Neural plasticity, the capacity for neurons to change their connections, is a fundamental property of the brain. It’s what allows us to learn. But as we age, the plasticity of the brain decreases. Indeed, there are developmental windows called ‘critical periods’ where the brain is especially plastic, usually when very young. The neural basis for vision, as an example, is laid down at during infancy, and doesn’t change easily thereafter. This is why kittens, raised in a visual environment of many vertical stripes, find it difficult as adult cats to see horizontal stripes – their brains, while plastic, had adapted for certain visual features, and found it difficult to adapt to new ones.
Recently, researchers at the University of California San Francisco have found a way to renew the infant-like plasticity of the mouse brain, allowing childlike learning to begin again. They did this by injecting embryonic mouse neurons (not to be confused with embryonic stem cells) into young mice, which had been raised with one eye deprived of light. Without the injection, mice that were deprived up until around a month old (the usual critical period for mouse ocular dominance) would have difficulty adapting to seeing through the eye that had been deprived of light. But with an injection of embryonic neurons into the brain, the mice went through sort of a second critical period, when the injected brain cells were about a month old, thereby aiding the mice to learn to see from their deprived eye.
This has profound consequences for enhancement of human intelligence. The obvious therapeutic outcome would be using embryonic human neurons, taken from human embryos or cultured from human embryonic stem cells, and injecting these into adult humans to give a second chance at a critical period of learning, which would be useful for re-learning to walk after an injury or learning to adapt to blindness (or, I don’t know…learning to control your new cyborg limbs?). But on a grander scale, if we could have a constant trickle of neural stem cells, developing into immature neurons, throughout life, we could very well keep our critical periods going for the rest of our lives, allowing our brains to stay young and malleable!
There is one caveat I can think of. There must be a reason why the brain has evolved to shut off critical periods of learning. Most likely, learning is a costly process in some way, thereby creating a pressure to keep learning periods are short as possible. If this is merely an increased energy cost, humans in the first world can probably deal with it (seeing as most of us eat too much food energy anyway). But if shutting down mechanisms of learning is necessary for enhancing the function of the newly learned circuits (that is, if we don’t perform as well if we keep ‘changing our minds’), there may be a question of whether learning is worth the cost.
I would assume in this ever changing technological environment and with our lives getting longer and longer (making what we learned during our critical period more and more irrelevant), that keeping the brain plastic would be very useful indeed.