Patients with Parkinson's disease can suffer from neuropsychological deficits which, in most cases, are already demonstrated at the very beginning (53). Primarily, they are characterized by impairments associated with frontal lobe function affecting psychomotor speed and mental flexibility. Furthermore they show circumscribed impairment of memory functions as well as deficits of visuospatial information processing. Sets of neuropsychological dysfunction of that kind are called subcortical dementia in the American literature. In addition, Parkinson's disease is quite often associated with depressive states which are independent of the neuropsychological deficits. Application of L-Dopa helps to improve the cognitive impairment.