Essential tremor (ET) is a frequent and disabling disorder with progressive worsening of postural tremor of the upper limbs that impairs most of the manual activities of every day life (feeding, drinking, etc.). Although the pathophysiology of essential tremor (ET) is not fully elucidated, tremor is associated with abnormal activity within different brain regions, in particular the thalamus and the cerebellum. Deep brain stimulation (DBS) of the ventral intermediate nucleus of the thalamus (VIM-Thal) reverses the symptoms of tremor but is an invasive procedure. Transcranial stimulation of the cerebellum may represent a non-invasive therapeutic option for ET patients. Here, the investigators propose to test the efficacy of cerebellar stimulation in 15 ET patients previously operated for DBS of the thalamus. To further understand how this treatment provokes tremor reduction, the investigators will analyse the brain neuronal activity in 13 others ET patients candidate to thalamic DBS by using combined electrophysiological recordings of the thalamus (with the electrodes implanted), the cerebellum and the cortex with magnetoencephalography.