Impermanence Often Wrongly Described as 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.
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 lost souls with erroneous assumptions and
unrealistic fantasies, but then, I also read rather naive comments about
plasticity in the neuroscience literature.
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. '
Neurons and glial cells are the brain cells that a manifest
all the properties of mind. The study of neurons could be considered ne plus
ultra, the quantum mechanics of biology. Neurons come in different shapes and
sizes but have the common property of constant changes receiving and
sending information. Neurons conduct discrete signals as electro-chemical
pulses, known as action potentials or “spikes.” The signal passes from one
neuron to another by the secretion of chemical neurotransmitters in synapses.
There are trillions of synaptic junctions in the human brain. Learning occurs at
least in part by changes in the number, strength and kind of synaptic
connections.
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.
Sleep is a transformative time of day. Cortical neurons are active, reviewing
events of the day. During slow-wave sleep, the cortex disconnects from other
parts of the brain and concentrates on memory consolidation. The emergent
properties of the sleeping brain are unpredictable. You could argue that the
events of each day will alter the brain during sleep and a new person wakes in
the morning.
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 and social activity, reduced caloric
intake and good nutrition are the keys to high functioning aging.
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. A the same time, 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.
A basic idea in neuroscience is 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. "
