Less than fifteen years ago, it was a known fact that the neural connections in the adult brain were hard-wired and the specific neurons in each brain area were only for that region’s form and function. Neuroscientists also believed that brain injury resulted in permanent loss of function because new neurons could not be created. In 1990, President George H. Bush, observing that "a new era of discovery is dawning in brain research" proclaimed the decade beginning January 1, 1990 as the Decade of the Brain.
Plasticity is an essential organizational feature of human brain function. Historically, the brain was thought to be hardwired following a critical period in development. This brain plasticity underlies normal development and maturation, skill learning and memory, recovery from injury, as well as the consequences of sensory deprivation or environmental enrichment.
The brains of infants and children are known to be plastic, undergoing spurts of neuronal development in response to stimulus exposure during critical periods (Mundkur, 2005). This development consists of the genesis of neurons, increased connectivity between neurons and the routing of new synaptic connections between previously unrelated neurons. There is an incredible increase in synapses occurs during the first year of life. The functional architecture of the brain is created through the development of these synapses. By the time a child is three years old, a baby's brain has formed about 1,000 trillion connections about twice as many as adults have. Near age eleven, a child's brain begins eliminating the extra connections in a process calling "pruning".
However, it is now known that neuroplasticity occurs with many variations, in many forms, and in many contexts. The discovery of the growth of new neural tissue in the adult human hippocampus, a brain region responsible for memory (Eriksson et al., 1998), led the widely accepted concept of the hardwired brain to be renounced. The ability of the adult brain to grow new neurons, once scoffed at, is now accepted as a reality. This determination complemented earlier data from primate studies demonstrating that sensory experience and learning new behaviors triggers neuron growth in the somatosensory and motor cortices, areas of the brain subserving tactile perception and limb movement (Jenkins, Merzenich, Ochs, Allard, & Guic-Robles, 1990; Nudo, Milliken, Jenkins, & Merzenich, 1996). Subsequent to the discovery of neurogenesis in the adult human brain, neuroscience has enthusiastically pursued additional investigation of these findings, aided by advances in brain imaging techniques such as functional magnetic-resonance imaging (fMRI) and diffusion tensor imaging (DTI) .
The growth of neurons in the brains of adults repeatedly exposed to a variety of experiences has been well documented. Taxi drivers develop the areas of their brains involved in spatial relationships by memorizing the streets and avenues of the cities in which they work (Maguire et al., 2000). Violinists evidence neural growth in the portion of their somatosensory cortex devoted to their fingering hand through hours of musical practice (Elbert, Pantev, Wienbruch, Rockstroh, & Taub, 1995), as do people who juggle. (Draganski et al., 2004).
Additionally, mental practice promotes neuroplasticity: neurogenesis occurs in the motor cortex just by imagining playing the piano (Pascual-Leone, Amedi, Fregni, & Merabet, 2005). While the underlying mechanisms are different, neuroplasticity research suggests that challenging learning experiences can lead to the development of brain tissue just as exercise stimulates muscle development. Studies have shown that mental practice has successfully used on stroke patients, who visualize motor movements through mental imagery, complementing other cognitive-based treatments. It has been suggested that patients should be taught to mentally visualize...