Have you ever wondered why you can't remember being born, riding in a stroller, having your diaper changed or really any thing about your existence before those few snippets of memories you have from your toddler years? OK, maybe you don't want to remember, but in case you're wondering the reason why you have no recollection of these events, it is likely thanks to your brain cells.
According to a new study, your new brain cells are erasing some old memories.
“The finding was very surprising to us initially," Sheena Josselyn, who led the study with her husband Paul Frankland, said, according to Nature. "Most people think new neurons mean better memory."
Conducting their research in newborn and adult mice, the scientists trained the animals to fear a certain environment. While animals of all ages learned to fear being electrically shocked, the infant mice only remembered for a day. Adult mice, however, would remember the fear of being shocked for weeks, Nature reported.
When the researchers stopped the growth of new neurons in infant mice, they remembered to fear the environment longer and vice versa. In adult mice, if they promoted more brain cell growth, then the older mice would forget the fear faster than they usually would.
“More neurons increase the capacity to learn new memories in the future,” Josselyn said. “But memory is based on a circuit, so if you add to this circuit, it makes sense that it would disrupt it.”
Here's the technical explanation of what's going on from the study's abstract published in the journal Science.:
Throughout life, new neurons are continuously added to the dentate gyrus. As this continuous addition remodels hippocampal circuits, computational models predict that neurogenesis leads to degradation or forgetting of established memories. Consistent with this, increasing neurogenesis after the formation of a memory was sufficient to induce forgetting in adult mice. By contrast, during infancy, when hippocampal neurogenesis levels are high and freshly generated memories tend to be rapidly forgotten (infantile amnesia), decreasing neurogenesis after memory formation mitigated forgetting. In precocial species, including guinea pigs and degus, most granule cells are generated prenatally. Consistent with reduced levels of postnatal hippocampal neurogenesis, infant guinea pigs and degus did not exhibit forgetting. However, increasing neurogenesis after memory formation induced infantile amnesia in these species.
Stanford University neuroscientist Karl Deisseroth, who was not involved with this study, called the findings "incredibly impressive," according to Nature.
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