wilson, an MIT professor of neuroscience, has been studying rats as they work and sleep for years, and has found out that they, too, replay their daily activities as they sleep.

Rat dreaming of running in circles... (MIT image)

In groundbreaking research published in 2001 in the journal Neuron1, Wilson and his colleague Kenway Louie were given an unprecedented glimpse into the dreams of rats by studying rats’ brain activity while they ran through mazes and then later on while they slept. (To clarify, the rats – not the researchers – were the ones who ran through the mazes. Sorry to disappoint you.)

To investigate what happens in the brain during rapid eye movement (REM) sleep, the type of sleep associated with dreaming, the researchers recorded the activity of neurons in the hippocampus (the area of the brain known to be critical to the formation and encoding memories) of four rats, both while the rats ran around circular mazes and then afterwards during REM sleep.

What they found out was striking.

As the rats ran through the mazes, the neurons fired in distinctive patterns that were dependent on where the rats were within the mazes. Then, when the researchers took comparable measurements of the rats’ brain activity later on during the rats’ REM sleep after a hard day of maze running, they found that the rats played back exactly the same neuron activity patterns as had occurred when they originally performed their tasks. More specifically, in 20 of the 45 REM sleep sessions that the researchers measured, they could detect prolonged periods (tens of seconds to several minutes in length) during which the same spatially-correlated hippocampal neurons fired in the same order, with the REM patterns essentially repeating the daytime patterns at approximately the same speed.

During REM sleep, we could literally see these rat brains relive minutes of their previous experience. It was like they were watching a movie of what they had just done.

In his terrific blog The Frontal Cortex2, Jonah Lehrer noted that Wilson was astonished by these results, quoting him as saying, “During REM sleep, we could literally see these rat brains relive minutes of their previous experience. It was like they were watching a movie of what they had just done.”

Wow!

More recently, Wilson and Daoyun Ji, a postdoctoral associate, extended these findings in research published in Nature NeuroScience3. In this newer study, the researchers focused on brain activity during slow-wave sleep (SWS), often referred to as deep sleep, a stage of sleep not characterized by dreaming but thought to be important to long-term memory formation. The researchers wanted to learn more about how the brain consolidates long-term memories during SWS, and whether it replays visual images from daytime experiences as part of the process. To test these matters, the researchers focused on the interaction between two separate areas of the brain: the hippocampus and the visual cortex, which is responsible for processing visual information.

As before, the researchers measured brain activity in four (presumably different!) rats as the rats ran in alternating directions through figure-eight shaped mazes, and then repeated the same measurements while the rats slept both before and after their maze-running sessions. This time, though, the researchers measured activity in both the visual cortex and the hippocampus.

Once again, the findings are notable.

As the rats ran through the mazes, neurons in both brain areas, the visual cortex and the hippocampus, acted similarly, firing in distinctive patterns that were dependent on where the rats were within the mazes. During subsequent SWS periods, the rats replayed these same firing patterns and sequences in both brain areas, much as they had done in the earlier experiment on hippocampus activity during REM sleep. Moreover, at all times, during maze-running activity and later on as the rats replayed their memories during periods of SWS, the brain activity in the visual cortex and the hippocampus were highly correlated.

By linking the visual cortex to this coordinated memory replay process, the researchers were thus able to show that not only were the rats replaying their daytime memories during sleep, but that they were reliving the same sensory experiences, the exact visual images that they had seen during their maze running!

Do these studies provide insight into the neurobiology of sleep, dreams and memory in humans and other animals? All mammals have similar brain structures that seem to operate similarly, and the research was designed to help us gain a better understanding of our own memory formation behavior, but it never hurts to ask these sorts of questions.

Assuming that these findings do have relevance for other species, then it may well be that when your Golden Retriever paddles his feet, rolls his eyes and twitches during sleep, he is indeed reliving that epic battle he recently had with an evil, slobber-covered tennis ball.

_____

1Louie, K., & Wilson, M. (2001). Temporally Structured Replay of Awake Hippocampal Ensemble Activity during Rapid Eye Movement Sleep Neuron, 29 (1), 145-156 DOI: 10.1016/S0896-6273(01)00186-6.

2The Frontal Cortex, “The Neuroscience of Dreaming,” December 19, 2006.

3Ji D, & Wilson MA (2007). Coordinated memory replay in the visual cortex and hippocampus during sleep. Nature neuroscience, 10 (1), 100-7 PMID: 17173043.

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3 Comments

  1. It would certainly be useful if we, like the maze-running rats, could use our dream-time to practice and store information from our daytime activities. And perhaps we do, a little (a boost for last-minute test-studying!) But aren’t human dreams mostly a mishmash of childhood memories, everyday events, a few traumas, some fantasies, tidbits of recent movies watched, random conversations, etc.? Did the rats do nothing all day long except run mazes? Did they run those same mazes more swiftly the next day after dreaming the pathways into longterm memory? Are there human techniques for adapting rat REM patterns as boosts to learning?

    Reply
    • Good questions. I believe the rats did do more than running in the maze during the day, but I don’t have any details. Note that in monitoring the the rats during REM sleep, the researchers found daytime patterns repeated in 20 out of 45 REM sleep sessions, meaning that in over half of the REM sessions, the rats presumably dreamed about things other than maze running. Also, the matching brain activity sequences varied in length, with some being a good bit shorter than a minute – perhaps this means that during the remaining portions of these REM sessions, other (random) thoughts and memories with the maze running replays. As to running the mazes more swiftly, the research papers didn’t discuss this topic, but I’m pretty sure that much other research has established that rats (and other animals) do improve their performance on these sorts of tasks over time; it seems likely to me that the process by which experiences are fixed into long-term memory plays a role in this sort of improvement. Finally, I’m not positive what you’re mean by adapting rat REM patterns as boosts to learning; I do believe that much has been published about improving learning through various sleep techniques, but I’m not sure whether there has been any particularly definitive research in this area.

      Reply
  2. Some long time ago, Margaret Mead (?) came across an island tribe that taught creative dreaming to its children; all aggressions were channeled through dreams, and the peaceful tribe lived happily, free from bullies. The ultimate “safe” dreaming is achieved when one dreams that one is dreaming — at that point, one can even commit murder, knowing that it is not an actual murder at all.
    But more recently, we’ve learned that playing “Baby Einstein” or “Baby Mozart” to infants doesn’t create noticeable benefit.
    So I wonder if the percentage of rats that DO re-live their maze experiences at night have some inner (developed?) ability somehow to file an experience into a dreaming slot for future reference. How wonderful if there could be a deliberate way of using our dream-time.

    Reply

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