Transcranial magnetic stimulation as a method for investigating the plasticity of the brain in Parkinson's disease and dystonia.

It is now possible to probe the plasticity of some neural circuits in the human motor cortex using transcranial magnetic stimulation (TMS). This article illustrates how changes in the plasticity of these circuits is linked to the expression of dyskinesias in Parkinson's disease, and may even underlie the tendency of some individuals to develop focal dystonia. Indeed, gradual normalisation of this excessive plasticity occurs after initiating deep brain stimulation of the internal globus pallidus in patients with generalised dystonia. It may therefore relate to the slow onset of clinical improvement that occurs over the first 6 weeks or so of treatment.