The relationship between dreams and brain activity has long been a fascinating mystery. A recent study published in Nature reveals that the brain reactivates past neural stimuli during rest, mind-wandering, or daydreaming, influencing learning and memory. Researchers from Harvard Medical School recorded the activity of 6,900 neurons in the visual cortex of mice and discovered that this "reactivation" not only aids memory consolidation but also predicts future neural response patterns. This study sheds light on how the brain processes information unconsciously and provides a scientific explanation for the phenomenon of "precognitive dreams.
It is said that humans spend about one-third of their lives asleep, during which the mind generates countless vivid and often mysterious dreams. For centuries, people have sought to interpret the meaning of dreams, with some even believing in their ability to predict the future.
In modern neuroscience, the study of the brain remains one of the most enigmatic fields. Countries worldwide have launched ambitious brain research initiatives, striving to unlock the mysteries of human cognition. One of the key questions in this field is how modern science can explain the formation, development, and function of dreams.
The Brain’s Replay Mechanism and Its Role in Learning
The concept of "mental replay" has gained attention as an effective learning strategy—reviewing past experiences to extract lessons and enhance understanding. This process reflects the brain’s ability to reactivate previously stimulated neural circuits in the absence of new external stimuli. Such reactivation often occurs when we daydream, lose focus, or sleep. Recent neuroscience studies have identified this replay mechanism in the hippocampus, amygdala, prefrontal cortex, and visual cortex.
However, previous studies on this phenomenon have typically focused on small neural populations, ranging from dozens to a few hundred neurons. The similarities and differences between initial stimulus responses and later reactivation events remained unclear.
To address this, in 2023, a groundbreaking study by Professor Andermann’s team at Harvard Medical School was published in Nature. The researchers recorded the activity of 6,900 neurons in the visual cortex of mice over several days, investigating how sensory experiences are reactivated in the brain. In simple terms, they explored what happens in our brain when we momentarily lose focus.
Figure 2: Distributed stimulus reactivation in lateral visual cortex during quiet waking
Investigating Neural Reactivation in Mice
The study involved eight mice that were exposed to two randomly selected visual stimuli (S1 and S2) each day from a set of 64. Using calcium imaging and viral injection techniques, researchers observed neuronal activity and found that lateral visual cortex stimulation played a crucial role in reactivation, independent of eye or body movement. These reactivating neurons were evenly distributed across four regions of the lateral visual cortex.
Activity levels remained consistent across different brain regions and cortical layers during both stimulus presentation and reactivation. The study also found that large-scale neuronal imaging significantly improved the ability to detect reactivations. When only 10% of the recorded neurons were analyzed, over two-thirds of reactivation events were missed, and false positives increased.
Interestingly, the frequency of reactivation declined over the course of training, suggesting an inverse relationship between reactivation likelihood and the frequency of recent exposure to the stimulus.
Figure 3: Single-session raster plots of S1 and S2 reactivation after presentation of S1 or S2
Factors Influencing Neural Reactivation
The study further revealed that stimulus novelty and surrounding arousal levels play a key role in regulating reactivation rates. To explore whether the same neurons involved in initial stimulus response were necessary for reactivation, the researchers used optogenetic techniques to suppress excitatory neurons in the lateral visual cortex. This suppression significantly reduced the subsequent reactivation rate, demonstrating that cortical activity during sensory experiences is essential for later reactivation.
Figure 4: Progressive dissociation of stimulus-response patterns correlates with reactivation rate
Previous studies have suggested that neural reactivation plays a role in memory consolidation and learning. However, the relationship between reactivation and long-term changes in cortical response patterns remained unclear. Using Pearson correlation analysis and ROICaT imaging, researchers found that evolving response patterns in the lateral visual cortex were closely linked to stimulus-specific reactivation rates. This suggests a potential relationship between reactivation and response pattern differentiation over time.
Figure 5: Reactivation predicts representational drift
Can Neural Reactivation Predict Future Sensory Responses?
The researchers sought to determine whether neural reactivation could predict future stimulus responses. By analyzing activity shifts between early and late trials, they found that both stimulus-evoked responses and reactivation patterns evolved in a consistent manner. Remarkably, these predictive reactivations remained stable throughout the study, even across different days.
Further analysis indicated that early reactivation patterns were strongly correlated with future sensory responses, suggesting that the rate and structure of reactivation play a crucial role in shaping the brain’s future response to stimuli. Using a heuristic model, the researchers demonstrated that these reactivation events were sufficient to predict the direction and rate of future stimulus-induced response changes.
Conclusion
In essence, past experiences leave traces in our brain, which resurface when we daydream, take a short nap, or lose focus. These reactivations help shape cognition, learning, and memory. What’s even more fascinating is that these changes in brain activity can be predicted—suggesting a scientific basis for what some might call "premonition dreams." This study, titled Cortical Reactivation Predicts Future Sensory Responses, provides a compelling look at how our brain continuously processes and prepares for future experiences, even when we are seemingly lost in thought.
