#CAN13 Why you don’t remember your days as a baby

This is part of a blog post series for the 2013 Canadian Association for Neuroscience annual conference. 

Poster: Inhibiting hippocampal neurogenesis protes memory persistence in infant mice. Axel Guskjolen, Sheena Josselyn, Paul Frankland. University of Torontothe hospital for sick children.

I don’t know if you were ever forced to watch your baby videos with an SO. It’s MORTIFYING. I remember staring in horror as baby-me yanked off her dress, ran around butt naked, chasing ducks and doing other unspeakable things (long story).

Thankfully, I don’t actually remember any of my baby days. Nor, probably, do you. This is due to something called “infantile amnesia”. We don’t really know why this happens. Some think it’s because babies need to develop language before they can describe and encode events. Others think babies don’t have a representation of themselves, so to babies, “there is no me”. Unfortunately, none of these theories tell us anything about what’s going on in the brain to mediate baby forgetfulness.

Researchers now think that neurogenesis may the key. After birth, our brains continually produce new neurons, rapidly at first, then gradually slowing down as we age. Adult neurogenesis adds more functional units to the brain, increasing its computational power. In adults, it’s tightly linked to the formation of new memories.

But neurogenesis may have a “dark side”. By elbowing their way into existing networks, new neurons may disrupt the old memories stored within. Baby brains – in mice and men – generate A LOT of new neurons. Is this why they forget?

Researchers used a genetic-chemical method to selectively kill off newborn neurons in a group of baby mice (17 days old, not yet weaned and super cute!). They then trained these mice, along with normal baby and adult controls, to associate a box with a nasty foot shock. Mice hate electrical shocks as much as you; they quickly learned to fear the box.

28 days later, the adult mice still recalled the trauma: when placed back into the box, they froze in fear. Normal baby mice, however, went about their merry ways – they’ve forgotten that the box is a dangerous place. Further testing showed that the memory was even shorter-lived than 28 days; in normal baby mice, it lasted more than a day, but less than a week.

New neurons generally need a week to mature and “infiltrate” into the brain’s network. Researchers think that is why the “creepy-box” memory still lasted a day in normal mice: the newborn neurons simply didn’t have enough time to “do its thing”. But give it a week, and the memory’s gone (or repressed so that the mouse can’t retrieve it. We don’t really know.).

After inhibiting neurogenesis, however, a completely different picture emerged. These mice froze 18% more than their normal peers, which is quite significant. This tells us that they, on average, remember that the box is a terrifying place. The only thing that sets them apart from normal baby mice is their inhibited neurogenesis. These results, then, strongly suggests that neurogenesis is one of the factors that drive babies to forget.

This is the first study to show that neurogenesis may be involved in infantile amnesia. It also offers a biological reason to why we forget. Next up, the researchers want to know if it’s possible for a baby memory can persist into adulthood by inhibiting neurogenesis. It also opens questions on the opposite end of the age spectrum. Elderly people have massively reduced rates of neurogenesis. If, and how does this influence their lifetime of memories?