Neural Replay as a Proposed Explanation for the Experience of Dreams

Post by Megan McCullough

What is neural replay?

Hippocampal neurons have been observed to spontaneously increase their firing rate during sleep. Recent studies have linked this display of brain activity with prior experiences; neurons that were active during an activity in an awake state are more likely to be reactivated during sleep. This is known as hippocampal neural replay. Neural replay, more broadly known as memory reactivation, occurs when there is a sequence of neuronal activity during rest or sleep that echoes the sequence of activity that occurred in an awake state. The evidence for this phenomenon was first discovered in maze exploration experiments with rodents; brain cells that were active when the rodents were exploring the maze also showed similar activity patterns during sleep. Recent technological advances in neuroimaging and electrical recordings have provided the first evidence for neural replay in humans. Neural replay during NREM has been shown to relay new information to the larger neural network, thus playing a key role in memory consolidation during sleep.  Interestingly, dreams share some features with neural replays, which has led to the idea that neural replays may be one mechanism underlying dreaming.

What is the link between neural replay and dreaming?

One proposed explanation for the purpose of dreams is that they support memory processes like consolidation, the process of transforming short-term memories into long-term ones. Since neural replays have also been shown to support memory consolidation, one hypothesis proposes that dreams are the subjective experience of neural replays that facilitate memory consolidation. Like dreaming, neural replays represent fragments of experiences, can combine multiple memories, and occur in both the hippocampus and cortical regions. Interestingly, neural replay has been shown to occur during sleep onset and NREM stages. These memory reactivations tend to occur for spatial memories, but can also occur for  motor, visual, and social memories.

Neural replay shares some features with the neural correlates of dreaming, but current research shows that memory activation is probably not the main explanation for dreams. Most neural replay events occur in earlier sleep stages, whereas dreams become most vivid in later sleep cycles. The timescales also differ; studies show that dreams occur on a timeline of seconds to minutes and are experienced at "life-like" timescales whereas neural replay occurs in the range of hundreds of milliseconds. These differences suggest that dreaming relies on other mechanisms than neural replay. Because of the number of shared features however, neural replay may relate to dreams in different ways. Dreams that include memories may rely on neural replay to an extent or neural replays could trigger dreaming. But since dreams most vividly occur in the REM stage, don't always include events that the dreamer experienced, and happen at a different timescale than neural replay events, memory activation events alone do not explain the neural basis of dreaming. 

Are there other possible explanations for the basis of dreams?

Beyond memory consolidation, there are other proposed explanations for why we dream, such as improved emotional regulation, future preparation, and the idea that dreams may have evolved to help us adapt to new sets of data. Although there are many hypotheses for why we dream, the neural correlates of dreaming remain unknown. Dreaming is a subjective experience and although new advances in electrical recordings and brain scanning have allowed scientists to monitor brain activity during sleep, the content of dreams is still studied through subjective measures such as dream journaling. More research is needed as we move into the future to further understand the reasons why humans dream, and its neural basis.

References +

Aleman-Zapata et al. Sleep deprivation and hippocampal ripple disruption after one-session learning eliminate memory expression the next day. PNAS (2022). Access the original scientific publication here

Freyja Olasfsdottir et al. The role of hippocampal replay in memory and planning. Current Biology (2018). Access the original scientific publication here

Hoel. The overfitted brain: Dreams evolved to assist generalization. Patterns: Cell Press (2021). Access the original scientific publication here

Mutz et al. Exploring the neural correlates of dream phenomenology and altered states of consciousness during sleep. Neuroscience of Consciousness (2017). Access the original scientific publication here

Picard-Deland et al. Memory reactivations during sleep: A neural basis of dream experiences. Trends in Cognitive Sciences: Cell Press (2023). Access the original scientific publication here

Ruby PM (2020) The Neural Correlates of Dreaming Have Not Been Identified Yet. Commentary on “The Neural Correlates of Dreaming. Nat Neurosci. 2017”. Front. Neurosci. 14:585470. doi: 10.3389/fnins.2020.585470