You probably remember your 18th birthday, but not your first – or your zeroth. Why is that?
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A lot of things are easy to remember: My high school graduation. My first summer job. That time I got arrested for emptying a bunch of Jello packets into Bryan Cranston’s gas tank – long story.
It doesn’t take a scientist to notice that adults don’t generally remember things that happened before the age of about 3 or 4. Why is that? Why can’t we remember the earliest events in our lives, up to and including birth?
OK, here’s an experiment: Try to remember what happened the last time you ate a burrito. Where were you? Who was with you? Was the burrito full of spiders?
These kinds of memories – being able to recall details of a particular event in the past – are called episodic memories. A person at age 60 will usually have some episodic memories from age 30 – she might not get all the details right, but she will be able to recall some events and explain what happened.
But if you take that same person at age 30 and ask her to describe something that happened during her first year of life, you’ll typically get nothing at all.
Sigmund Freud referred to this hole in our memory as “childhood amnesia,” or “infantile amnesia.” Freud, being Freud, explained it by saying we needed to repress memories from infancy because of their inappropriate or traumatic sexual content.
But sometimes a blank is just a blank, and contemporary scientists don’t tend to throw in with Freud on this one.
Another hypothesis that used to be popular says that babies can’t form episodic memories until they develop certain cognitive capacities, like language.
But there’s a major problem with the language-based hypothesis: Experiments have shown that animals like mice also display both long-term memory and infantile amnesia. Since childhood amnesia crosses species lines, it’s probably something to do with brain biology rather than language.
One possible answer would be to say that baby brains simply can’t make memories. It’s true that memory encoding isn’t as efficient in infant brains as it is in the brains of older children or adults – possibly because the prefrontal cortex of a baby’s brain hasn’t reached maturity yet.
But recent studies have shown that very young children can form some memories, leading scientists to think it’s not that we don’t make memories early in life, but that after a certain point, we can’t access them. The memories are made, but something happens to them: They get erased, or put behind some kind of memory blockade.
Patricia Bauer and Marina Larkina of Emory University have led research on this hypothesis: For example, in one study, researchers recorded children at age 3 describing a recent event, like a trip to a theme park.
Years later, the researchers followed up with those same children to see how much they remembered. At ages 5, 6 and 7, the children could recall more than 60 percent of the earlier events, but by ages 8 and 9, their recall was less than 40 percent. More research of this kind is needed, but this looks like watching the onset of childhood amnesia as it happens.
Another recent study has considered the role of neurogenesis in the hippocampus. The hippocampus is a part of the brain that’s crucial for creating and storing episodic memories. If you didn’t have either of your hippocampi, you could end up like the guy in “Memento” – unable to make new episodic memories.
Neuroscientists Sheena Josselyn and Paul Frankland have proposed a theory that childhood amnesia happens because of rapid formation of new cells in the hippocampus when children are young. This is known as hippocampal neurogenesis.
Basically, while your brain is manufacturing lots of the cells you will use to make memories for the rest of your life, it wipes away or obscures the memories you already created as a young child.