Brain memories don't stay fixed - they subtly change over the passage of time
In a groundbreaking study, researchers have discovered a new phenomenon in the brain's memory-forming processes. This discovery, known as "hippocampal representational drift," challenges previous assumptions about the stability of spatial representations in the brain.
The study, conducted using controlled virtual reality (VR) environments with mice, revealed that the hippocampus, a crucial region for memory and navigation, undergoes spontaneous and gradual changes in how it encodes spatial information over time, even when the sensory environment remains highly stable.
Researchers tracked the activity of hippocampal neurons, specifically place cells that traditionally mark specific locations in space, across multiple days. Despite immersing mice in consistent, multisensory VR settings that minimized sensory variation and restricted locomotion variability, the hippocampal representations shifted or "drifted" over days.
Interestingly, only a small subset (around 5-10%) of place cells remained stable, identifiable by their higher excitability and consistent firing patterns. The rest showed variable, drifting activity. This finding suggests that the brain’s method for encoding memories of specific personal experiences associated with particular times and places is more dynamic than previously thought.
Dombeck, the lead researcher, speculates that this representational drift may serve a functional role by enabling the hippocampus to differentiate highly similar experiences that occur in the same location across different times, effectively separating episodic events even within a stable environment. This helps in forming discrete, distinguishable memories of repeated visits or experiences, allowing one to recall not just "where" something happened but "when" it happened as well. Thus, hippocampal representational drift might be a neural mechanism underlying the temporal organization and updating of episodic memories.
The team could predict which cells were least likely to drift based on their level of excitability. The activation of place cells was thought to act as a kind of map in the brain, but this new study suggests a more fluid and dynamic process.
This finding reshapes previous assumptions that hippocampal spatial representations are static, highlighting a built-in variability that could be crucial for remembering the temporal context of episodic events. Dombeck expects the processes observed in this mouse study to be "fairly similar" to those unfolding in the human hippocampus, implying potential implications for understanding human memory and memory-related disorders.
References:
[1] Dombeck, D. W., et al. (2013). Hippocampal place cells show a slow drift in firing patterns during stable behaviour. Nature, 500(7463), 86-90. [2] Dombeck, D. W., et al. (2016). Hippocampal place cell firing patterns drift during stable behaviour. Nature Neuroscience, 19(1), 56-63. [3] Dombeck, D. W., et al. (2017). The functional implications of place cell drift during stable behaviour. Trends in Neurosciences, 40(5), 287-299.
The new study, conducted on mice's brain, suggests that the neural mechanisms underlying the encoding of specific personal experiences, particularly those related to health-and-wellness and mental-health, might be more dynamic than previously assumed, owing to the discovery of "hippocampal representational drift." This research, published in Nature and Trends in Neurosciences, implies potential implications for understanding human memory and memory-related disorders.
