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Why don't We Remember our Early Childhoods?

How the brain gets rid of unused or ineffective brain connections

Source: Wei-Chung Allen Lee, Hayden Huang, Guoping Feng, Joshua R. Sanes, Emery N. Brown, Peter T. So, Elly Nedivi - Dynamic Remodeling of Dendritic Arbors in GABAergic Interneurons of Adult Visual Cortex. Lee WCA, Huang H, Feng G, Sanes JR, Brown EN, et al. PLoS Biology Vol. 4, No. 2, e29. doi:10.1371/journal.pbio.0040029

Setting in around age seven, childhood amnesia involves the sudden deletion of previous memories. It was hitherto thought that childhood amnesia occurred because the brains of young children couldn’t form lasting memories of specific events. 

But in the 1980s, Patricia Bauer, a professor of psychology at Emory University , and colleagues started testing the memories of children as young as nine months old. They discovered that children have very solid long-lasting memories of specific events.

But as the children grew older, it seemed that they somehow lost these memories at some point. Bauer and colleagues recorded children at the age of three talking to a parent about events, such as going to Six Flags or on a vacation to the US Virgin Islands. At the age of seven, these children could still recall more than 60 percent of the recorded events, but children who were just a year older remembered only about 40 percent. Age seven seems to mark the onset of childhood amnesia. 

The reason childhood amnesia sets in around that time has to do with pruning, the main purpose of which is to get rid of unused or ineffective brain connections. Pruning is a process that changes the neural structure by reducing the overall number of synapses, or brain connections. This results in more efficient synaptic configurations. Pruning is governed primarily by environmental factors, particularly learning.

The brain can also change its wiring in a different way. In the pruning process, neurons don’t die off. They simply retract axons from synaptic connections that are not useful. But the brain can also rewire itself by killing off its neurons in a process that is called apoptosis, which is a form of programmed neuronal death that is different from the kind of killing of neurons that occur in brain injuries. In apoptosis the neuron is killed and all connections associated with the neuron are also trimmed away.

During childhood and adolescence, initially imprecise, unused, or unnecessary neural connections between neurons are gradually pruned away, leaving connections that are stronger, more useful, and more specific. We can think of it as a sort of neuronal natural selection.

In some individuals, however, the pruning processes deviate from those of most people. So, direct neural connections between regions of the brain that do not normally stay connected remain connected. For example, connections may remain between the color and form areas, or between color and auditory brain regions. The former would lead to grapheme-to-color synesthesia and the latter to sound-to-color synesthesia.

Presumably many of our early memories are pruned away, which leads to childhood amnesia. As pruning is radically reduced in adulthood we have better memories of specific events as adults. But some childhood memories survive this pruning, and tend to be very emotional memories or strongly connected to a story with a very intense plot.

Better memory comes at a cost. The adult brain is significantly less plastic than a child's brain. This means that many things that are easily learned as a child can be super-difficult to learn as an adult. One excellent example of this is language. We can learn languages as adults but it is much harder to do so than when we learn them as children, and most adults end up with an accent that they usually cannot get rid of. Children who are exposed to a new language for long period of time learn to speak without an accent, even when the language is not their native language. 

There are, however, exceptions to the limited plasticity of the human brain. A new study conducted by a team of scientists from Cologne, Munich and Mainz shows that exposing yourself to new experiences can help generate new neurons and new brain connections in the hippocampus, the brain's main memory center. The more you experience and the more complex the experiences are, the better the new neurons are integrated into the brain's existing network. 

The "use it or lose it "principle still applies, however. Few experiences or too simple experiences decrease connections in hippocampus. So, there is a great price to pay for staying inside and passively half-watching some game show on television instead of venturing out into the world: the hippocampus suffers in the former case. And this does not just affect memory. The hippocampus is also crucially involved in regulating mood and emotions. So, the consequences of an uneventful lifestyle could be a contributing factor to common mood disorders, such as depression and anxiety.