Why you can't remember your life as a baby

When Sigmund Freud met with his patients, he noted that they could never recall any memories from their earliest childhood years. He went on to give this phenomenon a name – infantile amnesia. Ever since, psychologists have taken great interest in infantile amnesia and, across studies (and indeed cultures), a pretty universal pattern has emerged. For most adults, their earliest memories are from when they were four or five years old, with the majority of us remembering very little of what happened when we were only two or three.

However, the absence of memories from this period doesn’t seem to mean that children at this stage are unable to make memories at all. My daughter, for example, would tell any willing listener all about her recent trips to the zoo when she was three years old. But now eight, she has no recollection of those excursions. These memories appear to have been lost. What is responsible for this rapid forgetting of childhood memories?

Some years ago, experiments in my lab suggested that the ongoing generation of new neurons (or neurogenesis) in the hippocampus might be responsible for this rapid infantile forgetting. Clearly, neurogenesis is essential for building our brains during development. However, in most brain regions the production of new neurons peters out soon after birth. In contrast, in the hippocampus – an area that is key for forming and storing memories of events (like trips to the zoo) – neurogenesis continues throughout life, and neurogenesis rates are especially high during infancy.

“We wondered whether this rewiring might overwrite memories already stored in the hippocampus and lead to forgetting.”

As these newly generated neurons integrate into hippocampal circuits they establish new connections, essentially ‘rewiring’ the hippocampus. We wondered whether this rewiring might overwrite memories already stored in the hippocampus and lead to forgetting.

We tested this idea in two ways. First, we took adult mice (with relatively low levels of neurogenesis). We trained them to navigate a maze and later artificially boosted hippocampal neurogenesis after training. As predicted, increasing neurogenesis after learning led to forgetting, which suggests that excessive ‘rewiring’ overwrote those maze memories.

We then turned to infant mice. They too can learn mazes, but in contrast to adult mice, they rapidly forget these maze memories. Since infant mice have much higher levels of hippocampal neurogenesis, we speculated that excessive rewiring might be responsible for this rapid forgetting. To test this idea, we again trained mice in a maze, and then artificially reduced (rather than increased) hippocampal neurogenesis. The striking result was that these mice no longer forgot so quickly. In other words, reducing neurogenesis (and the amount of rewiring of the hippocampus) led to the relative preservation of memories.

These experiments suggest that we rapidly forget our earliest memories because the brain, and in particular the hippocampus, is still growing during early childhood.