Abnormal Movements, Cerebellum and Sensorimotor : Oculomotor Study

Dystonia is a movement disorder characterized by involuntary, sustained, often repetitive muscle contractions of opposite muscles that lead to abnormal twisting movements or odd postures. Essential tremor is a slowly progressive neurologic disorder characterized by the appearance of a tremor during the voluntary movement. The pathophysiology of dystonia or essential tremor is not fully elucidated. Dystonia and essential tremor are associated with dysfunction of the sensorimotor basal ganglia-cortical network and involvement of the cerebellum and cerebellar pathways has also been recently suggested. The investigators propose to study 30 patients having a primary dystonia (15 DYT11 genetically documented), 15 patients having an essential tremor without deep brain stimulation and 15 patients having an essential tremor with deep brain stimulation.A group of 30 healthy volunteers will be recruited and tested according to the same modalities. They will be paired in sex and age. 30 patients having a Parkinson disease will be also tested. Eye position will be sampled with a video-based monocular eye tracker (SMI, Germany) before and immediately after an adaptation task. Saccade adaptation is evaluated as the percentage change in the mean saccade amplitude between pre-test and post-test. Expected results: * no or fewer alteration of the performance to the adaptation task in the Parkinson group than in the Essential Tremor group/ dystonia group. * abnormal reactive saccade backward adaptation in the Dystonia group and Essential Tremor group, providing further neurophysiological evidence of cerebellar dysfunction.