Though we learn so much during our first years of life, we can't, as adults, remember specific events from that time. Researchers have long believed we don't hold onto these experiences 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. But new Yale research finds evidence that's not the case.
In a study, Yale 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. And the researchers are now looking into what happens to those memories over time.
Our inability to remember specific events from the first few years of life is called "infantile amnesia." But studying this phenomenon is challenging.
"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," said Nick Turk-Browne , professor of psychology in Yale's Faculty of Arts and Sciences, director of Yale's Wu Tsai Institute, and senior author of the study.
For the study, the researchers wanted to identify a robust way to test infants' episodic memories. The team, led by Tristan Yates, a graduate student at the time and now a postdoctoral researcher at Columbia University, used an approach in which they showed infants aged four months to two years 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," said Turk-Browne. "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, said Turk-Browne.
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, Turk-Browne suspected that episodic memory may appear later in infancy, around one year or older. He argues 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," he 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, says Turk-Browne. 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. And Turk-Browne suspects it may be the latter.
In ongoing work, Turk-Browne's 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.
The new findings, led by Yates, provides an important connection.
"Tristan's work in humans is remarkably compatible with recent animal evidence that infantile amnesia is a retrieval problem," said Turk-Browne. "We're working to track the durability of hippocampal memories across childhood and even beginning to entertain the radical, almost sci-fi possibility that they may endure in some form into adulthood, despite being inaccessible."
