We intend to carry out a study in 10 to 12 parkinsonian patients before and after pallidotomy. These patients, selected because of severe motor fluctuations, will undergo unilateral pallidotomy contralateral to the most akinetic limbs. In addition to the evaluation of parkinsonian symptoms by the mean of clinical scales, we are going to evaluate more precisely two aspects of the effects of pallidotomies. Firstly, we are going to study the influence of pallidotomy on postural reflexes and balance, with electrophysiological tools. Secondly, we are going to evaluate the modification of the premotor potentials, including the
Bereitschaftspotentials with the technique used in the laboratory. A part of the Bereitschaftspotentials is presumed to reflect the activation the supplementary motor area and is impaired in Parkinson's disease. The different evaluations will be carried out, before and after surgery, in two conditions: after an overnight fasting and after the administration of a levodopa dose.