Plasticity (brain)

Brain plasticity refers to the changes that occur in the organisation of the brain, and in particular changes that occur to the location of specific information processing functions, as a result of the effect of experience. The term cortical plasticity is more commonly used, however there is no particular restriction of the phenomenon to the cortex. A common and surprising consequence of plasticity is that the location of a given function can "move" from one location to another in the brain.

The concept of plasticity can be applied to molecular as well as to environmental events. The phenomenon itself is complex and involves many levels of organization. To some extent the term itself has lost its explanatory value because almost any changes in brain activity can be attributed to some sort of "plasticity". Plasticity should be more restricted to adaptive events in the central nervous system rather than merely indicating any change in response to environmental input. For example, after a traumatic brain injury, if the organism can recover to normal levels of performance, that adaptiveness could be considered an example of "positive plasticity". However, an excessive level of neuronal growth leading to spasticity or tonic paralysis, or an excessive release of neurotransmitters in response to injury which could kill nerve cells, would have to be considered perhaps as a "negative or maladaptive" plasticity.

The main thing to know is that even the adult brain is not "hard-wired" with fixed and immutable neuronal circuits. Many people have been taught to believe that once a brain injury occurs, there is little to do to repair the damage. This is simply not the case and there is no fixed period of time after which "plasticity" is blocked or lost. We simply do not know all of the conditions that can enhance neuronal plasticity in the intact and damaged brain, but new discoveries are being made all of the time. There are many instances of cortical and subcortical rewiring of neuronal circuits in response to training as well as in response to injury. There is now solid evidence that neurogenesis, the formation of new nerve cells, is possible in the adult, mammalian brain--and such changes can persist well into old age.

Brain plasticity and cortical maps
Cortical organization, especially for the sensory systems, is often described in terms of maps. For example, sensory information from the foot projects to one cortical site and the projections from the hand target in another site. As the result of this somatotopic organization of sensory inputs to the cortex, cortical representation of the body resembles a map (or homunculus). Interestingly, cortical maps are not fixed, but rather plastic. The work of John, Pomeranz, Kaas, Merzenich, Diamond, Ebner, Nicolelis and many other researchers has shown that cortical maps change after manipulations with peripheral inputs (e.g., sensory nerve transection): cortical representations deprived of sensory input are 'filled' by adjacent representations. Similar plasticity of cortical maps results from changes in sensory experience.

An interesting phenomenon involving cortical maps is the incidence of phantom limbs. This is most commonly described in people that have undergone amputations in hands, arms, and legs, but it is not limited to extremeties. The phantom limb feeling, which is thought to result from disorganization in the homunculus and the inability to receive input from the targeted area, may be annoying or painful. Incidentally, it is more common after unexpected losses than planned amputations. There is a high correlation with the extent of physical remapping and the extent of phantom pain. As it fades, it is a fascinating functional example of new neural connections in the human adult brain.

Suggested Reading

Ramachandran & Hirstein. (1998). The perception of phantom limbs Brain, 121:  1603-1630.

Flor, H. (2002). Phantom limb pain: Characteristics, causes, and treatments. Lancet Neurology, 1: 182-189.

Brain plasticity during operation of brain-machine interfaces
Brain-machine interface (BMI) is a rapidly developing field of Neuroscience. According to the results obtained by Mikhail Lebedev, Miguel Nicolelis and their colleagues (Lebedev, M.A., Carmena, J.M., O’Doherty, J.E., Zacksenhouse, M., Henriquez, C.S., Principe, J.C., Nicolelis, M.A.L. (2005) Cortical ensemble adaptation to represent actuators controlled by a brain machine interface. J. Neurosci. 25: 4681-4693), operation of BMIs results in incorporation of artificial actuators into brain representations. The scientists showed that modifications in neuronal representation of the monkey's hand and the actuator that was controlled by the monkey brain occurred in multiple cortical areas while the monkey operated a BMI. Initially, monkeys moved the actuator by pushing a joystick. After the monkey started using its brain activity to directly control the actuator, the activity of individual neurons and neuronal populations became less representative of the animal's hand movements while representing the movements of the actuator. Presumably as a result of this adaptation, the animals could eventually stop moving their hands yet continue to operate the actuator. Thus, during BMI control, cortical ensembles plastically adapt to represent behaviorally significant motor parameters, even if these are not associated with movements of the animal's own limb. Active laboratory groups include those of John Donoghue at Brown, Richard Andersen at Caltech, Andy Schwartz at Pitt, and Miguel Nicolelis at Duke. Donoghue and Nicolelis' groups have independently shown that animals can control external interfaces in tasks requiring feedback, with models based on activity of cortical neurons, and that animals can adaptively change their minds to make the models work better. Donoghue's group took the implants from Richard Normann's lab at Utah (the "Utah" array), and improved it by changing the insulation from polyimide to parylene-c, and commercialized it through the company Cyberkinetics. These efforts are the leading candidate for the first human trials on a broad scale for motor cortical implants to help quadriplegic or trapped patients communicate with the outside world.

Suggested reading " Nat Neurosci. 2002 Nov;5 Suppl:1085-8.
 * Donoghue JP, "Connecting cortex to machines: recent advances in brain interfaces.
 * Lebedev, M.A., Carmena, J.M., O’Doherty, J.E., Zacksenhouse, M., Henriquez, C.S., Principe, J.C., Nicolelis, M.A.L. (2005) [http://www.jneurosci.org/cgi/content/full/25/19/4681 Cortical ensemble adaptation to represent actuators controlled by a brain machine interface.
 * Monkeys Treat Robot Arm as Their Own
 * Monkeys treat robot arm as bonus appendage
 * Monkey See, Robotics Do