Babies Form Memories. Why Do Adults Forget Them?   

An infant learns many things in the first few years of life—movements, languages, relationships, and more—all of which require memory formation. Yet, if an adult is asked to recall individual experiences from this dynamic period, they would hardly remember any. This paradoxical phenomenon, called infantile amnesia, has intrigued scientists for more than a century.¹

One important way the brain remembers the world is by forming episodic memories to immortalize experiences, which include multiple components such as places, objects, and people involved. Within the brain, a banana-shaped structure called the hippocampus is crucial for the formation of episodic memories.² On studying patients suffering from amnesia with hippocampal lesions, scientists observed similarities between their memory capabilities and those of infants. Based on these findings and the fact that the hippocampus is immature until adolescence, the prevailing theory suggests that infantile amnesia emerges from the inability of the hippocampus to encode experiences during early years of life.² Now, in a study published in Science, researchers at Yale University reported that infants as young as one year old can encode episodic memories, overturning the longstanding hypothesis.³

Studying the neural mechanisms underlying infantile amnesia comes with challenges, key among which is working with tiny humans who aren’t always interested in following instructions. However, Nicholas Turk-Browne, a neuroscientist at Yale University and coauthor of the study, has pioneered ways to peer into the brains of awake infants while they form memories. “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,” said Turk-Browne in a statement.

One method for assessing episodic memory in adults relies on individuals observing a series of words or images and subsequently performing a memory test. Since infants cannot perform the latter half of this paradigm, Turk-Browne and his team adapted their experiment such that they could ascertain memory encoding passively. Each child was initially shown a series of pictures of objects, faces, and places. This was followed by a test phase where the researchers showed the infants two pictures, one that they had seen before and one that was new to them. Infants tended to look at pictures that they knew for longer—the team used this behavior as a readout for memory. Simultaneously, Turk-Browne and his colleagues recorded the infants’ hippocampal activity using functional magnetic resonance imaging (fMRI) and observed higher neural activity in the posterior hippocampus for images that would later be remembered by the infants, as compared to those that were forgotten. While the study cohort included children between the ages of four and 24 months, the team observed memory formation preferentially in those who were about 12 months or older.

Scientists have previously teased apart the mechanisms of infantile amnesia in mice, showing that the hippocampus encodes episodic memories, which the animals promptly forget in adulthood.⁴ However, these memories can be retrieved through optogenetic activation of the hippocampal neurons. The findings from Turk-Browne’s study suggest that the inability to remember infant experiences is not because babies can’t form such memories, but could stem from the lack of their consolidation into stable long-term memories or lapses in their retrieval in adulthood.

“[Our] 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.”