Can't remember that childhood blankee you toted everywhere? Blame it on all those new brain cells you were making. The formation of brain cells normally helps us learn information, but it may also come with a downside – forgetting.
Few adults can remember anything from before their third birthday, a phenomenon known as childhood amnesia, but the reason has always been unclear. Now work in mice suggests that there are so many neurons being born in the brains of young children that this interferes with the storage of long-term memories.
Until the 1990s, it was thought that our brains have no capacity for regeneration and we are born with our lifetime supply of neurons. Then it was found that extra neurons are born throughout life. One place where neurogenesis happens is the hippocampi, structures involved in memory formation. Neurogenesis in the hippocampi is at its peak in the first few years of life, then the rate drops off sharply.
Mice show a similar tailing off of neurogenesis after birth and have their own version of childhood amnesia. If they are taught to link a certain environment with a mild electric shock as infants, they forget the memory by adulthood – which doesn't happen if they are taught as adults.
In adult mice, boosting neurogenesis can promote learning, if the growth of new neurons occurs before memories are laid down. But a study by Katherine Akers at the Hospital for Sick Children in Toronto, Canada, and her colleagues, now suggests that neurogenesis has the opposite effect on pre-existing memories.
She and her team taught mice of different ages to associate a particular environment with a mild electric shock. They then got some of the adult mice to run on a wheel, because this has been shown to promote the growth of new neurons.
When mice were placed back in the threatening environment, adult mice that had boosted their neuron numbers by running were less likely to freeze to the spot – a sure sign of fear – than a control group with no access to an exercise wheel.
Drugs to help you remember
This suggests that forming brain cells caused the mice to forget the electric shocks. Akers's team then gave a group of mice just a few weeks old a drug that inhibits neurogenesis. These mice were more likely to remember the electric shock than a control group.
One theory is that as youthful neurons form connections, they could disturb existing wiring in the hippocampi, which would promote forgetting, says Akers "This could interfere with the storage of pre-existing memories," she says. "Our experiments provide pretty good evidence for that." The high levels of neurogenesis in human children might therefore contribute to childhood amnesia, she says.
If we want to understand memory better, "neurogenesis in the hippocampus might be an interesting avenue to pursue", says Mark Howe, a memory researcher at City University London. But he adds that there are several other possible explanations for childhood amnesia, such as altered connectivity between various areas of the brain.
Journal reference: Science, DOI: 10.1126/science.1248903
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