Why are early childhood memories not remembered?

THAT'S THE WHY:What's your earliest memory? The first one I can time-stamp is on my fourth birthday: it's of my aunt Sol handing me a present of a toy car with the number four on it. Before that? Perhaps the odd flicker of a memory, but not much.

Yet in those earlier years our brains are extraordinarily busy learning how to talk, read and walk and all about how to interact with our environment. Why can we not remember more events from those times?

It’s partly because our ability to remember events is down to a memory system in the brain that doesn’t mature until about the age of four or five, according to Shane O’Mara, professor of experimental brain research at Trinity College Dublin.

We have different memory systems that develop at different rates, he explains. One type - “implicit” or “procedural” memory – allows us to remember how to do things, and it kicks in pretty much from birth.

Another type, the “explicit” or “declarative” system, is involved with consciously remembering things such as what we had for dinner yesterday or whether we can speak Russian. “It has to do with anything you can declare or say out loud,” explains O’Mara.

This explicit memory is centred on a brain structure called the extended hippocampal formation, which is immature in early childhood.

It also appears to depend on language. By around age four or five, our brains and language abilities are typically ready for us to be able to remember events.

As an aside, O’Mara notes that for a memory to be a real recollection, it has to be at least seen through your own eyes – an experiment he ran in the Science Gallery last year found that many people could see themselves in their proclaimed earliest memories, suggesting they weren’t true memories after all.