When you think about it, it is true that recalling our first steps, the first candle we blew out, or our first teeth coming through is impossible. Our family is there to remind us about these important events from our childhood, but despite that, whatever we do we can't remember these events. This known phenomenon, called infantile amnesia, may be explained by a recent study.

Katherine Akers, from the Toronto hospital for sick children, carried out this study on mice and guinea pigs with the aim of studying how neurogenesis, or the creation of new neurons, might affect our memories. To do this, the team of researchers trained mice to fear a particular environment with little electric shocks. So each time the researchers put the mouse back into this environment, they were able to measure the extent to which the mouse remembered it. They then gave some of these mice access to exercise wheels, an activity which is known to stimulate neurogenesis. A few weeks later, most mice that had been trained and then put back into the original environment had forgotten their fear, unlike the mice that had not exercised. What's more, when researchers slowed down neurogenesis using a drug, they noticed that the mice held on to their old memories better. Identical results were obtained with guinea pigs.

These results suggest that the creation of new neurons, which generally help with forming memories, could have the opposite effect if neurogenesis takes place too quickly, and could make us forget things. In other words, forgetting is the consequence of forming new memories.

However, this conclusion is not the only explanation for infantile amnesia. Mark House, a researcher specializing in memory at the City University London, points out that there could be other explanations for this phenomenon such as changes to connectivity between different areas of the brain.