Plasticity of the motor cortex in Parkinson's disease patients on and off therapy.

We used the paired associative stimulation (PAS) technique to investigate associative plasticity of the sensorimotor cortex in 16 Parkinson's disease (PD) patients off and on therapy and in 10 age-matched controls. After PAS, motor evoked potential (MEP) amplitudes increased more and the cortical silent period showed a reduced prolongation in patients off therapy than in controls. These changes lasted for at least 30 minutes. In addition, MEP amplitudes increased in a less focal manner in patients off therapy than in controls. After patients received dopaminergic therapy, these abnormalities normalized. The abnormal responsiveness of sensorimotor cortex neurons to PAS in PD patients off therapy probably reflects disordered plasticity within the motor cortex.