Cognitive disorders in Parkinson's disease (PD) are not as obvious as cognitive disorders in Alzheimer disease and their diagnosis tends to be delayed. If neuropsychological assessment of cognitive decline in Parkinson's disease is well established, the thresholds of cognitive testing corresponding to a dementia are unknown. Recently, new diagnosis criteria of dementia associated with PD have been proposed by the movement disorder society. In this study two groups of patients with idiopathic PD will be separated and compared according to their Mini Mental State Examination (MMSE) score (upper or lower than 26). Included patients will have clinically idiopathic PD, will be over 65 years old and will present cognitive complain. Cognitive and mood disorders as well as motor symptoms will be assessed using validated scales and a neuropsychological assessment dispatched in two visits will be performed . Differences in the distribution of data from the two groups of patients will be assessed in statistically analysis with non parametric tests. The purpose of this study is to determine the most effective tests and their threshold value corresponding to a pathological cognitive decline.