Theories abound as to why few of us have any true memories of life as an infant. Here's one with some new scientific grounding: We don't have good, solid recall of our earliest days because our brains were in such a state of rapid formation that any memories that began to establish themselves in the brain's neuronal circuitry were quickly swept away as newly made brain cells scrambled to find their places.
Think of memory-making as a process of laying down a new bridge across a stream. With the water flowing gently, the bank-to-bank (or neuron-to-neuron) connection can be progressively fortified and made permanent. But if the stream becomes a raging torrent while the bridge is still under construction, the rising water can loosen its stanchions. The delicate structure can be washed away before it's become established.
Laying down enduring memories in a baby's brain, a new study suggests, is like trying to build a bridge across a river swollen with rain: As baby observes her surroundings and neurons begin lashing themselves together to form new memories, the fragile bridges under construction are constantly being swept away by a flood of newly created brain cells.
In the brain, this process is called "competitive circuit modification."
In an infant--as in a newborn mouse--the pace of neuronal production is far higher than it is as the human (or mouse) brain ages. In a series of experiments reported in Friday's edition of the journal Science, neuroscientists in Japan and Canada showed that the slower the pace of neurogeneration, the more (and more firmly) memories can be laid down. And the faster the pace of brain-cell production, the flimsier memories become.
To do so, they conditioned 17-day-old mice (infants, in other words) to fear a certain place in a special cage, by shocking them whenever they were in that spot. Essentially, the newborn mice, whose brains were generating new cells at a furious pace, forgot within a week that the special cage was a frightening and dangerous place. Older mice--whose brain cells were being replaced at a far statelier pace--had no difficulty remembering where they'd been shocked before, and avoided the spot like the plague.
Then the scientists, led by neuroscientists Katherine Akers of Toronto's Hospital for Sick Children, tried switching things up. They infused baby mice with agents that slow down the pace of brain-cell generation and found the subjects were far better at remembering, from week to week, that spot where they'd been shocked before.
They also performed the experiment on two kinds of rodents whose brains were largely developed before birth--guinea pigs and degus. Sure enough, these animals formed early and lasting memories of the frightening spot in which they'd been shocked. And when those animals were given agents that boosted their brain's rate of cell regeneration, they became distinctly more forgetful.
Until a couple of decades ago, scientists were convinced that we were born with all the brain cells we'd ever have--that no replacements were produced after birth. Now, the brain's ability to regenerate lost brain cells is well-established, and that churn is key to the human brain's remarkable ability to adapt and learn. It may be no coincidence that one of only two factories for those new brain cells is located in the brain's hippocampus--a structure key to human memory--inside a smaller structure called the dentate gyrus.
In short, the ability to regenerate brain cells lost to disease, injury or the ravages of time is a good thing. It makes us smart and flexible and resilient. But in periods when our brains are under aggressive construction--or reconstruction--something's got to give, as they say. And if a few memories are lost, well, that may just be the cost of progress.