What’s the earliest memory you can recall? While many people’s recollections of the past may stretch back into childhood, research shows that the trip down memory lane generally hits a wall once you reach infancy. In some ways, this doesn’t make much sense—after all, the first years of a baby’s life are when they learn foundational psychological concepts, form relationships with caregivers, and gain a sense of self.
Experts have long attributed this “infant amnesia” to the development timeline of the hippocampus, the region of the brain responsible for retaining memories. But according to new evidence from a team at Yale University, the explanation for early our memory blocks may be a bit more complicated. Humans generate memories during their very first months of life, but where do those memories go?
Researchers detailed their work in a study published March 20 in the journal Science. To begin their experiment, scientists showed infants a series of new images before later testing to see if they remembered them. Recognizing an image from the past is an example of an episodic memory. As an adult, these can take the form of remembering specific events, like watching a sports match or taking a vacation. But judging episodic memory capabilities is more difficult when there’s a pretty obvious communication barrier between adults and a bunch of babies.
“The hallmark of [episodic memories] is that you can describe them to others, but that’s off the table when you’re dealing with pre-verbal infants,” Nick Turk-Browne, a psychology professor, director of the Yale Wu Tsai Institute, and study senior author said in an accompanying statement.
Instead of trying to learn babytalk, the team recorded hippocampal activity during both test phases using functional magnetic resonance imaging (fMRI). The results showed that an infant appeared more likely to remember an image if the hippocampus activated more during the first exposure. They also examined a baby’s physical cues for further confirmation.
“When babies have seen something just once before, we expect them to look at it more when they see it again,” 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.”
Infant behavior corresponded to their fMRI scans. A more active hippocampus while first looking at an image meant a baby looked at it longer when they saw it again later. Meanwhile, the posterior portion of the hippocampus that lit up is the same region associated with an adult’s episodic memory banks. The strongest evidence came from infants who were 12 or more months old, indicating a timeline of how the brain develops in these first few years.
Previous work by Turk-Browne’s team already revealed babies exhibit what’s known as statistical learning even earlier—around three months old. While an episodic memory has specificity, statistical learning relates to more holistic concepts, such as understanding what a building looks like or which traditions are celebrated on certain holidays. Each also relies on separate neuronal pathways in the hippocampus. Combined with their latest findings, researchers are better understanding the progression from a baby’s earlier statistical learning formations to the development of episodic memories. According to Turk-Browne, this makes a great deal of sense.
“Statistical learning is about extracting the structure in the world around us. This is critical for the development of language, vision, concepts, and more,” he said. “So it’s understandable why statistical learning may come into play earlier than episodic memory.”
But the larger question still remains: What happens to these earliest memories? Turk-Browne believes there are multiple possible explanations, including the theory that those recollections simply never make it to the long-term storage regions of the brain. However, the study’s senior author thinks another reason is more likely: Those memories remain encoded in our brains, but we simply can’t access them. That’s what the team hopes to explore in the future.
“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,” Turk-Browne said.
