You’re sitting calmly when your mind abruptly tunes out the world and drifts to someplace altogether different – possibly a recent experience or an old recollection. You were having a daydream, as almost everyone does.
However, despite the widespread occurrence of this experience, neuroscientists are largely at a loss as to what exactly goes on in the brain during daydreaming. A team led by researchers at Harvard Medical School has now taken a step closer to understanding it thanks to a study done on mice.
“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 PhD student in neurobiology in the Blavatnik Institute at Harvard Medical School (HMS).
Formation of Visual Memory
The researchers monitored the activity of neurons in the visual cortex of mice 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, implying that the mouse was thinking – or daydreaming – about the image.
Furthermore, the activity patterns of a mouse’s first few daydreams of the day predicted how the brain’s response to the image would change over time.
The research provides tantalizing, if preliminary, evidence that daydreams can shape the brain’s future response to what it sees. The team cautioned that further research is needed to confirm this causal relationship, but the findings provide an intriguing hint that daydreaming during quiet waking may play a role in brain plasticity, or the brain’s ability to remodel itself in response to new experiences.
Scientists have spent a lot of time studying how neurons replay past events in the hippocampus, a seahorse-shaped brain region that is important for memory and spatial navigation. In contrast, little research has been conducted on the replay of neurons in other brain regions, including the visual cortex. Such research would provide vital insights into the formation of visual memory.
“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 neurobiology at HMS.
Imaginary Images
The mice were shown one of two images in the study, each with a different checkerboard pattern of gray and speckled black and white squares. The mice looked at a gray screen for a minute in between photos. The researchers observed activity from approximately 7,000 neurons in the visual cortex at the same time.
When a mouse looked at an image, the neurons fired in a specific pattern, which was distinct enough to distinguish image one from image two.
More importantly, when a mouse glanced at the gray screen in between images, the neurons fired in a similar, but not identical, way as when the animal looked at the image, indicating that it was daydreaming about the image. These daydreams only happened when mice were comfortable, as evidenced by calm behavior and tiny 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.
But what the researchers found next was completely unexpected.
Non-random Drifting
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.
The patterns linked with the images became increasingly distinct over time, until each engaged an almost totally independent set of neurons. Notably, the pattern seen during a mouse’s first few daydreams about a picture predicted the pattern seen when the mouse later gazed at the image.
“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.
Hippocampus Replay Activity
Finally, the researchers observed that visual cortex daydreams occurred concurrently with hippocampus replay activity, implying that the two brain regions were communicating during these daydreams.
The researchers believe that these daydreams are actively involved in brain plasticity based on the study’s results.
“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. He added that learning to differentiate between the images should help the mouse respond to each image with more specificity in the future.
These findings are consistent with a growing body of evidence in rodents and humans that entering a state of quiet wakefulness following an experience improves learning and memory.
Unexplored Brain Activity
The researchers will next use their imaging tools to visualize the connections between individual neurons in the visual cortex and investigate how these connections change when the brain “sees” an image.
“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,”
Andermann said.
It’s unclear whether daydreams in people entail similar activity patterns in the visual cortex, and more research is needed to find out. However, preliminary data suggests that when individuals recollect visual imagery, an analogous mechanism happens.
Randy Buckner, the Sosland Family Professor of Psychology and Neuroscience at Harvard University, found that when people are asked to recall an image in detail, brain activity in the visual cortex increases. Other investigations have found flashes of electrical activity in the visual cortex and the hippocampus during such recollection.
According to the researchers, the findings of their study and others imply that it is crucial to allow for moments of peaceful waking that lead to daydreams. This could be pausing from staring at a series of photos for a mouse, or pausing from scrolling on a smartphone for a human.
“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.
Abstract
Many theories of offline memory consolidation posit that the pattern of neurons activated during a salient sensory experience will be faithfully reactivated, thereby stabilizing the pattern. However, sensory-evoked patterns are not stable but, instead, drift across repeated experiences. Here, to investigate the relationship between reactivations and the drift of sensory representations, we imaged the calcium activity of thousands of excitatory neurons in the mouse lateral visual cortex. During the minute after a visual stimulus, we observed transient, stimulus-specific reactivations, often coupled with hippocampal sharp-wave ripples. Stimulus-specific reactivations were abolished by local cortical silencing during the preceding stimulus. Reactivations early in a session systematically differed from the pattern evoked by the previous stimulus—they were more similar to future stimulus response patterns, thereby predicting both within-day and across-day representational drift. In particular, neurons that participated proportionally more or less in early stimulus reactivations than in stimulus response patterns gradually increased or decreased their future stimulus responses, respectively. Indeed, we could accurately predict future changes in stimulus responses and the separation of responses to distinct stimuli using only the rate and content of reactivations. Thus, reactivations may contribute to a gradual drift and separation in sensory cortical response patterns, thereby enhancing sensory discrimination.
Reference:
- Nghia D. Nguyen, Andrew Lutas, Oren Amsalem, Jesseba Fernando, Andy Young-Eon Ahn, Richard Hakim, Josselyn Vergara, Justin McMahon, Jordane Dimidschstein, Bernardo L. Sabatini, Mark L. Andermann. Cortical reactivations predict future sensory responses. Nature, 2023; DOI: 10.1038/s41586-023-06810-1