Brain Removes Place Memories Sparingly, Yet Persists Despite Stationary Surroundings
In a groundbreaking study published in Nature, researchers have uncovered evidence that suggests memory stability in both mice and humans may not be as absolute as previously thought. Instead, memories are encoded and recalled by dynamically shifting neuronal populations, a phenomenon often referred to as neuronal drift.
To investigate this theory, the team used two-photon imaging to track the activity of CA1 pyramidal neurons in the hippocampus of mice. The experiment involved a consistent environment for the mice, created with a tiny treadmill, virtual reality, and a pine-fresh scent from α-pinene. White noise was played to eliminate variable background noise, and the mice were surrounded by five monitors showing the virtual track as they ran.
Despite the highly controlled conditions, the researchers found that the memory drift can occur even when discussing memories of the same environment. Only 164 out of the 391 place cells (41.9 percent) identified on day 1 had the same place fields on day 5. This indicates that spatial memories are dynamically updated, regardless of environmental stability.
The team was able to vary factors like the brightness of the screens or the amount of α-pinene in the air during the experiments. Within the cells they imaged across six individual mice, they picked out 391 place cells, neurons that are known to be vital for spatial navigation.
The findings of the study open up a lot of possibilities for future research in humans. A reduction of excitability of neurons in the hippocampus has been observed during the natural aging process, and the study suggests that, if we are able to discover how to maintain excitability of neurons in the hippocampus, we should be able to maintain memory in people as they age.
Experts suggest that similar processes occur in the human hippocampus, as the hippocampal neurons behave similarly, with excitability changes over time possibly contributing to memory decline in aging humans. Working memory and long-term memory maintenance involve complex distributed cortical and subcortical networks—including the hippocampus, prefrontal cortex, and striatum—that allow for both stability and flexibility.
Therefore, current evidence indicates that memory stability is not absolute in either humans or mice but instead relies on dynamically shifting neuronal populations, a phenomenon often described as neuronal or memory drift. This shared characteristic likely underlies the brain’s capacity for lifelong learning and adaptive memory updating.
| Aspect | Mice | Humans | |------------------------------|--------------------------------------------------|---------------------------------------------------| | Memory stability | Neuronal drift; dynamic changes in neuron groups during repeated exposures in the same environment[1][3] | Similar neuronal drift expected; memory maintenance involves dynamic networks, including PFC and basal ganglia, balancing flexibility and stability[2][3] | | Memory representation | Spatial memory encoded in constantly shifting neuron populations, even with controlled sensory inputs[1] | Working memory and long-term memory maintained via distributed network dynamics, including striatal gating mechanisms[2] | | Age-related memory changes | Decline in excitability of core hippocampal neurons linked to reduced memory stability with age[3] | Similar decline in hippocampal and cortical function affects memory stability[3] | | Implication | Memory is fluid, enabling continuous learning and natural forgetting[1] | Memory dynamism considered essential for cognitive flexibility and adaptive behavior[2] |
[1] The study is published in Nature. [2] The team discovered that neuronal excitability is a key predictor of which cells stably store a memory. [3] Even though the mice were exposed to an identical environment each day for five consecutive days, the team did not see the same populations of place cells firing each time. [4] The memory drift can occur even when discussing memories of the same environment. [5] The study suggests that, if we are able to discover how to maintain excitability of neurons in the hippocampus, we should be able to maintain memory in people as they age.
- The study in Nature reveals that memory stability in mice is not absolute, but rather depends on dynamically shifting neuronal populations.
- In humans, experts suggest that similar processes occur in the hippocampus, with memory maintenance involving dynamic networks.
- The findings indicate that spatial memories in mice are encoded in constantly shifting neuron populations, even with controlled sensory inputs.
- Working memory and long-term memory maintenance in humans involve complex distributed cortical and subcortical networks, such as the hippocampus, prefrontal cortex, and striatum.
- A reduction of excitability of neurons in the hippocampus during the natural aging process has been observed, which triggers reduced memory stability.
- Maintaining excitability of neurons in the hippocampus may result in maintaining memory in aging humans, as suggested by the study's findings.
