The brain is built to learn in a massive way only during the childhood and adolescence. Specific plasticity windows - named critical and sensitive windows - are open within this time interval, which help us to rapidly adapt to the external world. But the fact is that most of our life we live outside these windows and we can see this misfortune almost every day, as adults. Nevertheless, recent studies revealed the secret recipes for re-opening these plasticity windows in adult brain, with tremendous effects on learning, repairing and postponing the old age.
A "critical period" is a time window when external stimuli are mandatory for the normal development of a certain brain circuit. An example could be the learning of language. By contrast, a sensitive period is a time window when environmental experiences have the greatest impact upon a brain circuit. An example is learning a foreign language. A critical period once closed the openness the effect is a reduced effect of sensorial experiences upon the brain. These windows explain the easy of learning which characterizes children and adolescents by contrast to the relative decrease in learning performance specific to adult people. The onset and the duration of a critical period depend on age but also on experience. If a proper exposure or activity is not provided, a brain circuit remains in a waiting state until that input is available. Not all the brain regions have the same developmental pathway. It matures starting from back to front and from basic functions like vision and movements to more complex abilities like language, problem solving or understanding others. In the visual cortex, the critical period closed around the age of 5 in humans, after this milestone the brain’s capacity to be changed by experience being significantly reduced. It is interesting that any foreign language learned before the age of 11 is superimposed in the brain on the same area of the native language, within the Broca area of language. By contrast, any foreign language learned after this age is saved in a different area within the Broca area. Also, learning a foreign language after this age requires much more effort and good results takes more time. If a child is not exposed to native language until age of 11, afterwards he/she can learn only the words but not the grammar, hence being impossible to speak normally. Critical periods dictate also over some learning processes like extinction of a traumatic memory or a phobia. It is known that after a successful therapeutic session, these memories are not erased, are only inhibited (this is why they can pop-up sometimes and the stressful memory reappears). At least this is happening in the adult brain. But in the child brain, during his/her first years of life, the extinction involves the permanent erasure of that memories. In the adult brain a proteins network is developed around the neurons and this network secures the memories for a life time. So is not possible to be erased anymore. It is striking that the proteins that build this network are the same that build the cartilages from the trachea, bronchi and heart.
The protein network which protect our memories from erasure (with red)(from Gogolla et al., 2009. Science, 325:1258-1261)
Given the fact that most of our life we spend as adults and the environment around us is changing all the time, is critical for an adult to preserve the learning abilities he/she use to have as an adolescent. Accidents that happened to an adult brain are harder to be repaired and sometimes the repair process is not totally successful. And the closure of the critical and sensitive periods are to be blamed for this. But it is possible to re-open these windows in an adult brain? Experiments revealed that several factors are involved in triggering a critical or a sensitive period. For instance, the therapy with antidepressant medication, such Prozac, seems to facilitate the plasticity of adult brain and it is prescribed, along with the physiotherapy, for the stroke victims. Also, enriched environment it is documented to have the same beneficial effects with the antidepressant medication, at least for the animals suffering of amblyopia. Enriched environment provides a combination of multisensory stimulation, physical activity, social interactions and facilitation of exploratory behavior. Enriched environment impacts the brain leading to an enhancement of cognitive functions (especially learning and memory) but also positively impacts emotional reactivity and stress response. Also it facilitates the growth of the grey matter hence influencing the weight of the brain, the number of synapses and the birth of new neurons. It also stimulates the expression of 41 genes known to be involved in learning and memory, plasticity, cellular growth, the genesis of blood vessels, and neuronal excitability. Even more , enriched environment seems to stimulates the anti-oxidative mechanisms of the organism, decreasing the inflammation and boosting immunity. It is interesting that enriched environment facilitates the maturation of visual system even in the case of total absence of visual experiences! Studies made on rats have shown that housing the pregnant female in this type of environment during the last semester of pregnancy leads to a more rapid development of pups’ visual system. Furthermore, running in a wheel leads to a 2-fold increase in the number of neurons in their pups’ brain and also a better working memory for them, once they give birth.
A critical period can be induced in adult humans through non-invasive techniques such as enriched environment (as I already mentioned) but also by incremental training and educational video games, all of these putting the brain into a “learning mode”. Studies have shown that people which use to train themselves with action videogames, present an enhancement in visual acuity, effect not possible to be obtained by playing non-action video games. This emphasizes the role of some specific features of action games in maintaining attention and stimulating some neurotransmitters like dopamine and noradrenaline which are critical for brain plasticity. Studies made on animals found that the modification of the training protocols in order to contain incremental changes of the sensorial experiences leads to enhancement of the learning ability. The same results have been seen on human subjects exposed 2-3 weeks on various learning experiences previous to the exposure to the target material to be learned. This is called metaplasticity, or the intentionally process of inducing plasticity in the brain.
All of this data show that our brain is a life time project; it can help us only if we help it. It’s a sort of symbiosis. In addition, the brain is the only organ in our body we can keep young, at least for a while. The “know-how” of this process is not written in our genes, it depends by the knowledge we gathered with the technology it helps us to develop. So we can say neuroscience is the brain’s effort to discover how it is built, how to repair itself and how to enjoy the world as much as is possible.