Plasticity induction and modulation of the human motor cortex in health and disease

In the past few years, the development of non-invasive techniques for brain stimulation have made it possible to modulate the brain excitability through synaptic plasticity-like mechanisms in the conscious human brain. Among the paradigms of brain stimulation, theta burst stimulation (TBS) based on repetitive transcranial magnetic stimulation (rTMS) is the most efficient protocol for exploring plasticity. In particular, the pattern of delivery of TBS is crucial in determining the direction of change in cortical excitability. The amount of plasticity is known to be controlled by several mechanisms, and plasticity induced in the motor cortex may interact with physical activities through, for example, metaplasticity. Metaplasticity should therefore be seriously considered in the design of combination with physical therapy and brain stimulation. Moreover, plasticity could be erased by a stimulation given within a certain time-window after the plasticity is induced. The reversal of plasticity, including depotentiation and de-depression, is considered to be important in remodeling and fine tuning our learning. Abnormal function of plasticity, metaplasticity and plasticity reversibility can therefore result in diseases, e.g. dystonia and Parkinson's disease.