Despite the long-held belief that the only change that can be seen in an adult brain is degeneration due to age and/or injury we now know that our brains retain a level of neural plasticity throughout life. Not only can new neural connections be grown through the process of collateral sprouting but entirely new neurons can be produced, as evidenced in studies on the adult hippocampus. The process can be spurred on and ensured through environmental enrichment efforts: Depending on the location and type of brain damage these can be language training, various cognitive tasks targeting the regions affected, or rehabilitation through physical therapy such as the constraint-induced movement therapy.
In the later, a healthy part of the body, a limb usually, is restrained to prevent a patient from compensating for a disability in its counterpart limb or body part in order to force the use and therefore rehabilitation of the affected one. Thus a patient’s unaffected arm may be immobilized with specialized restraints to force the use of the paralyzed arm. In some such cases researches have reported a 75% mobility restoration through regular and persistent repetitive movement training of the disabled arm after only a couple of weeks of therapy.
Another approach that has shown success is a setup where the patient is put in front of mirrors arranged in such a way as to only reflect the unaffected side of the body, with that reflection doubled and positioned to make it appear that the patient’s entire body is reflected. When the person is asked to move both of their arms it appears that they’ve succeeded even though they are in reality only moving the unaffected arm – at first. Impressively, having the visual feedback that one is capable of moving their paralyzed arm despite knowing the opposite to be true seems to help the patient override the reluctance and pessimism that such an achievement is possible and regain full control of their self-rehabilitation resources.
Something more may be at play however: mirror neurons have been shown to activate a person’s corresponding motor pathways on merely observing another person’s movement, thus providing a neural pathway exercise of sorts and a type of physical therapy in its own right (Tyson, 2005). In cases where rehabilitation therapy and enriched environment is simply not enough another more complex option is to implant new neurons in damaged areas using stem cells - embryonic, umbilical-cord-blood-derived, or possibly even patient’s own cells regressed to their stem stage - which then position themselves and assume needed functions based on chemical guidance from their neighbors.
There are however certain types of abilities whose full potential can only be reached within certain early critical periods, after which the skill and its underlying mechanisms can only be activated and developed in a very limited way. Such is the case with both vision and language: a person born blind would not be able to see if whatever malfunction was the cause was repaired or bridged in adulthood – their brain would get the proper stimuli but it would simply have no way of interpreting them correctly as it never learned how during the early critical period when it was still possible to do so.
The same applies to attempts to restore hearing in an adult deaf from birth. Also, learning one’s first language must be done during an early critical childhood period as well while the proper brain structure to accompany language acquisition can still be developed – otherwise, only very limited speech can be achieved and one will never reach anything resembling conversational fluency.
Learning a second language however is a different story as the necessary circuitry has already been set. Adults can learn additional languages but it becomes increasingly difficult to do so and they will never reach the perfect fluency of a native speaker. Children who acquire their second language during the critical language development period will have the fluency of a native speaker for it as well, and using the second language will activate largely corresponding brain pathways of the first language. The larger the time delay between learning the first and second language the less of a regional activation overlap will the use of each have.
Children’s brain plasticity in general is remarkable and very much superior to that seen in adults, and nowhere is this more evident than in extreme surgical interventions sometimes seen in attempts to relieve severe epilepsy. There are cases where the entire left brain hemisphere is removed and yet children are able to over time regain full language capacity as well as restore any other function initially affected, while intellect does not appear to be affected. This is light-years away from the kind of results an adult undergoing the same procedure can hope for – the language ability at the very least usually remains drastically impaired.
Biology and Human Behavior: The Neurological Origins of Individuality, 2nd edition, Robert Sapolsky, Stanford University, 2005 (DVD)
PBS series: The Secret Life of the Brain, 2001 (DVD)
Tyson, Peter. (2005). Research Update: Daniel Glaser’s latest study with ballet and capoeira dancers. Retrieved November 16, 2007, from Nova Science Now website: http://www.pbs.org/wgbh/nova/sciencenow/3204/01-resup.html