Neuroplasticity
Neuroplasticity, also known as neural plasticity or just plasticity, is the medium of neural networks in the brain to change through growth and reorganization. Neuroplasticity refers to the brain's ability to reorganize and rewire its neural connections, enabling it to adapt and function in ways that differ from its prior state. This process can occur in response to learning new skills, experiencing environmental changes, recovering from injuries, or adapting to sensory or cognitive deficits. Such adaptability highlights the dynamic and ever-evolving nature of the brain, even into adulthood. These changes range from individual neuron pathways making new connections, to systematic adjustments like cortical remapping or neural oscillation. Other forms of neuroplasticity include homologous area adaptation, cross modal reassignment, map expansion, and compensatory masquerade. Examples of neuroplasticity include circuit and network changes that result from learning a new ability, information acquisition, environmental influences, pregnancy, caloric intake, practice/training, and psychological stress.
Neuroplasticity was once thought by neuroscientists to manifest only during childhood, but research in the later half of the 20th century showed that many aspects of the brain exhibit plasticity through adulthood. The developing brain exhibits a higher degree of plasticity than the adult brain. Activity-dependent plasticity can have significant implications for healthy development, learning, memory, and recovery from brain damage.
History
Origin
The term plasticity was first applied to behavior in 1890 by William James in The Principles of Psychology where the term was used to describe "a structure weak enough to yield to an influence, but strong enough not to yield all at once". The first person to use the term neural plasticity appears to have been the Polish neuroscientist Jerzy Konorski.One of the first experiments providing evidence for neuroplasticity was conducted in 1793, by Italian anatomist Michele Vincenzo Malacarne, who described experiments in which he paired animals, trained one of the pair extensively for years, and then dissected both. Malacarne discovered that the cerebellums of the trained animals were substantially larger than the cerebellum of the untrained animals. However, while these findings were significant, they were eventually forgotten. In 1890, the idea that the brain and its function are not fixed throughout adulthood was proposed by William James in The Principles of Psychology, though the idea was largely neglected. Up until the 1970s, neuroscientists believed that the brain's structure and function was essentially fixed throughout adulthood.
While the brain was commonly understood as a nonrenewable organ in the early 1900s, the pioneering neuroscientist Santiago Ramón y Cajal used the term neuronal plasticity to describe nonpathological changes in the structure of adult brains. Based on his renowned neuron doctrine, Cajal first described the neuron as the fundamental unit of the nervous system that later served as an essential foundation to develop the concept of neural plasticity. Many neuroscientists used the term plasticity to explain the regenerative capacity of the peripheral nervous system only. Cajal, however, used the term plasticity to reference his findings of degeneration and regeneration in the adult brain. This was controversial, with some like Walther Spielmeyer and Max Bielschowsky arguing that the CNS cannot produce new cells.
The term has since been broadly applied:
Research and discovery
In 1923, Karl Lashley conducted experiments on rhesus monkeys that demonstrated changes in neuronal pathways, which he concluded were evidence of plasticity. Despite this, and other research that suggested plasticity, neuroscientists did not widely accept the idea of neuroplasticity.Inspired by work from Nicolas Rashevsky, in 1943, McCulloch and Pitts proposed the artificial neuron, with a learning rule, whereby new synapses are produced when neurons fire simultaneously. This is then extensively discussed in The organization of behavior and is now known as Hebbian learning.
In 1945, Justo Gonzalo concluded from his research on brain dynamics, that, contrary to the activity of the projection areas, the "central" cortical mass, would be a "maneuvering mass", rather unspecific or multisensory, with capacity to increase neural excitability and re-organize the activity by means of plasticity properties. He gives as a first example of adaptation, to see upright with reversing glasses in the Stratton experiment, and specially, several first-hand brain injuries cases in which he observed dynamic and adaptive properties in their disorders, in particular in the inverted perception disorder . He stated that a sensory signal in a projection area would be only an inverted and constricted outline that would be magnified due to the increase in recruited cerebral mass, and re-inverted due to some effect of brain plasticity, in more central areas, following a spiral growth.
Marian Diamond of the University of California, Berkeley, produced the first scientific evidence of anatomical brain plasticity, publishing her research in 1964.
Other significant evidence was produced in the 1960s and after, notably from scientists including Paul Bach-y-Rita, Michael Merzenich along with Jon Kaas, as well as several others. An attempt to describe the mechanisms of neuroplasticity, an early version of the computational theory of mind derived from Hebb's work, was put forward by Peter Putnam and Robert W. Fuller in that time.
In the 1960s, Paul Bach-y-Rita invented a device that was tested on a small number of people, and involved a person sitting in a chair, embedded in which were nubs that were made to vibrate in ways that translated images received in a camera, allowing a form of vision via sensory substitution.
Studies in people recovering from stroke also provided support for neuroplasticity, as regions of the brain that remained healthy could sometimes take over, at least in part, functions that had been destroyed; Shepherd Ivory Franz did work in this area.
Eleanor Maguire documented changes in hippocampal structure associated with acquiring the knowledge of London's layout in local taxi drivers. A redistribution of grey matter was indicated in London Taxi Drivers compared to controls. This work on hippocampal plasticity not only interested scientists, but also engaged the public and media worldwide.
Michael Merzenich is a neuroscientist who has been one of the pioneers of neuroplasticity for over three decades. He has made some of "the most ambitious claims for the field – that brain exercises may be as useful as drugs to treat diseases as severe as schizophrenia – that plasticity exists from cradle to the grave, and that radical improvements in cognitive functioning – how we learn, think, perceive, and remember are possible even in the elderly." Merzenich's work was affected by a crucial discovery made by David Hubel and Torsten Wiesel in their work with kittens. The experiment involved sewing one eye shut and recording the cortical brain maps. Hubel and Wiesel saw that the portion of the kitten's brain associated with the shut eye was not idle, as expected. Instead, it processed visual information from the open eye. It was "…as though the brain didn't want to waste any 'cortical real estate' and had found a way to rewire itself."
This implied neuroplasticity during the critical period. However, Merzenich argued that neuroplasticity could occur beyond the critical period. His first encounter with adult plasticity came when he was engaged in a postdoctoral study with Clinton Woosley. The experiment was based on observation of what occurred in the brain when one peripheral nerve was cut and subsequently regenerated. The two scientists micromapped the hand maps of monkey brains before and after cutting a peripheral nerve and sewing the ends together. Afterwards, the hand map in the brain that they expected to be jumbled was nearly normal. This was a substantial breakthrough. Merzenich asserted that, "If the brain map could normalize its structure in response to abnormal input, the prevailing view that we are born with a hardwired system had to be wrong. The brain had to be plastic." Merzenich received the 2016 Kavli Prize in Neuroscience "for the discovery of mechanisms that allow experience and neural activity to remodel brain function."
Neurobiology
There are different ideas and theories on what biological processes allow for neuroplasticity to occur. The core of this phenomenon is based upon synapses and how connections between them change based on neuron functioning. It is widely agreed upon that neuroplasticity takes on many forms, as it is a result of a variety of pathways. These pathways, mainly signaling cascades, allow for gene expression alterations that lead to neuronal changes, and thus neuroplasticity.There are a number of other factors that are thought to play a role in the biological processes underlying the changing of neural networks in the brain. Some of these factors include synapse regulation via phosphorylation, the role of inflammation and inflammatory cytokines, proteins such as Bcl-2 proteins and neutrophorins, energy production via mitochondria, and acetylcholine.
JT Wall and J Xu have traced the mechanisms underlying neuroplasticity. Re-organization is not cortically emergent, but occurs at every level in the processing hierarchy; this produces the map changes observed in the cerebral cortex.