A2 Only: Plasticity and functional recovery of the brain after trauma.
The ability to adapt to a new and changing environments is an essential skill for survival. It is therefore important for the brain to be able to learn throughout a person’s lifetime, which is achieves through brain plasticity.
The term ‘brain plasticity’ refers to the brain’s ability to adapt and change at any age as the result of experience – for better or worse. These physical changes can be in the form of grey matter shrinking or thickening, and new connections between neurons forming (through experiences and learning) or weakening (as happens in the process of forgetting). These changes have a significant impact on our abilities.
What is brain plasticity?
Up until the 1960s scientists and psychologists believed that the brain could only grow and adapt during childhood; that once a person reached adulthood, their abilities were then set. This is where the phrase “you can’t teach an old dog new tricks” comes from. Indeed, there are some abilities that are more easily learnt during childhood before synaptic pruning occurs. Each child is born with around 2500 synaptic connections per neuron, increasing to 15,000 per neuron by the age of 3. An adult, on the other hand, has around half that number of connections due to synaptic pruning – new experiences strengthen connections, whilst under-used connections are eliminated, or ‘pruned’. By developing new connections and pruning away weak ones, the brain is able to adapt to the changing environment (Doige, 2007).
One of the best examples of how synaptic pruning occurs in children is via language learning. Case studies have shown that children who fail to learn a language before the age of around 7 years old struggle to learn and use complex grammar as adolescents or adults (see the case studies of Genie and Victor, the Wild Boy of Aveyron).
As often occurs in the field of Science, a psychologist called William James demonstrated foresight into brain plasticity much further back in history, yet was largely ignored. In his book The Principles of Psychology (1890) he wrote, “Organic matter, especially nervous tissue, seems endowed with a very extraordinary degree of plasticity.” That brain plasticity was not a common belief prior to the 1960s is necessarily surprising since advances in technology had not reached a point whereby structural changes in the brain could be observed empirically (you can check out the history of neuroimaging here) and the effects of brain damage seemed irreversible at this time.
In the 1960s psychologists investigating stroke victims first began to notice that some brain functioning could be regained and the brain appeared more malleable that was previously thought. Other clear evidence for brain plasticity exists in the fact that, given clear practice, adults can improve in perceptual, motor and cognitive skill domains (Green & Bavelier, 2008); first through ‘fast’ learning within minutes, then via ‘slow’ learning, whereby gains in learning occur much more slowly via sustained practice.
What we now know about brain plasticity
We now know that brain plasticity can occur:
- As an infant: when the immature brain organises itself.
- Following brain injury: to compensate for lost functions or maximise remaining functions.
- Throughout adulthood: whenever something new is learned or memorised
The two main types of plasticity are functional and structural.
- Functional plasticity: arguably the most interesting, this is the ability of the brain to move functions from a damaged area of the brain to other, undamaged areas.
- Structural plasticity: the ability of the brain to change its physical structure as a result of learning
Brain Injury & Plasticity
Norman Doidge, a leading researcher into brain plasticity, described a case study of a surgeon in his 50s who suffered a stroke, resulting in the paralysis of his left arm. During rehabilitation, he had his good arm and hand immobilised during a series of tasks such a cleaning a table, forcing his brain to ‘remember’ how to use his left arm. The surgeon eventually regains the use of of his left arm and is able to write again, because the functions mapped to the brain area damaged by the stroke are transferred to healthy regions.
Ribot’s Law of retrograde amnesia (1881) states that there is a time gradient in retrograde amnesia, so that recent memories are more likely to be lost than the more remote memories following brain trauma.
Plasticity in adults
A lot of recent research using brain imaging techniques has demonstrated structural and functional changes in adult brains as a result of learning experiences. Maguire et al. (2006) compared the brains of London taxi drivers (who have to learn every road name in the city of London for three years and pass a memory test called ‘The Knowledge’) had larger hippocampi than a control group of London bus drivers, who tend to drive the same routes regularly. The hippocampi play a significant role in learning and memory, particularly acquiring and using complex spatial information in order to navigate efficiently. The study was important in demonstrating that brain areas could ‘grow’ as a result of frequent use.
The role of brain plasticity in adults who learn a second language is particularly interesting, as studies have demonstrated that learning a second language can reduce the risk of developing age-related memory disorders, such as Alzheimer’s. A Swedish study on army recruits with a flair for languages showing that after learning Arabic, Russian or Dari intensively, MRI scans showed that parts of their brain increased in size. The study also utilised a control group of medical students who were studying hard at the same time, but saw no particular increase in brain structures during the course of the study. Interestingly, those language students who saw an increase in the size of their hippocampi during the study (as opposed to other regions for other learners, such as the motor region), had the biggest gains in their new language ability.
How can research into brain plasticity be utilised?
Research into brain plasticity is already being used in second language learning. Adult native speakers of Japanese find it difficult to hear differences between the letters ‘r’ and ‘l’, since the Japanese language does not differentiate between these distinct sounds. In fact, a single phoneme represents both sounds. fMRI has been used to show that the same area of native Japanese speaker’s brains lights up when presented with each of these sounds. The only way for these individuals to thus learn to differentiate between the sounds is through the re-wiring of their brains. A software programme has recently been created, which exaggerates between the ‘r’ and the ‘l’ sounds, so they become distinct enough to be recognised. One study has found that after just an hour’s worth of practice using this programme, native Japanese speakers can identify between the ‘r’ and the ‘l’ sounds in normal speech. This is an exciting area of new research!
Rehabilitation After Brain Injury
By far one of the most important areas of research and real life application of neuroplasticity is in the rehabilitation of patients who have suffered from brain injury, trauma or disease. Research shows that for cognitive and functional recovery following a stroke or injury, intensive rehabilitation is absolutely vital is helping the brain to restructure and repair itself.
Unlike most other cells in the body, brain cells do not regenerate when they are destroyed. However, this does not mean that no recovery can occur. As we have seen above, the brain is somewhat flexible and is able to reorganise itself, to an extent, in order to regain lost function. During recovery, other areas of the brain take over the activities of the damaged areas and new nerve pathways can be established using undamaged brain cells. Engaging in activity helps these alternative pathways to develop. Cognitive Rehabilitation aims to help the brain learn alternative ways of working in order to minimise the long-term impact of the brain injury.
Studies of patients, such as JC and Julia, suggest that rehabilitation of memory is most successful if the patient is young, intelligent and motivated. It is significantly easier for younger patients of brain trauma to rehabilitate brain function following injury as their brains are more plastic.
In order to investigate at a cellular levels the effects of rehabilitation on the brain, researchers looked at rats who were relearning physical skills (Wang et al. 2015). It was found that rats who received intensive therapy over time, following deliberate brain lesions, experienced an impressive 50% recovery of function, whilst those rats receiving not therapy failed to recover function entirely. Those rats undergoing intensive therapy also experienced significant restructuring of their brains, with surviving neurons sprouting greater numbers of dendritic spines to form connections with other neurons. The researchers also discovered that the cholinergic system played an essential role in this brain plasticity. As the cholinergic system declines with age, the researchers suggest that a class of drugs called cholinesterase inhibitors (which boosts levels of acetylcholine) may improve functional outcomes for patients who suffer from brain injury; particularly the elderly. This is an new and exciting area of research, which has yet to be explored.
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