Feb 28 2007

Plastic Brains

Recently I was chatting with my sister-in-law, who is a psychiatric APRN, about how plastic the brain is when my 7-year-old daughter, who had been listening in, asked incredulously, “Our brains are made of plastic?” I had to explain to her that I didn’t mean that plastic, but rather that our brains can change and adapt with use.

It is a very fortunate property of vertebrate brains that they can “rewire” themselves to some degree based upon need and use – a property known as plasticity. The brain can remember and learn stuff, so it’s not surprising that it changes with use. That is the essence of learning. But plasticity is different than just memory. If refers to the rewiring of the structure of the brain for a new type of use, not just forming memories in established brain tissue.

Brain plasticity has two primary functions – developmental and damage repair. From an evolutionary point of view, the former is most likely more important, meaning that it is what leads to the evolution of plasticity. As the brain forms embryologically a key component of its development is adaptation to use. As we use our eyes they map to the visual cortex and cause it to develop. As we hear language our dominant temporal lobe develops its language hardwiring. If someone is born congenitally blind the part of their brain that would otherwise be dedicated to vision is wired instead for other uses.

So the growth and development of the brain is dependent upon brain regions developing in response to use. This makes sense – it allows the brain to adapt to usage and for brain tissue to be maximally utilized. It would not make sense for a blind person to have a huge chunk of their occipital lobe remain dedicated to vision and go unused. (This, by the way, is just one line of argument against the silly notion, still strangely common, that humans only use 10% of their brains.)

But the same mechanisms that allow brains to adapt themselves to usage during the development phase also allow the brain to rewire itself in response to injury. It is not clear if this was selected for as a specific evolutionary advantage or just a nice side benefit of developmental brain plasticity. So if the language part of the brain is damaged by a stroke, for example, then other surrounding parts of the brain may take on some language function and over time become rewired for better language function. This is an important component of recovery, usually takes months to years, and has been documented by fMRI studies.

Plasticity is not unlimited, however. Like everything it becomes less robust as we age. Fetuses and infants have extreme plasticity, as their brains are still forming. There are cases of fetuses having strokes in utero and losing a large piece of one hemisphere, and the other hemisphere develops to take over much of the otherwise lost function, for example. Children have remarkable recuperative powers. Adults have significant plasticity, but less than children. And older adults have progressively less plasticity, and so their recovery from brain damage is likely to be much less than younger individuals.

It is not exactly clear why we lose plasticity as we age, but there are some likely factors. There is a process known as pruning that occurs as we age, especially after age 50, in which unused or perhaps little used neurons die off. This is reflected in progressive brain atrophy that occurs normally with age. So there are fewer brain cells around to take over for damaged areas. Also, brain cells just age metabolically like all cells in the body, and are less robust in all ways.

From a practical point of view plasticity points to one primary reason for rehabilitation – trying to perform the lost task or ability will trigger the brain’s plasticity and help with recovery. However, this (like plasticity in general) is limited. Studies of the effects of rehabilitation are difficult to interpret because there are many beneficial effects that do not derive from brain plasticity. However, it seems that there is a certain minimal amount of activity necessary to promote neurological recovery, but that going beyond that point does not have continued neurological benefits (although there may be other benefits in terms of practice and conditioning, muscle strength, flexibility, etc.). So you probably cannot exceed the brain’s current plasticity by engaging in heroic rehab.

Researchers are now seeking ways to maximize the brain’ plasticity. I have written before about stem cells for certain neurological diseases. It may be possible to simply inject stem cells capable of turning into neurons (brain cells that make connections) and glia (support and modulating cells). These new cells could then be recruited to boost the brains plasticity. This may be an early application for neural stem cells, and may one day significantly aid in recovery from stroke and traumatic brain injury.

The second type of research I have seen involved neural implants or prostheses, for example cochlear implants to replace lost hearing. Basically we want to be able to put a computer chip inside someone’s brain to replace a lost function (or enhance current function). But these chips have to make meaningful connections to the rest of the brain, and that’s where plasticity comes in. So researchers are trying to figure out how to maximize the brain’s plasticity so that such implants will be integrated more effectively into overall brain function, and they are already making meaningful progress.

It’s all very exciting. I for one can’t wait for my computer enhanced cognitive function. Maybe then we really will have plastic brains.

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