Our memories are stored in brain connections called synapses. And according to a new Nature study, synapses aren’t permanent structures: They only last as long as the memories they store.
The hippocampus keeps memories of events that happen and conversations we have that might be forgotten over time if we don’t recall them. These are known as episodic memories. When mice experience a new episode, such as meeting a new cage mate, or learn to navigate a maze for food, that memory is stored in the hippocampus for about a month. (In people it’s stored for a bit longer than that.) If the hippocampus is disrupted within a month of forming a memory, by surgery for example, that memory will be lost. But if that same disruption occurs after a month is up, the memory is retained since it had been relocated to the neocortex. Memories are gradually moved around the brain.
However, only some of the connections in the neocortex are permanent. Researchers previously monitored "spines", the bulbous protrusions from the branched projections of neurons known as dendrites. Spines come and go as synaptic connections between neurons are formed and broken. Only half of the spines in the neocortex are permanent; the rest turn over every 5–15 days. "The interpretation was that about half the spines in the neocortex are long-term repositories for memories while others retain malleability for new memories or forgetting," Stanford’s Mark Schnitzer said in a statement.
Now Schnitzer’s team wants to monitor the lifespan of synapses between hippocampal neurons. But unlike the neocortex, which is near the surface, the hippocampus lies deep within the center of the brain and its spines are densely packed. So they combined a stable, high-resolution imaging technique using an optical needle called a microendoscope, with a mathematical model that accounts for the limitations in resolution.
Connections between hippocampal neurons in live mice last for about 30 days – this is about the duration of episodic memories in mice. The spines all turn over every three to six weeks. The lifetimes of hippocampal synapses, they found, match the longevity of hippocampal memory.