Our most complex artifact, the digital computer, has long been used as a metaphor to explain how the brain works.
Now, brain scientists are increasingly researching "neuroplasticity": how the brain changes in ways that are utterly un-computer-like.
Brain cells and their connections are not static. The brain grows new cells, these form ever-changing networks, which are to a large extent the physical basis of our psychological and emotional lives. Learning and memory take place because of dynamic alterations in the strength of the connections between cells, the grouping of cells into networks and the connections between networks.
In one sense, biologists and psychologists have known about these changes for a long time, and the label "neuroplasticity" is just a new name for old ideas.
However, there is a change in emphasis that the term points to. In particular, evidence that adult brains grow brand new nerve cells was very much in doubt just ten or twenty years ago.
This and other developments have forced the textbooks to be rewritten. Scientists are also learning more details about changes that take place in the brain in response to healthy and harmful stimuli. Together, these new findings give us an updated picture of the brain as made of dynamic, "self organizing" sets of networks that grow new components and which change in response to stress, damage, learning and novelty.
Are we wired, like microchips, or are we made of something more complex?
Unlike fixed, rigid computer electronics, brains are dynamic, can recover in response to trauma and show the ability to change structure without losing function ("plasticity").
A single mote of dust can ruin manufacture of a microprocessor. Circuits in electronic devices eventually fail and do not regenerate; they are replaced if hardware is to function properly. Yet the human brain, presented with stress or damage, can display amazing powers of self-organization, dynamism, response to change at different scales and the ability to "re-wire" itself.
Except that the brain has no wires. It is vastly more complex than any computer ever designed, and is the most complex system known to exist in nature.
Nowadays, one may still encounter the idea that the brain is "hard wired" to engage in language, face recognition, memory and so forth. Psychologists, neuroscientists and cognitive scientists emphasize these functions are innate and instinctual. This is not so much wrong as slightly misleading. We do have instincts and we did evolve to be good at certain tasks, but our brains are not fixed and unchanging the way a microchip is.
Where did the idea that the brain is "hard wired" come from?
Around the time of World War II and its immediate aftermath, when brilliant early computer scientists coaxed their whirring vacuum-tube machinery to solve complex mathematical problems, the idea that the brain was a sort of computer took hold. The finding that neurons are electrical and generate all-or-nothing binary-like bursts of current made the brain seem like nature's version of an electronic computing device.
Over the decades, a different emphasis on how the brain works has emerged, especially in recent years. One still reads about humans being "hard wired" by our genes to do this and that, but an increasing number of researchers use the term "neuroplasticity" to refer to the way the brain responds to change.
Dr. Jill Kays and colleagues explain the shift in thinking that has occurred among scientists and clinicians: "Until fairly recently, the adult brain was considered largely fixed and stable. Although it was accepted that changes occurred in the context of learning and memory, the general consensus was that major processes essential to normal brain development (e.g., generation of new neurons, neuron migration, pruning) ceased once full development was reached."
The brain's 100 billion neurons grow into each other like bushes sharing the same space. The branches of the neurons connect with each other to form networks. Cells reach out to each other via electrochemical synapses, which are microscopic, fluid-filled connective zones.
A family of hormones called nerve growth factors are periodically released and bind to specialized protein structures called receptors. Growth factors enable nerve cells to persist and to dynamically respond to the changing chemical environment in human physiology. These are only one of a series of hormones that are released in response to new situations, emotionally significant experiences, stressful contexts, brain damage and more. Nothing of the sort occurs with microprocessors.
Like making a new friend that you start hearing from more and more, synaptic connections can grow stronger. Yet they also may weaken, if not used enough. The strengthening and weakening of connections over time, which form the cellular basis of learning and memory, show the brain to be quite unlike a microchip, which is utterly rigid and has fixed circuits.
One of the difficulties with the term "plasticity" is finding a workable definition. Dr. Christopher Shaw and colleagues in Toward a Theory of Neuroplasticity define it as "induced change in some property of the nervous system that results in a corresponding change in function and/or behavior".
Plasticity refers to the range of changes that brain cells can generate as their physiochemical environments shift. However, neurons cannot handle stress beyond a certain point. For instance, traumatic brain injury can shear the long cable-like axons of brain cells, producing serious neurological damage, coma or even death.
The clinical relevance of neuroplasticity
In previous eras of neurology, psychology and medicine in general, it was believed that adults did not develop new neurons. This may seem a technical point of interest primarily to anatomists or physiologists, but it was consistent with a certain emphasis on how development is constrained by genetics, inheritance and the pre-determined "hard wiring" of the brain.
Overall, the emerging view is one in which the brain is understood to self-organize based on genetic instructions, to grow new cells, to adapt to new conditions, to respond to damage by a sort of rerouting, thereby compensating for damage in one area through other networks taking over that function. A computer must be programmed by someone external to itself, but brains have genetic instructions that allow it to self-organize, to grow and to make new connections.
Stress, trauma, as well as learning and novelty can affect brain structure and function. Stress hormones such as cortisol are critical for the "fight or flight" response. However they can alter the way the brain's cortex and hippocampus work together, and subvert the ability form new memories. This is a form of plasticity that clinicians would like to be able to prevent, and medications may in fact help with protecting these vital connections.
A stroke can deprive nerve cells of oxygen and kill them. Areas of "necrosis" (dead tissue) from a stroke or a blow to the head will tend to compromise mental functioning. Yet the plasticity of the brain can allow people to heal by recruiting other populations of cells to handle these processes. New networks can form to compensate for the loss of the old.
Dr. Kays and colleagues describe the relevance of neuroplasticity to clinical situations involving sensory disorders: "Both blind and deaf individuals often demonstrate superior skills in their remaining senses, as compared with individuals with all senses intact. Also, areas of brain normally dedicated to the missing sense can be recruited for use by other sensory modalities. Braille reading, for example, has been shown to require participation of visual cortex."
Learning and new, meaningful experiences novelty are associated with the formation of new synapses. These synapses can connect cells into new networks. For a person who speaks one language only, learning a new language can push the brain to develop new synapses and new networks. For a person who already knows ten languages, this effect would not be as pronounced.
Kays and team express some optimism that patients with psychological disorders may benefit from neuroscientist's research into the growth of new cells and the brain's dynamic ability to reorganize: "Researchers have begun examining ways to harness neuroplasticity to promote healing and recovery. Although these efforts are still in the beginning stages, there is promising evidence that the dynamic qualities of the brain may play a pivotal role in how one copes with stress and mental illness."
"Neuroplasticity" is a newish name for an old concept: brains can change structure and function over time. Stress, trauma, as well as learning and novelty can affect brain structure and function.
Plasticity enables the human brain to bounce back from injuries, can display un-computer-like dynamic changes to stimuli, and grow new neurons. Unlike a microprocessor in a computer, human brains can reroute and use other networks of cells to compensate for reduced function elsewhere, such as when due to stroke or a sensory disorder.
By investigating how cells form new synaptic connections in response to stress, trauma and learning, the field of neuroplasticity research may help provide new treatments for people with psychological disorders. Patients should empower themselves to learn more about this leading-edge branch of science to find out whether they can benefit from this research.