Rats! Bad, old memories stay with us
Early childhood is a vivid experience that all of us eventually forget. In the scientific community, this phenomenon goes by the name “infantile amnesia.” For more than a century, scientists have puzzled over whether we actually form memories in the first years of life and, if so, where those memories go. A new study moves us closer to answering those questions thanks in large part to rats.
The paper, published in July in Nature Neuroscience, finds that with the right reminders, rats can recall painful experiences from infancy that they’d previously forgotten. It also zeroes in on the aspects of brain development that make those earliest memories so difficult to recall.
“There’s a kind of a paradox, the memories are lost quickly, yet apparently they have a long-term influence even in adult life,” says Cristina Alberini, a neuroscientist at New York University and coauthor of the study.
To see how that influence persists, the researchers subjected infant rats to a scary experience. They placed the animals in a box with two rooms, one well-lighted and the other dark. The rats began in the well-lighted room, but eventually explored the dark room, and once there they received a shock to their feet. The infant rats learned their lesson quickly. “In one single trial, they learn that the dark side is paired with a foot shock,” says Alberini.
But, then, they forgot. In subsequent tests a week later, at which point researchers think the rats had aged out the infantile amnesia period (one week is a long time for rats, which have a life expectancy of one to two years), the researchers established that the rats no longer remembered the connection between the dark room and the shock. Once they were confident that infantile amnesia had wiped the memory, the researchers tried to retrieve it.
They first exposed the rats to the same two-room setup from their infancy and observed that the rats wandered into the dark room (which was no longer electrified) as if they’d never seen it before. Then they gave the rats a foot shock in a completely different setting, out of view of the two rooms. Finally, they placed the rats back into the two-room box, and that’s where the big moment took place: All of a sudden, the rats started avoiding the dark room again. This strongly suggested that the combined experiences of seeing the dark room and later receiving a shock were enough to reinstate the forgotten learning experience from their infancy.
“If the room and the shock were shown together, the experience could have just been re-learned. Instead, by being shown separately, they can prove that the rat is using the room and the shock as powerful cues to bring their memories back,” says Simona Ghetti, a professor of psychology at the University of California Davis who studies early memory formation.
The researchers went a step further and tracked this process of forgetting and retrieval to changes in the hippocampus, the home of conscious memories in the brain. They observed and tinkered with molecular pathways in the hippocampus and found that the region undergoes a critical period of development during infancy. This period is driven by the formation of memories themselves — each time the infant rats have an experience, they form memories that mature the neural circuitry in the hippocampus. After enough time (around age 3 in humans), the hippocampus reaches its mature state, at which point it’s capable of holding onto long-term memories.
But, memories formed before that point are more loosely fixed and, in a sense, get overwritten as the neural architecture of the hippocampus matures. Researchers have previously established that a similar process happens with sensory regions of the brain, like the visual cortex. But this is the first time that a critical period has been described in hippocampal memory formation.
The study provides evidence for how infant memories recede on a neural level. It also provides clues about how they can be retrieved. In short, they’re dredged up by contextual similarities between the present and the past that remind you: I’ve been here before. Yet finding those contextual analogs is hard because it’s hard to re-create the world as it appeared to a baby.
“Even if you had the same room, your proportions inside that room would be quite different before and after the infantile amnesia period so that would make even the perception of the context quite different,” says Kimberly Cuevas, a psychologist at the University of Connecticut who runs experiments with human infants.
Cuevas notes that a change in context can be challenging for infant memory retrieval in the first place. They don’t have language, which can be an important dimension we use to gain access to our memories. Other research has shown that a chasm opens which makes it difficult to put words to memories acquired during a pre-verbal period of development.
“It’s not that the memories aren’t there, it’s that we don’t have the retrieval cues,” she says.