Sigmund Freud, the famous psychologist, was the first to investigate this phenomenon and coined it 'infantile amnesia'. His view was that memories of the first few years of life were repressed because they were too traumatic to deal with at such a young age, as the relevant coping mechanisms for a confusing and often cruel world had not developed yet. Nowadays not many people give much weight to Freud's explanations of childhood development (mostly because it's complete garbage).
In more recent times, Simcock and Hayne (2002) hypothesised that long term memory formation was dependant on language, based on the fact that infantile amnesia seemed to disappear at about the same time as language developed. However, many other animals also exhibit infantile amnesia so language development cannot be the full answer (although it may play a role).
|I hope this baby remembers to be this photogenic in later life!|
A study published recently seems to have the answer though, and it is very interesting. In a small part of the brain called the Hippocampus (Latin for seahorse, because it looks one apparently...), which is incredibly important in forming long term memories, there is a small region that generates new neurones (brain cells) called the Dentate Gyrus. However, this "neurogenesis" tends to have mostly stopped by the ages of 3-4- the same age where infantile amnesia seems to fade away as well!
Josselyn and Frankland conducted a series of experiments to investigate whether neurogenesis is actually interfering with making long term memories. First, they placed some adult mice in an enclosure different to their normal living space; with a vinegary smell and occasional mild shocks to the feet. Unsurprisingly, the mice didn't really like their new enclosure and when they were put back in after a few days they were still fearful of being inside the new enclosure. However, when infant mice were put in the same enclosure they tended to forget the new enclosure and didn't show the same fear as the adults.
Next they tried some new adult mice in the box, but once they were out of the box they were put in enclosures with exercise wheels (exercise has been shown to increase adult neurogenesis) and found that when adult mice had exercised they showed less fear of the new box- because the new neurones had interfered with forming the long term memories. They also repeated this using other ways of stimulating neurogenesis in adult mice, such as injecting proteins that increase neurogenesis.
Finally, they placed infant mice in the box and then reduced the amount of neurogenesis by administering a drug called ganciclovir. They found that these mice remembered the box better the next time.
The idea behind this is that, while new neurones are needed to lay down new memories, they can mess up old memories by making new connections between neurones. This suggests that old and less important memories become harder and harder to recall because the new connections take over.
In an interview with Science News, neuroscientist Richard Morris says that the hippocampus might be something like a computer cluttered with files, “every so often we all sit down and do a little tidy-up,” he says. “Maybe that’s what neurogenesis is all about. It’s the hippocampus’s very own spring cleaning system.” This actually serves as quite a good metaphor, most of the stuff we experience isn't really all that important and instead of accumulating and clogging up the system, neurogenesis allows the brain to get rid of the junk while still keeping the important stuff.
Agers et al. Hippocampal Neurogenesis Regulates Forgetting in adult and infant mice, Science Vol 334, 2014 p 598
Simcock & Hayne (2002) Breaking the Barrier? Children Fail to Translate Their Preverbal Memories into Language. Psychological Science, vol. 13 no. 3 225-231