By Alex Fradera
Can we form memories when we are very young? Humans and non-humans alike show an “infantile amnesic period” – we have no memory of anything that happens during this time (usually up to age three or four in humans) which might suggest we can’t form very early memories. But of course it might be that we can form memories in these early years, it’s just that they are later forgotten. The idea that at least something is retained from infancy is consistent with the fact that disorders present in adult life can be associated with very early life events.
Now Nature Neuroscience has published a paper confirming that in rats some kind of memories are created during the amnesic period, but that these operate differently and are produced by different brain chemistry from adult memories. What’s more, such events may have a role in kickstarting memory system maturation.
The research team from University of New York and Mount Sinai Hospital looked at rats aged either 17 or 24 days – the former still in the infantile amnesic period and who I’ll henceforth refer to as infants, the latter grown to non-amnesic status – and specifically how they were affected by the experience of being electrically shocked when exploring a novel, dark chamber adjoining their brighter home.
The infant rats showed almost no indication of being able to retain a memory of the shock. Given the chance, they sometimes returned to the chamber within the next 30 minutes, And after 24 hours, any memory of the unpleasant experience seemed entirely absent, in that they showed no hesitation in re-entering the chamber. This suggests that, at least in rats, infantile amnesia in due to a failure to store memories in the first place.
But with further tests, Alessio Travaglia’s team showed that some kind of memory had in fact been formed. First they gave the rats an opportunity to wander back into the chamber, which was now a safe place (again they were happy to do this, suggesting they’d forgotten the earlier shocks). A few days later the researchers gave the rats a shock in a different location – crucially, after this unpleasant experience, the rats showed a new, persistent (days long) reluctance to enter the dark chamber when given the chance, even though it was now safe. It’s as if a memory of the earlier shocks in the chamber had been reawakened by the later shocks somewhere else.
What was going on? Firstly, whatever was happening involved the hippocampus, a brain structure involved in the laying down of normal memories in mature brains. We know this because, just before the initial shocks, the researchers used a chemical to block normal functioning in the hippocampus of some rats, and for these animals, no amount of jogging and reminders were able to bring back any indication of remembering.
The researchers also uncovered some signs of learning and memory in the infant rats’ brains at a molecular level. Infant rats have less of a specific version of a chemical receptor in their hippocampus than mature rats. Analysing the extracts of brain tissue taken from the trained infant rats showed that after the initial training shocks in the dark chamber, they showed a sudden increase in this specific receptor (an effect not seen in the mature rats).
Let’s make sense of this. Imagine that we enter the world with just a pared-down version of memory, appropriate to being newborn, where the necessities of life, milk and warmth, are just there without you having to understand much about the world. Within weeks, this system is to be superseded by one using the same functional circuitry but fine-tuned, ready to capture novel information and store it accessibly. Until then, the system operates in a bare way, but still preserves what it can about significant events. Crucially, such events accelerate the transition to the full system, by triggering the development of more of the chemical receptors that are seen in adults. It’s too late to capture an accurate memory of this threat – the horse has bolted – but the brain gets busy building a better stable for the next one.
We don’t know, of course, whether still earlier stages in the amnesic period involve the capacity to capture memory information – it’s hard to do this kind of work with animals too small to explore their environment. But this new research suggests that in rats, and likely in infant humans too, the system is far more active than expected, not only retaining some information (which the researchers dub a “latent memory” to account for it’s difficulty in retrieval) but also acting as a developmental critical period, akin to the way infant visual systems begin to change themselves in response to light. Given this sensitivity, the researchers speculate that early wrong experiences – or deprivation from experience – may harm us in later life through an upset of this critical period, and may contribute to neuropsychiatric disorders as a consequence.