Memory – an elusive, enigmatic entity yet perhaps one of the most powerful forces on Earth. We each have our own collection, a flip-book of times gone by that shape our present and mold our future.
Yet why do memories fade? Why do memories slip from our minds – forgotten, lost? Why do we forget our earliest years in life, arguably the most formative?
This mystifying phenomenon is known as childhood amnesia. It is the inability of adults to unlock the years before the age of around 3.5 years old. When this “amnesia” hits is a bit different for everyone, with some not remembering anything until the age of six.
Researchers have dipped into this conundrum, with some clarifying results. Most, however, remain as vague as the memories themselves. Still, there are hints that tell us where to look next.
At birth, a baby’s brain contains around 86 billion neurons, roughly a quarter as many cells as there are stars in the Milky Way. Using this neural machinery, babies soak up information, their brain forming an estimated 700-1,000 new synaptic connections per second. It’s an incredible feat of learning and a rapid period of development. Yet perhaps this is also its downfall.
In a rather simple but compelling experiment, Paul Frankland and colleagues placed infant and adult mice in separate cages they had never seen before and zapped their feet with mild electric shocks. Anytime the rodents returned to the cage that had previously zapped them, a fear response kicked in.
The fear in baby mice, however, began to slip away after a day; the adult mice never forgot. That is, until the team stimulated neurogenesis – the formation of new neurons in the brain – by running the adult mice on a wheel. In doing this, they could make the adult mice start to mirror the infant amnesia.
To check that this effect goes both ways, they then hindered neurogenesis in the infants with genetic engineering and drugs. In doing so, the mice formed much more stable memories.
While you may think that’s the end of the story, the team dived even further into the mice's tiny brains and inserted a fluorescent tag into newly formed brain cells. This revealed an interesting process – the mice’s memories were not being replaced, just modified with new memories.
What does this mean? Well, it hints at the possibility that our memories have not vanished, but instead changed in a way that makes them inaccessible. Of course, further work on humans will need to be done to verify this for ourselves.
That’s perhaps one piece of this complex puzzle, but another study on memory led researchers to another equally compelling theory: As babies, we just don’t have the mental equipment necessary yet to form lasting memories. Ironically, one of the strongest pieces of evidence for this theory comes not from a child but an adult.
A famous 27-year-old patient named HM had a lobectomy to “cure” his epileptic seizures – possibly the result of a bicycle accident when he was 7 years old. After a good chunk of his brain was removed, including a good portion of his hippocampi, he could no longer recall new events.
Ask him what happened yesterday and he couldn’t say. Ask him to draw a star by looking at it in a mirror, however, and he improved with practice, despite not remembering he had practiced at all. This holds intriguing similarities to babies – they can learn new information, but they can’t remember the process by which they gained it.
Are the memories gone? Or are they inaccessible, the key to their retrieval lost? Researchers don’t know for certain, but some hypothesize that the memories are there and that the connections to access them are gone, perhaps in a process of synaptic pruning.
By now you may notice that the study of memory, and in particular the lack of remembrance, is a tricky endeavor. This is especially true in light of recent discoveries revealing how fallible and prone to error our brains truly are.
Case in point: false memories. These are memories that seem as real as all the others, yet never actually happened. This can be for reasons as innocuous as our mother telling us a story over and over again until it sinks into our minds as if it was our own memory. It can also lead us astray in the criminal justice system as evidenced by one prominent cognitive psychologist.
The experiments of Professor Elizabeth Loftus of the University of California, Irvine, have become renowned worldwide. In one, she and her colleagues provided subjects with true stories from their childhoods – except for one false story. The tale of being lost in a mall as a child was mostly made up. Yet a quarter of participants said they recalled it; what’s more, when told that one of the stories had been invented, some couldn’t pinpoint which memory was the fabricated culprit.