The birth of new hippocampal neurons may help explain infantile amnesia — the fact that adults can’t remember experiences from before age 3. Lots of new neurons get added to hippocampal circuitry at that age, disrupting existing connections and causing us to forget experiences. In adults, new neurons pop up more slowly, but the forgetting continues, just to a lesser degree, and may serve to clear away meaningless and irrelevant information. “Luckily, young kids don’t forget useful skills like walking or talking,” says Paul Frankland of the Hospital for Sick Children in Toronto. “They only forget how they learned those skills.”
Putting a Place to a Face
Itzhak Fried at the University of California, Los Angeles, has shown that when patients recall a video clip, their neural networks activate in the same way as when they first saw it. In other studies, Fried actually saw associations forming — neurons that originally fired for celebrities (like Clint Eastwood) began to also fire for landmarks (like the Hollywood sign) after patients saw pictures of the celebrity-landmark pairings. This shows that neural networks can change quickly to associate new information with old memories.
Without realizing it, we often make inferences to fill in gaps or remember being somewhere we weren’t because we’re so familiar with the story. It’s likely these false memories get reinforced the same way real ones do: During the recall process, the circuit gets fortified, strengthening the inaccuracies. Henry Roediger at Washington University in St. Louis, who studies false memories, says the brain can’t tell the difference between real and false memories, making our fabricated memories seem authentic.