Dreaming and Remembering

Ed Yong has a typically great post on a new Current Biology paper that investigates the link between dreaming and memory consolidation:

The last decade of research has clearly shown that sleep is one of the best aide memoires that we have. During this nightly time-out, our brain can rehearse information that it has picked up during the day and consolidate them into lasting memories. Wamsley's new study supports that idea but it also shows that dreaming while you nap can strengthen our memories even further.

She asked 99 volunteers to learn the layout of a complex virtual maze so that they could reach a specific landmark after being dropped at a random starting point. Five hours later, they were tested again. Those who had stayed awake in the intervening time beat their previous times by 26 seconds, but those who had had a 90-minute nap improved by a whopping 188 seconds.

But those who dreamt about the task fared even better. Wamsley either asked her recruits directly about whether they dreamt about the labyrinth, or asked them to give an open-ended report of everything that was going through their mind while they were asleep. Either way, those who had thought about the maze during their short nap improved far more than those who didn't. They also beat those who mentally replayed their training again while awake. These striking results suggest that there's something special about the mental rehearsals that happen during dreaming sleep.

However, the dreams weren't straightforward replays of previous experiences. When the volunteers described their dreams, they didn't mention specific objects, locations or routes through the maze. Instead, some talked about isolated parts of their experience, like the music or the prospect of a re-test. Others discussed tangential memories, like other mazes or being stuck in a bat cave (heh). Interestingly, scientists have found the same thing in rodents. A sleeping rat will show similar brain activity to its prior bout of wakefulness, but the two patterns won't quite match up.

Last month, I wrote about some related research in the New York Times.

So why are dreams so much more than literal playbacks of the day just passed? Why the non-sequiturs, the long forgotten characters and the unexplained state of public undress? Wilson speculates that dreams are also an attempt to search for associations between seemingly unrelated experiences, which is why it's so important for the controlling conscious self to disappear. What does this maze have to do with that maze? How can we use the lessons of today to get more food pellets tomorrow? This suggests that the strangeness of our nighttime narratives is actually an essential feature, as our memories are remixed and reshuffled, a mash-up tape made by the mind.

In recent years, scientists have discovered that R.E.M. sleep isn't just essential for the formation of long-term memories: it might also be an essential component of creativity.

In a 2004 paper published in Nature, Jan Born, a neuroscientist at the University of Lübeck, described the following experiment: a group of students was given a tedious task that involved transforming a long list of number strings into a new set of number strings. This required the subjects to apply a painstaking set of algorithms. However, Born had designed the task so that there was an elegant shortcut, which could only be uncovered if the subjects saw the subtle links between the different number sets. When left to their own devices, less than 25 percent of people found the shortcut, even when given several hours to mull over the task. However, when Born allowed people to sleep between experimental trials, they suddenly became much more clever: 59 percent of all participants were able to find the shortcut. Born argues that deep sleep and dreaming "set the stage for the emergence of insight" by allowing us to mentally represent old ideas in new ways.

One of the main remaining controversies for sleep researchers is whether or not REM dreams are a mere side-effect of a subterranean process - this would suggest that the narratives themselves don't matter - or are actually a core feature of the sleep-remembering cycle. This is an academic question with plenty of practical relevance, as it will determine whether or not it's worth recounting our dreams in polite conversation.