Infantile amnesia might be a memory retrieval problem, Yale study finds 

Our inability to remember specific events from the first few years of life is called “infantile amnesia,” and it’s a phenomenon that Yale researchers have demystified, suggesting that it could simply be a memory retrieval problem.

In their new study the researchers found that infants can encode specific memories, busting the myth that as adults, memories from infancy haven’t been able to be ‘held’ because the part of the brain responsible for saving memories — the hippocampus — is still developing well into adolescence and just can’t encode memories in our earliest years. 

The latest research has found that this is not the case, after researchers showed infants new images and later tested whether they remembered them. When an infant’s hippocampus was more active upon seeing an image the first time, they were more likely to appear to recognize that image later. 

The findings, published March 20 in Science, indicate that memories can indeed be encoded in our brains in our first years of life, with the researchers now looking into what happens to those memories over time.

“The hallmark of these types of memories, which we call episodic memories, is that you can describe them to others, but that’s off the table when you’re dealing with pre-verbal infants,” Professor Nick Turk-Browne, senior author of the study, said.

Led by graduate student Tristan Yates, researchers used an approach in which they showed infants aged four months to two years of age an image of a new face, object, or scene. Later, after the infants had seen several other images, the researchers showed them a previously seen image next to a new one.

“When babies have seen something just once before, we expect them to look at it more when they see it again,” Professor Turk-Browne said. “So in this task, if an infant stares at the previously seen image more than the new one next to it, that can be interpreted as the baby recognizing it as familiar.”

In the new study, the research team, which over the past decade has pioneered methods for conducting functional magnetic resonance imaging (fMRI) with awake infants (which has historically been difficult because of infants’ short attention spans and inability to stay still or follow directions), measured activity in the infants’ hippocampus while they viewed the images.

Specifically, the researchers assessed whether hippocampal activity was related to the strength of an infant’s memories. They found that the greater the activity in the hippocampus when an infant was looking at a new image, the longer the infant looked at it when it reappeared later. And the posterior part of the hippocampus (the portion closer to the back of the head) where encoding activity was strongest is the same area that’s most associated with episodic memory in adults. 

These findings were true across the whole sample of 26 infants, but they were strongest among those older than 12 months (half of the sample group). This age effect is leading to a more complete theory of how the hippocampus develops to support learning and memory, Professor Turk-Browne said.

Previously, the research team found that the hippocampus of infants as young as three months old displayed a different type of memory called “statistical learning.” While episodic memory deals with specific events, like, say, sharing a Thai meal with out-of-town visitors last night, statistical learning is about extracting patterns across events, such as what restaurants look like, in which neighborhoods certain cuisines are found, or the typical cadence of being seated and served.

These two types of memories use different neuronal pathways in the hippocampus. And in past animal studies, researchers have shown that the statistical learning pathway, which is found in the more anterior part of the hippocampus (the area closer to the front of the head), develops earlier than that of episodic memory. Therefore, researchers suspected that episodic memory may appear later in infancy, around one year or older, arguing that this developmental progression makes sense when thinking about the needs of infants.

“Statistical learning is about extracting the structure in the world around us,” Professor Turk-Browne said. “This is critical for the development of language, vision, concepts, and more. So it’s understandable why statistical learning may come into play earlier than episodic memory.”

Even still, the research team’s latest study shows that episodic memories can be encoded by the hippocampus earlier than previously thought, long before the earliest memories we can report as adults. So, what happens to these memories? 

There are a few possibilities. One is that the memories may not be converted into long-term storage and thus simply don’t last long. Another is that the memories are still there long after encoding and we just can’t access them. Professor Turk-Browne suspects it may be the latter.

In ongoing work, his team is testing whether infants, toddlers, and children can remember home videos taken from their perspective as (younger) babies, with tentative pilot results showing that these memories might persist until preschool age before fading.