Brain plasticity--What is it and how extraordinary can it be?

Brain plasticity (also known as cortical plasticity or neuroplasticity) is the ability of the brain to reorganize itself – forming new brain cells and new information processing connections between those cells, and new functions for those cells.

Neuroplasticity is known to occur in the brain in the following situations:

• During infancy and childhood, and into young adulthood: when the immature brain organizes itself.
• Through adulthood: whenever we form a new memory, learn something new, or develop a new skill.
• In response to brain injury, disease or congenital brain disorders such as blindness: to compensate for lost functions or maximize remaining functions.

Recent evidence demonstrates that the brain is capable of remarkable widespread change and adaptation throughout the lifespan – much more than had previously been thought.

One remarkable type of brain plasticity occurs when parts of the brain that usually serve one cognitive function are taken over by other parts of the brain to serve other, completely different, functions.

A general principle of brain function is what is called localization: different perceptual and cognitive functions are performed by different parts of the brain. Damage to one part of the brain can result in a highly specific and selective deficit, while leaving remaining cognitive functions intact.

Blindness from birth and ‘cross modal plasticity’

Blind individuals have to make some major adjustments in order to cope with and flourish in a world that is designed for the sighted. Blind individuals develop heightened abilities in the use of their remaining senses in order to compensate for their loss of sight: their superior skills in tasks involving touch and hearing has been established as a fact in laboratory settings.

In normal brains the back (anterior) region of the two hemispheres of the brain is specialized for processing visual information. This part of the brain in each hemisphere is called the occipital lobe. Damage to occipital lobes – perhaps in a car accident, or through a stroke – results in blindness, but will leave all other sensory modalities and higher level functions such as language and personality thought unaffected.

What do the blind use the occipital lobes for if they can’t see? Magnetic Resonance Imaging (fMRI) and other brain imaging techniques have enabled us to answer this question. As normally sighted individuals we use our vision – and our occipital lobes - to read. Blind individuals, by contrast, use the sense of touch to read braille. Recording the brain activity in individuals blind from birth when they read braille shows that their occipital lobes light up – it has been recruited to read brain by touch. Since one modality (touch) is substituting for another modality (vision), this kind of plasticity is called ‘cross modal’.

Additional experiments have also shown that blind individuals’ occipital lobes are used for speech processing, a function usually performed in the temporal and frontal lobes.

These discoveries are perhaps not that surprising, given how ‘plastic’ the brain can be during childhood. A similar kind of brain reorganisation can be seen by comparing bilingual vs monolingual speakers: an area in the left  parietal lobe is significantly larger in bilingual brains than monolingual brains.

But the brain’s plasticity is more remarkable than that. And here are some examples of its power to change adaptively.

Blindfolding Experiments

Just five days of blindfolding in normal sighted adults leads to the occipital lobe processing information for touch and auditory (sound) processing - functions normally performed by the parietal and temporal lobes. It begins to light up in brain imaging studies, with touch and sound stimuli! This is a rapid, temporary rearrangement of brain structure and function, designed to adapt to new demands. The occipital lobe returns to normal functioning once the blindfold is removed.

Memory and Expertise: taxi drivers and musicians

When you become an expert at something, the areas in your brain that deal with this kind of skill increase in volume.  For example London taxi drivers – who have a highly developed memory for routes through London - have been shown to have a larger hippocampus in the temporal lobe than London bus drivers. The hippocampus is specialized for spatial coding in navigation.

Brain reorganization has also been shown in musicians. There are several areas of the brain involved in playing music – in the frontal lobe, parietal lobe and temporal lobe. Professional musicians who practice at least 1hour per day) have a highest volume of neural tissue in these areas. Amateur musicians in turn have more brain mass in those areas than non-musicians.

Brain plasticity as a result of skill acquisition and extensive learning in specialist areas is the norm. Cortical territory shifts over time in response to the knowledge and skills we acquire.

Short term changes: studying for exams

In one brain imaging study, the brains of German medical students were scanned 3 months before their medical exam and right after the exam. Their brains were compared them to brains of students who were not studying for an exam. The medical students’ brains showed showed changes in regions of the parietal lobe as hippocampus – areas of the brain known to have memory and learning functions.

Training intelligence and changing brain connections 

Recently a brain training exercise has been successfully developed that has been demonstrated to improve what is called fluid intelligence – our ability to reason and problem solve in new situations – as measured by standard IQ tests (link). Training with this software has also been shown to increase neural activity in part of the frontal lobe known to be involved in higher cognitive functioning, and to increase the density of neurons’ receptors for dopamine, a neurotransmitter involved in higher cognition. This is called synaptic plasticity – it occurs at the level of the connections between individual neurons.

The most extraordinary example of brain plasticity of them all: hydrocephaly

The brain is suspended in a fluid called cerebrospinal fluid which baths the whole of the central nervous system. Without cerebrospinal fluid the brain would collapse under its own weight. Hydrocephalus – more commonly known as 'water on the brain' – is a condition in which there is an abnormal build up of cerebrospinal fluid. Without treatment, it can lead to brain damage and death. Pockets of cerebrospinal fluid in the brain called ventricles can expand and in extreme cases the entire brain can be pushed by the fluid into a small layer close to the skull.

The British neurologist John Lorber has documented over 600 scans of people with hydrocephalus and has categorized them into four groups:

• those with nearly normal brains
• those with 50-70% of the skull filled with cerebrospinal fluid
• those with 70-90% of the cranium filled with cerebrospinal fluid
• those with 95% of the cranial cavity filled with cerebrospinal fluid – the most severe group
  
  

Of the last group, which made up 10% of the study, half were profoundly retarded.  The remaining half – and this is the extraordinary testament to brain plasticity – had IQs greater than 100 (the average IQ level). One young man in this category of ‘virtually no brain whatsoever’ had an IQ of 126 and got a first class honours degree in mathematics! Brains are normally around 1.5kgs. But these cases prove that 50-100 gram brains may perform at a normal and even superior level.

Below is an MRI scan of a French civil servant’s brain! The image shows that this 44-year-old man’s brain is little more than an outer shell in comparison to a normal brain. The region labled LV is just fluid in this brain - normally it is packed with neural tissue!

“He was a married father of 2 children, and worked as a civil servant,” Dr Lionel Feuillet and colleagues at the Universite de la Mediterranee in Marseille wrote in a letter to the The Lancet medical journal.

Image: Feuillet et al/The Lancet

Almost all that neuroscientists know about brain anatomy and function are turned on their head in these extraordinary cases. Many neurologists feel that this is a tribute to the brain's incredible plasticity and its built in ‘redundancy’- the idea that much brain tissue has a ‘backup’ function. Others disagree.  Patrick Wall, professor of anatomy at University College, London states “To talk of redundancy is a cop-out to get around something you don't understand”.

One way or another, it is clear that the brain has amazing powers of plasticity. And the research tells us that our longer-term commitments in life - what skills we invest time into acquiring, what talents we want to cultivate, and what we want to learn in depth - will result in profound changes in the very structure and organisation of our brains.