Saturday, November 28, 2009

Impermanence & Plasticity

Everything changes. The largest chunk of uncertainly is impermanence. There are constant paradoxes and contradictions built into our brain function. We must be alert to notice and respond to changes but, at the same time, attempt to be stable and consistent. Our visual system is designed to notice minute changes but ignores most of the movement around us to create the illusion of a stable world in consciousness. Growth, development, and aging are the main expressions of predetermined impermanence that combines DNA programming with environmental opportunities and hazards.

You could argue that brain growth and development changes are most vigorous in the first 20 years of life; later, after a brief period of relative stability, degenerative changes take over, accelerating with advancing age.

A big problem we have is that while the world around us changes, we also change and the biggest changes occur in our brain. The idea of one personality remaining stable over many years is actually absurd, but we are tempted to believe in an enduring self. An astute observer will notice that each day brings forward a series of different personalities within one body. I call these personalities eigenstates. The self is not one entity but rather consists of a collection eigenstates that serve different needs, roles and capabilities. Some eigenstates are built it others are learned and remain open-ended, evolving with changing circumstances.

Learning, in the best case, is adaptive impermanence that requires changes to brain structure and function. We will consider, for example, that learned movements are generated from dynamic cortical maps based on fields of activity that converge and diverge in complex patterns. Over time, the pieces of the map change with learning and practice, so that the construction of cortical connections is always in flux. This impermanence allows us to learn at all stages of life, to adjust to changing environments and, to some extent, to work around disabilities that arise from brain injury and disease.

Among affluent self-indulgent humans, there is conspicuous age denial and much promotion of anti-aging products and procedures. While, in the best case, humans can continue to learn into old age, the facts are not so encouraging. All brain functions decline with age and degenerative brain diseases appear with increasing frequency as the years advance. Slogans such as use it or lose it may contain some truth, but it is never obvious that high functioning elderly humans are doing well because of brain exercise with crossword puzzles rather than by luck, cleaner air, better DNA and superior diet. It is more obvious that sustained physical activity, reduced caloric intake and good nutrition are the keys to high functioning aging.

Too often, I am an unwilling victim of television news nonsense and plasticity is a current favorite topic. Brain damaged survivors are shown with plausible mental abilities, as if their example refuted neuroscience beliefs. The term plasticity has crept into neuroscience jargon and should be erased from the vocabulary. I am not aware of the source of plastic metaphor and can only assume that it refers to a material that can be coaxed into different shapes by heat and pressure using a variety of machines. I cannot see any connection between the malleability of plastic and the constant flux that characterize brain function.

Even smart, educated humans participate in these media delusions. For example, I was surprised to read a report by Allison Gandey from a meeting of the American Academy of Pain Medicine that revealed basic ignorance among a group of smart professionals. She stated: " Some suggest the discovery of neuroplasticity is the most important breakthrough in neuroscience since the revelation of the brain's basic anatomy. Proponents say the brain is pliable and can alter its structure and function. " One MD even admitted:" We used to think the brain was wired after about the first 3 years and what you had was what you got and you work within that because there was no chance of changing it. If on top of that the brain was damaged, you had to live with that damage. Neuroplasticity says that's not so — the brain is changing all the time."

It is true that the brain is changing all the time, but it is not true that this is a discovery or a breakthrough. It is also not true that lost function is easy to recover. While it might be true that limited recovery of function is possible after brain injury, it is more true that loss of function tends to be permanent after the initial recovery in the first few months. You might consider that some physicians are just like everyone else, entertaining erroneous assumptions and unrealistic fantasies, but then, I also read rather naive comments about plasticity in the neuroscience literature.

Let me restate what should be a basic premise of neuroscience: All learning is adaptive impermanence that requires changes to brain structure and function. Another premise is that if learned skills are not refreshed through practice, skilled performance deteriorates.

You can fantasize opportunities to intervene with new technologies in the future to compensate for lost brain function, but progress to date is minimal. While there are limited populations of stem cells in the brain, their proliferation presents a hazard (aka cancer)more than a solution for degenerative brain diseases.

There is a growing body of knowledge about the growth and development of the brain from conception through adolescence; one important feature of childhood and adolescence is the pruning of synaptic connections. To make real sense from the facts as we known them is that brain structure and function is in turbulent flux with abundant opportunities for things to go wrong for 20 years. In the best case, a confused, rebellious adolescent will become a responsible adult who is a little more stable for the next 20 years and then begins a descent into cognitive decline. To believe that the brain is a finished organ at any age is nonsense. At the same time, you need to know that neurons are long lived cells that can survive from their origins in the fetus through old age. The cell body of the neuron must endure for its synaptic connections to change. The most dynamic structures are spines on dendrites and the synapses themselves. Damaged axons can regenerate if the cell body is still alive.

One basic idea in neuroscience is that the old brain is preprogrammed with maximal automaticity and stability whereas the neocortex is built to be modified. Survival depends on the stability of neuronal circuits in the oldest part of the brain. The critical controllers of respiration and cardiac function must be reliable or you die. You might compare the neocortex with dynamic random access memory in a computer that is programmable, stores memory, and can be erased.

There are time critical episodes in early development that leave no opportunity for recovery if things go wrong. Knudsen stated:" during a critical period, a neuronal pathway awaits specific instructional information encoded by impulse activity to continue developing normally. This information causes the pathway to commit irreversibly to one of a number of possible patterns of connectivity. There are critical periods for the development of form vision and stereopsis and for the development of appropriate social responses to members of the same species. "

See Neuroscience Notes by Stephen Gislason MD