The amazing durability of infant memory: Three-year-olds show recognition of a person they met once at age one

The fate of our earliest memories is something of an enigma. As adults, most of us are unable to recall memories from before we were age three or four. And yet, as toddlers we are perfectly capable of storing and recalling memories from before that age. To solve this mystery, we need to understand more about how infant memory works. Now a clever study has provided a test of just how durable infant memories can be. Osman Kingo and his colleagues in Denmark have demonstrated that three-year-olds display recognition of a person they met just once when they were aged one.

To maximise the chance of uncovering long-term memory through infancy and into early childhood, the researchers devised a scenario involving many many prompts – what they described as "massive cueing". Kingo and his team first renewed contact with parents and their children who'd taken part in an earlier study when the children were age one. That earlier research involved the infant children interacting with one of two researchers for 45 minutes – either a Scandinavian-Caucasian man or a Scandinavian-African man.

Now two years on, 50 of these parents and children – the latter now aged three – were invited back to the exact same lab (hopefully cueing their earlier memories). Here the children were shown two simultaneous 45-second videos side by side. One video was a recording of the researcher – either the Scandinavian-Caucasian or Scandinavian-African man – interacting with them two years earlier; the other video showed the other researcher (the one they hadn't met) interacting with a different child in the exact same way. The children themselves were not visible in these videos.

The key test was whether the three-year-olds would show a preference for looking at one video rather than the other. Amazingly, the children spent significantly more time looking at the video that featured the researcher they'd never met. This is not due to the children having a bias for either the white or black man, because for some of these children the previously unseen researcher was Scandinavian-African  and for others he was Scandinavian-Caucasian. All background features and behaviours in the videos were identical, so this result provides strong evidence that the children had some recognition of the researcher they'd met, and were drawn more strongly to look at the unfamiliar researcher.

Importantly, this same looking pattern was not observed among a control group of 36 three-year-olds who hadn't taken part in the original research two years' earlier. In fact, these children showed a bias toward looking at the black researcher. This is unsurprising because young children often show a bias towards looking at other-race faces. The fact that the three-year-olds in the experimental group didn't display this pattern shows that the influence of their memory overrode the usual other-race bias.

What is the nature of the children's memory of the researcher they met two years' previously? When asked which man they'd met before, the children's answers were no better than guessing. This suggests that their memory is "implicit" or not accessible to consciousness. But the researchers were not to so quick to form that conclusion. They said it remains an open question. One argument is that just because the children can't verbalise their recognition doesn't mean it's not consciously experienced by them in some way.

"The finding that three-year-olds have any kind of memory for a single event experienced more than two years earlier calls for increased attention to the question of why such seemingly resistant memory-traces are so difficult to verbally report later in life," the researchers concluded, "and to conditions that will allow the memory traces to become explicitly available."

_________________________________

Kingo OS, Staugaard SR, and Krøjgaard P (2014). Three-year-olds' memory for a person met only once at the age of 12months: Very long-term memory revealed by a late-manifesting novelty preference. Consciousness and cognition, 24, 49-56 PMID: 24413559