Researchers in Germany have found that when the human brain is in a resting state—like daydreaming or mind wandering—that patterns of neuronal activity which represent past events reappear spontaneously. The researchers believe that these 'daydream induced' neuronal recurrences are necessary to consolidate short-term memories to long-term memory.
The most widely held theory of memory formation assumes that long-term memories are stored and hard-wired in a gradual manner. At first, the brain stores new information in a temporary holding space. For memories to become long-term, another step—now being linked to both nighttime and daytime resting states—is required. The new study titled “Memory Consolidation by Replay of Stimulus-Specific Neural Activity” was published in the December 2013 issue of The Journal of Neuroscience.
Recurring Neural Activity of a Memory Hardwires That Memory
Dr. Nikolai Axmacher, who is lead author of the study and a researcher in the Department of Epileptology at the University of Bonn, explains, “We call it consolidation. We do not know exactly how this happens. However, studies suggest that a process we call reactivation is of importance. When this occurs, the brain replays activity patterns associated with a particular memory. In principle, this is a familiar concept. It is a fact that things that are actively repeated and practiced are better memorized. However, we assume that a reactivation of memory contents may also happen spontaneously without there being an external trigger.”
Axmacher and his team tested this hypothesis in an experiment with ten healthy participants whose average age was 24. The test subjects were shown a series of pictures, which displayed—among other things—frogs, trees, airplanes and people. Each of these pictures was associated with a white square as a label at a different location. The subjects were asked to memorize the position of the square each time.
At the end of the experiment, all images were shown again, but this time without the label. The study participants were then asked to indicate with a mouse cursor where the missing mark was originally located. Memory performance was measured as the distance between the correct and the indicated position.
“This is an associative task. Visual and spatial perceptions have to be linked together,” Axmacher explains. “Such tasks involve several brain regions. These include the visual cortex and the hippocampus, which takes part in many memory processes.” Brain activity was recorded by fMRI during the entire experiment, which lasted several hours and included resting periods and a nap inside the neuroimaging scanner.
If you'd like to read more on how memories are specifically "geotagged," check out my recent Psychology Today blog post, "How Does the Brain Rembember the Places of Your Past?"
For the data processing of the findings a pattern recognition algorithm was devised to look for similarities between neuronal patterns observed during initial encoding and the same patterns appearing at later occasions. “This method is complex, but quite effective," Axmacher says. “Analysis showed that neuronal activity associated with images that were shown initially did reappear during subsequent resting periods and in the sleeping phase.”
Memory performance was found to correlate with the replay of neuronal activity patterns. Axmacher summarizes the findings saying, “The more frequently a pattern had reappeared, the more accurate test participants could label the corresponding image. These results support our assumption that neural patterns can spontaneously re-appear and that they promote the formation of long-lasting memory contents. There was already evidence for this from animal studies. Our experiment shows that this phenomenon also happens in humans.”
Mind Wandering Strengthens Memories
A 2010 study from New York University also found that memories are strengthened during periods of mind wandering while we are awake. The research built on previous studies which have shown this process occurs during sleep, but not during times of daydreaming. "Taking a coffee break after class can actually help you retain that information you just learned," explained Lila Davachi, an assistant professor in NYU's Department of Psychology and Center for Neural Science. "Your brain wants you to tune out other tasks so you can tune in to what you just learned."
To determine if memory consolidation occurred during periods of awake rest, the researchers did fMRI brain imaging the hippocampus, a brain structure known to play a significant role in memory, and cortical regions during periods of awake rest.
The NYU experiment tested subjects' associative memory by showing them pairs of images containing a human face and an object (e.g., a beach ball) or a human face and a scene (e.g., a beach) followed by periods of awake rest. Subjects were not informed that their memory for these images would later be tested, but, rather, were instructed to rest and simply think about anything that they wanted, but to remain awake during the resting periods.
The researchers used functional magnetic resonance imaging (fMRI) to gauge activity in the hippocampus and cortical regions during the task and during the ensuing rest period. The experiment uncovered two noteworthy results:
First, the researchers found that during rest after the study experience (after the visuals were shown), there was a significant correlation between brain activity in the subjects' hippocampus and cortical regions that were active during the initial encoding of each stimulus pair. This boost in brain correlations was only seen following experiences that were later memorable suggesting these parts of the brain act in tandem for a purpose—to consolidate memories during rest.
Second, when examining each subject individually, it was found that subjects who had greater resting correlations between the hippocampus and cortex, also exhibited better performance on a subsequent associative memory test and those whose brain correlations were weaker, had worse memory—in other words, the greater the connectivity between the hippocampus and cortical regions, the stronger the memory.
"Your brain is working for you when you're resting, so rest is important for memory and cognitive function," Davachi concluded. "This is something we don't appreciate much, especially when today's information technologies keep us working round-the-clock.
Are Smartphones Making Us Less Intelligent?
Like most people, I am addicted to having my smartphone near me at all times. I am like a Pavlovian dog who salivates when I hear my phone ding with an incoming message. It's ridiculous. One of the problems of living in this hyper-connected digital age is that we are constantly fed a stream of new information and spend less and less time daydreaming. How is the lack of daydreaming affecting our long-term memory formation?
We have become so accustomed to being constantly barraged with new stimuli that it has become like a drug. Our attention spans are shorter and we get bored easily. Without time to contemplate and reflect on past events, our ability to form long-term memories is impaired and we end up floating in a cyber vortex that has less long-term roots to memories. The vaccuum is filled with a constant stream of new data pouring in from our smartphones and iPads. When was the last time you actually allowed yourself to feel ‘bored’ without reaching for your smartphone? Could this lack of mind wandering be short-circuiting our ability to consolidate long-term memories en masse?
One reason that working out regularly is more important than ever in the 21st century is that when you are doing aerobic activity you are forced to disconnect from the screen of your smartphone. Exercise creates a daydreaming state that allows you to consolidate memories. For many people (like myself) listening to music while working out actually facilitates a waking dream state and I would encourage you to work out with music if you enjoy it.
As a triathlete and ultrarunner, I realize that running, biking and swimming—or any type of aerobic exercise—leads to a constructive state of mindwandering. Like clockwork, within 7-12 minutes of rhythmic aerobic activity there is a very predictable shift in your thought processes. This is neurobiological and happens to everybody. Your thinking becomes more fluid and all parts of your brain synchronize in a way that helps you problem solve and come up with solutions. This is a universal phenomenon that anyone who exercises regularly experiences.
Aerobic exercise creates a waking state of mental rest that allows you to connect ideas in new and useful ways while also consolidating memories. Surely, if neuroscientists could have people do aerobic activity in an fMRI they could better understand the specific changes in brain function caused by exercise.
Conclusion: More Research on Memory Consolidation Is Needed
The new study from Germany indicates that mental resting periods where you are not taking in new information improve memory performance. Axmacher noted, “Though, our data did not show whether sleeping had a particular effect. This may be due to the experimental setup, which only allowed for a comparatively short nap [inside an fMRI]. By contrast, nighttime sleep is considered to be beneficial for the consolidation of memory contents. But it usually takes many hours and includes multiple transitions between different stages of sleep. However, other studies suggest that even short naps may positively affect memory consolidation.”
There is still some speculation as to whether recurring brain patterns are triggered by conscious memories or whether they remained below the threshold of conscious perception. Nikolai Axmacher said, “I think it is reasonable to assume that during resting periods the test participants let their mind wander and that they recalled images they had just seen before. But this is a matter of subjective perception of the test participants. That’s something we did not look at because it is not essential for our investigation.”
Axmacher concludes, “The strength of our approach lies rather in the fact that we look at memory contents from the outside, in an objective manner. And that we can evaluate them by pattern recognition. This opens ways to many questions of research. For example, brain patterns that recur spontaneously are also of interest in the context of experimental dream research.”
If you'd like to read more on this topic, check out my Psychology Today blog posts: