“We were chasing this 99 percent of unexplored brain activity and discovered that there’s so much richness in the visual cortex that nobody knew anything about,” the authors write.

While seated in tranquility, your mind may unexpectedly drift away from the present, venturing into a past memory or a recent event. This phenomenon is known as daydreaming.

The mechanics of daydreaming in the brain have long puzzled scientists. However, recent research from Harvard Medical School, detailed in a Nature publication on December 13, offers new insights.

In this study, scientists observed the neural activities in the visual cortex of mice during periods of calm wakefulness. They noticed that sometimes these neurons exhibited patterns similar to those observed when the mice viewed real images.

This activity suggests that the mice might have been ‘daydreaming’ about those images. Intriguingly, the neural patterns observed during these early daydreaming episodes appeared to influence how the brain responded to similar images over time.

This finding hints at a potential connection between daydreaming and brain plasticity—the brain’s capacity to adapt and change with new experiences. Although these are preliminary findings, they open up the possibility that daydreams, particularly during quiet moments, could be significant for learning and memory.

“We wanted to know how this daydreaming process occurred on a neurobiological level,” points out Nghia Nguyen, a PhD student in neurobiology at the Blavatnik Institute at HMS and the study’s lead author, “and whether these moments of quiet reflection could be important for learning and memory,”

Exploring the Visual Cortex: Beyond the Hippocampus

While the hippocampus, a seahorse-shaped part of the brain crucial for memory and spatial navigation, has been the focus of extensive research regarding how neurons process past events and form memories, the visual cortex has received far less attention. This region’s role in shaping visual memories remains largely unexplored.

“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,” adds Professor Mark Andermann, a senior author from Beth Israel Deaconess Medical Center and a neurobiology professor at HMS.

In their groundbreaking study, they exposed mice to two distinct checkerboard patterns and simultaneously monitored about 7,000 neurons in the visual cortex. They observed that specific neuronal patterns emerged when the mice viewed each image and differed enough to distinguish between the two patterns.

Interestingly, when the mice faced a gray screen, a period of apparent rest with calm behavior and small pupils, their neurons occasionally mirrored the patterns seen when viewing the images, hinting at daydreaming. These daydreams, more frequent for the most recently viewed image and more common earlier in the day, were a notable discovery.

However, the most surprising finding was the ‘representational drift’ observed over time. The neural patterns evolved, becoming increasingly distinct for each image and eventually engaging almost entirely different neuron sets. Early daydream patterns appeared to foreshadow these future shifts.

“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 adds.

Furthermore, this study revealed that these daydream-like states in the visual cortex coincided with replay activities in the hippocampus, suggesting a potential interaction between these two brain areas during such moments.

The Role of Daydreams in Brain Plasticity

The findings from this recent study indicate that daydreams could play an active role in brain plasticity.

According to Nghia Nguyen, a key researcher in the study, “daydreaming may guide this process by steering the neural patterns associated with the two images away from each other.”

Repeatedly seeing two distinct images seems to trigger the brain to separate the neural patterns related to each image, a process potentially guided by daydreaming. However, Nguyen notes that further research is needed to confirm this relationship.

According to Nguyen, this differentiation could enhance the mouse’s ability to respond more precisely to each image in the future. This hypothesis is in line with other studies in both rodents and humans, suggesting that a state of restful wakefulness following an experience enhances learning and memory.

The research team plans to delve deeper into this phenomenon, aiming to visually map the connections between individual neurons in the visual cortex and observe how these connections evolve in response to visual stimuli.

Mark Andermann, another senior researcher in the study, emphasized the uncharted potential of the visual cortex.

“We were chasing this 99 percent of unexplored brain activity and discovered that there’s so much richness in the visual cortex that nobody knew anything about,” adds Mark Andermann, another senior researcher of the study.

The question of whether human daydreams mirror this activity in the visual cortex remains unanswered. Preliminary evidence suggests a similar process might occur in humans recalling visual imagery.

Randy Buckner, a professor at Harvard University, found increased activity in the visual cortex when people are asked to remember an image in detail. Additionally, other studies have noted surges in electrical activity in both the visual cortex and the hippocampus during such recall tasks.

For the researchers, these findings underscore the importance of allowing time for quiet wakefulness, which can lead to daydreams. In mice, this might mean taking breaks between viewing images, and for humans, it could involve stepping away from constant engagement with digital screens.

Andermann concludes, “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.”

Source: 10.1038/s41586-023-06810-1

Image Credit: Photo by Max Harlynking on Unsplash