Do you know what happens in brain while daydreaming?

A study led by Harvard Medical School researchers is one step closer to understanding what happens in the brain during daydreaming. The findings of the mouse study were published in Nature.

The researchers monitored the activity of neurons in the visual cortex of mouse brains while the animals were awake and silent.

They discovered that these neurons occasionally activated in a fashion similar to that seen when a mouse gazed at a real image, suggesting that the mouse was thinking--or daydreaming--about the image.

Furthermore, activity patterns during a mouse's first few daydreams of the day indicated how the brain's response to the image would develop over time. The study presented tantalising, if preliminary, evidence that daydreams can influence the brain's future response to what it perceives.

The authors noted that further study is needed to establish this causal association, but the findings provide an intriguing hint that daydreaming during peaceful waking may play a role in brain plasticity, or the brain's ability to rebuild itself in response to new experiences.

"We wanted to know how this daydreaming process occurred on a neurobiological level and whether these moments of quiet reflection could be important for learning and memory," said lead author Nghia Nguyen, a Ph.D. student in neurobiology at the Blavatnik Institute at HMS.

Scientists have spent considerable time studying how neurons replay past events to form memories and map the physical environment in the hippocampus, a seahorse-shaped brain region that plays a key role in memory and spatial navigation.

By contrast, there has been little research on the replay of neurons in other brain regions, including the visual cortex. Such efforts would provide valuable insights about how visual memories are formed.

"My lab became interested in whether we could record from enough neurons in the visual cortex to understand what exactly the mouse is remembering--and then connect that information to brain plasticity," said senior author Mark Andermann, professor of medicine at Beth Israel Deaconess Medical Centre and professor of neurobiology at HMS.

In the new study, the researchers repeatedly showed mice one of two images, each consisting of a different checkerboard pattern of grey and dappled black and white squares.

Between images, the mice spent a minute looking at a grey screen. The team simultaneously recorded activity from around 7,000 neurons in the visual cortex. The researchers found that when a mouse looked at an image, the neurons fired in a specific pattern, and the patterns were different enough to discern image one from image two.

More importantly, when a mouse looked at the grey screen between images, the neurons sometimes fired in a similar, but not identical, pattern, as when the mouse looked at the image, a sign that it was daydreaming about the image. These daydreams occurred only when mice were relaxed, characterised by calm behavior and small pupils.

Unsurprisingly, mice daydreamed more about the most recent image and they had more daydreams at the beginning of the day than at the end, when they had already seen each image dozens of times.

Throughout the day and across days, the activity patterns seen when the mice looked at the images changed--what neuroscientists call "representational drift." Yet this drift wasn't random. Over time, the patterns associated with the images became even more different from each other, until each involved an almost entirely separate set of neurons.

Notably, the pattern seen during a mouse's first few daydreams about an image predicted what the pattern would become when the mouse looked at the image later. "There's drift in how the brain responds to the same image over time, and these early daydreams can predict where the drift is going," Andermann said.

Finally, the researchers found that the visual cortex daydreams occurred at the same time as replay activity occurred in the hippocampus, suggesting that the two brain regions were communicating during these daydreams. Based on the results of the study, the researchers suspect that these daydreams may be actively involved in brain plasticity.

"When you see two different images many times, it becomes important to discriminate between them. Our findings suggest that daydreaming may guide this process by steering the neural patterns associated with the two images away from each other," Nguyen said while noting that this relationship needs to be confirmed.

Nguyen added that learning to differentiate between the images should help the mouse respond to each image with more specificity in the future. These observations align with a growing body of evidence in rodents and humans that entering a state of quiet wakefulness after an experience can improve learning and memory.

Next, the researchers plan to use their imaging tools to visualise the connections between individual neurons in the visual cortex and to examine how these connections change when the brain "sees" an image.

"We were chasing this 99 per cent of unexplored brain activity and discovered that there's so much richness in the visual cortex that nobody knew anything about," Andermann said.

Whether daydreams in people involve similar activity patterns in the visual cortex is an open question, and the answer will require additional experiments. However, there is preliminary evidence that an analogous process occurs in humans when they recall visual imagery.

Randy Buckner, the Sosland Family Professor of Psychology and Neuroscience at Harvard University, has shown that brain activity in the visual cortex increases when people are asked to recall an image in detail. Other studies have recorded fluctuations in electrical activity in the visual cortex and the hippocampus during such recalls.

For the researchers, the results of their study and others suggest that it may be important to make space for moments of quiet waking that lead to daydreams. For a mouse, this may mean taking a pause from looking at a series of images, and for a human, this could mean taking a break from scrolling on a smartphone.

"We feel pretty confident that if you never give yourself any awake downtime, you're not going to have as many of these daydream events, which may be important for brain plasticity," Andermann said.