[Anecdote addressing rehabilitation from brain injury]
This change in the brain due to learning is an example of neuroplasticity. Neuroplasticity is the ability of the brain and nervous system in all species to change structurally and functionally as a result of input from the environment (Mundkur 855). Essentially, the brain is constantly changing. New neural pathways are created, while unused ones may wither away. Neural plasticity ranges in magnitude from cellular changes involved in learning, as observed above, to large-scale changes involved in cortical remapping in response to injury. Experience changes both the anatomy (physical) and physiology (function) of brain. There are two types of neuroplasticity: structural and functional. Structural plasticity refers to the brain's ability to actually change its physical structure as a result of learning, and functional plasticity is the brain's ability to move functions from a damaged area of the brain to other undamaged areas. The realization of brain plasticity ushered in a new age of psychology and neuroscience, showing there is hope for recovery for those affected with psychological disorders or brain trauma.
The prevailing belief in neuroscience was that the adult human brain is essentially unchangeable - fixed in form and function, so that by the time we reach adulthood no change in the brain is possible. The accepted theory was that neurogenesis (neuron birth) stopped past the age of mid-childhood (History). This was one of modern neuroscience’s founding principles (Lehrer 1). This theory lowered expectations about the value of rehabilitation for adults who had suffered brain damage from a stroke or about the possibility of fixing the pathological wiring that underlies psychiatric diseases. If the brain was hardwired by adulthood, there would be no way to correct any brain injuries or disorders, for there would be no change possible. Besides some alleviation from a reduction in swelling or some other small factor, they basically weren’t expected to recover.
However, research now shows that the brain retains impressive powers of "neuroplasticity"--the ability to change its structure and function in response to experience (The Brain 1). For example, if a stroke destroys part of the motor cortex that moves the right arm, a technique called constraint-induced movement therapy can condition nearby regions to take over the function of the damaged area, thus bringing motion back to the arm. Neuroplasticity was mentioned as early as 1890, but did not gain wide acceptance until the 1970s (History).
William James presented the first theory of neuroplasticity in 1890 in his book "Principles of Psychology”. In it James brought up his theory of brain plasticity, saying that ““Organic matter, especially nervous tissue, seems endowed with a very extraordinary degree of plasticity.” He is credited with first suggesting that the human brain is capable of reorganizing. James suggested that any habit becomes ingrained in the brain and becomes part of the normal brain (James). The brain, according to James, does this because 1) “habit simplifies the movements required to achieve a given result, makes them more accurate and diminishes fatigue”, and 2) “habit diminishes the conscious attention with which our acts are performed.” By streamlining the pathways to certain repetitive actions and decreasing the thought processes reaching those processes, the brain saves energy and processing power for other tasks.
In 1923, Karl Lashley performed experiments on rhesus monkeys demonstrating changes in neural pathways (History). He concluded that these changes were the proof of brain plasticity, but despite his concrete evidence other neuroscientists were not persuaded, and so neuroplasticity remained an outlandish theory.
The first documented person known to use the term “neuroplasticity” was Polish Neuroscientist Jerzy Konorski in 1948. Konorski suggested that over time neurons that had coincidental activation due to the vicinity to the firing neuron would after time create plastic changes in the brain (History). His theory on synapses strengthening due to use is similar to later Hebbian theory.
Donald Hebb sought to understand how the function of neurons contributed to psychological processes such as learning. He was influential in the neuropsychological field for his theory connecting the mind and the brain. His Hebbian Law is best summed up by the quote “When an axon of cell A is near enough to excite cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased” (Hebb). This is commonly substituted with the phrase “neurons that fire together, wire together”, meaning that the brain can form fresh pathways by having new unique patterns of neural cells firing together.
The first discoveries of neuroplasticity came from studies of how changes in the messages the brain receives through the senses can alter its structure and function. One of the most influential proponents of neuroplasticity was Paul Bach-y-Rita. He was one of the first neuroscientists to actually create modern applications for what had been studied, and also one of first to introduce sensory substitution as a tool to treat patients suffering from neurological disorders. He believed that if one sense were to sustain damage, a person’s other senses might be able to compensate (History). Bach-y-Rita’s most noted work was with stroke patients and patients with balancing issues due to vestibular damage (inner ear). Bach-y-Rita first became interested in stroke victim therapy when his father suffered a stroke and subsequent physical impairment. Bach-y-Rita believed from what he knew about the brain's ability to reorganize through neuroplasticity that healthy brain areas could take over functioning of brain's areas damaged by the loss of blood flow due to the stroke (History). He developed therapeutic exercises for his father, who made an almost full recovery of the language and physical impairments he had incurred.
Bach-y-Rita also helped to develop treatment for people with vestibular damage that causes severe balancing issues. He developed a device called Brainport, which helped rectify their balancing issues and even helped blind people see (History). It consisted of a group of accelerometers positioned on the patient and linked to a computer. Information is processed and fed to a small plate which is positioned on the patient's tongue (due to the high density of sensory receptors), and stimulation of different areas of the tongue enabled the patient to stay balanced (History). He took advantage of these sensors to send the impulses as visual signals and redirect to the visual cortex. Once treated, patients could stand on their own almost immediately while being connected to the device and even for short periods after being disconnected (History). After several weeks of treatment the patients were able to stand without being connected to the device – completely cured of balance problems (History). Bach-y-Rita demonstrated application of neuroplasticity in treating neurological disorders, and also the ability of the brain to adapt to repeated stimuli.
Edward Taub’s research helped create the most technologically advanced methods for treating people that have had strokes, other cardiovascular accidents, or neurological disorders that cause severe physical impairments or paralysis. He developed constraint-induced movement therapy (CI therapy) that treats both upper and lower limb paralysis in such patients (Taub). As a result of the patient engaging in repetitive exercises with the affected limb, the brain grows new neural pathways and use of limb redevelops. Neuroimaging and TMS showed that intensive CI therapy resulted in cortical reorganization that increases the area of cortex involved in the movement of the more-affected limb (Taub).
Joseph Altman first discovered adult neurogenesis in adults in 1962, but his research was ignored until 1999, when Elizabeth Gould displayed similar findings. She found evidence of neurogenesis in primates, specifically in the prefrontal, inferior temporal, and posterior parietal cortexes (. While neurogenesis had been discovered in the primate hippocampus before, this was much more dramatic, as the cerebral cortex is the largest and most complex part of the brain (Schultz). Resistance to the idea of neuroplasticity slowly eroded in the face of more and more empirical evidence throughout the twentieth century, and while it was accepted at this time, the discovery of neurogenesis in the cortex had major implications for theories about how the brain develops.