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Addition of new neurons makes us forget old memories

LiveMint logoLiveMint 09-05-2014 Nikita Mehta

Why can’t we remember our first day at school, or the day we walked for the first time?

Scientists have found out how we lose our childhood memory as we grow older, and the answer lies in the process of addition of new neurons in the brain.

Newly formed neurons in the hippocampus — a major brain component responsible for memory formation — could knock out established memories, scientists said in a study published in the journal Science on Thursday. While earlier studies have established that production of new neurons help make new memories, this study explains how new neurons also clear old memories.

The study was carried out in The Hospital for Sick Children, Toronto, Canada

Neurogenesis, the process of producing new neurons in the brain, continues throughout the lives of humans, most rapidly at the pre-natal development stage and slower as we get older.

“The finding was very surprising to us initially. Most people think new neurons mean better memory,” said Sheena Josselyn, lead author of the study in Nature journal. “More neurons increase the capacity to learn new memories in the future, but memory is based on a circuit, so if you add to this circuit, it makes sense that it would disrupt it.”

The researchers also observed that in adult mice, regular exercise for four to six months would lead them to forget the fear of electric shocks they had been trained to do at the start of the study. Previous studies have shown that regular exercise can cause increased production of neurons in the hippocampus.

After the scientists conducted experiments on memories of fear on infant guinea pigs and degus, they observed increasing neurogenesis after memory formation would lead to removal of memories in these species.

But the researchers may find it hard to find such results with humans. “The genetic and chemical manipulations cannot be applied readily to humans, so the findings will be difficult to pursue in people,”says Josselyn. But she added that both mice and humans have pronounced experiences of forgetting early life experiences which make these results relevant.

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