This is based on a review of existing research on mice at the University of Toronto and the Hospital for Sick Children by married researchers with a 2 year old subject... er, daughter. They conclude that high levels of neuron growth in young children, particularly in the hippocampus, which is implicated in long term memory storage, is responsible for the phenomenon Freud once dubbed "infantile amnesia."
My question is, what exactly did they ask the mice to yield this conclusion? Could they recall their first day at school? A trip they had when they were three? The name of their childhood dentist? Their grandma's phone number? (Mine's 783-3168; haven't used it in 30 years.)
I recall playing in the backyard with my own kids when their ages were calculated in months, wondering that all the rich experiences they were having were going to be forgotten in the course of time. But frankly, although remembering that, I don't recall the particular experiences themselves, myself, without photographs or video. As for short term memory, kids are fantastic: I remember others at a drop-in centre being skeptical when, while looking fruitlessly for something I had seen him playing with, I asked my pre-verbal child where it was. Like a little puppy, he led us right to the spot. No so surprising, but because he couldn't respond verbally most people wouldn't have asked at all.
To determine memory in mice, they are trained in some task and then tested on whether they can do it some time later. On a variety of tasks, mice have been found to "remember" better the older they are. The authors predict that if one could increase neurogenesis in a mature mouse it would cause them to remember less well, and the opposite would be true if you could slow neuronal growth in the infant hippocampus. (How long to mice live, anyway?) They suggest ways such experiments might be done, but they're pretty hard.
I searched the word "pruning" in their paper and was surprised no hypothesis was mentioned based on that. Perhaps mice don't experience neural pruning the way humans do. The reliance on memory of how to do things rather than recall of things that happened to you (hard to ask a mouse) also bothers me, as declarative memory is not the same as procedural memory. I fear that none of these hypotheses, however, is going to solve their primary goal of figuring out how to preserve their daughter's recall of her formative years. it is heartbreaking that they don't remember, but I suspect the solution lies less with neuroscientists and more with someone like Andy Clark, who says the human mind is not just inside the body any more, it incorporates our media, the way we use things to help us think.
In other words, get thee a video camera, and enjoy your children's childhood as much as you can, while you can. The rest, as Hillel, said, is commentary.